[RFC,4/4] ld: bfd: sframe: fix incorrect r_addend in RELA entries

  With the fix in GAS, we now use a different PC-relative RELA for
updating the SFrame FDE function start address: The value is the offset
of the start PC of the function from the start of the SFrame section.

When RELAs are output (e.g. for relocatable links), there is need to
adjust the r_addend.  This is because the r_addend values still have the
correct values for the _input_ SFrame section being linked / relocated.
The values must now be (before outputing the RELAs) with respect to the
_output_ SFrame section.

PS: This patch should be merged with the previous commits before final
commit (Otherwise the tests will fail).  It is currently a separate
patch as I would like to check if this is OK to do.  If such a "addend
fixup" is risky or wrong, it seems we will need a new type of RELOC for
SFrame sections.

bfd/
        * elf-bfd.h: New declaration.
        * elf-sframe.c (_bfd_elf_sframe_section_addend): New
	definition.
        * elflink.c (elf_link_input_bfd): Fixup the r_addend of
	SFrame RELAs.
---
 bfd/elf-bfd.h    |  2 ++
 bfd/elf-sframe.c | 31 +++++++++++++++++++++++++++++++
 bfd/elflink.c    | 24 ++++++++++++++++++++----
 3 files changed, 53 insertions(+), 4 deletions(-)
  

On 08.03.2025 08:38, Indu Bhagat wrote:
> With the fix in GAS, we now use a different PC-relative RELA for
> updating the SFrame FDE function start address: The value is the offset
> of the start PC of the function from the start of the SFrame section.
> 
> When RELAs are output (e.g. for relocatable links), there is need to
> adjust the r_addend.  This is because the r_addend values still have the
> correct values for the _input_ SFrame section being linked / relocated.
> The values must now be (before outputing the RELAs) with respect to the
> _output_ SFrame section.
> 
> PS: This patch should be merged with the previous commits before final
> commit (Otherwise the tests will fail).  It is currently a separate
> patch as I would like to check if this is OK to do.  If such a "addend
> fixup" is risky or wrong, it seems we will need a new type of RELOC for
> SFrame sections.

> diff --git a/bfd/elf-sframe.c b/bfd/elf-sframe.c

> @@ -574,6 +574,37 @@ _bfd_elf_sframe_section_offset (bfd *output_bfd ATTRIBUTE_UNUSED,
>   	  + out_fde_idx * sizeof (sframe_func_desc_entry));
>   }
>   
> +/* Get the "canonicalized" addend for the symbol reference corresponding to the
> +   relocation at RELOC_INDEX.  E.g.,  for the following reloc for the SFrame

Nit: s/reloc/relocs/

> +   FDE function start address:
> +	Offset          Type          Sym. Name + Addend
> +	00000000001c  R_X86_64_PC32   .text + 1c
> +	000000000030  R_X86_64_PC32   .text + 3b
> +   The canonicalized addend are 0 and b respectively as the relocs are for
> +   symbols (.text + 0) and (.text + b) respectively.

Maybe it would help to use the function symbols foo and bar in the example:

    relocation at RELOC_INDEX.  E.g.,  for the following annotated relocs for the SFrame
    FDE function start address of FDE[0] and FDE[1] for functions foo and bar:
	Offset          Type          Sym. Name + Addend
	00000000001c  R_X86_64_PC32   .text + 1c   // FDE[0] for foo at .text + 0
	000000000030  R_X86_64_PC32   .text + 3b   // FDE[1] for bar at .text + b
    With:
      1c = 0 + sizeof(sframe_header) + 0 * sizeof(sframe_fde)
      3b = b + sizeof(sframe_header) + 1 * sizeof(sframe_fde)
    The canonicalized addend are 0 and b respectively as the relocs are for
    symbols (.text + 0) and (.text + b) respectively.

> +
> +   This is used to manually adjust the RELA addends to ensure correct values
> +   for relocatable links.  */
> +
> +bfd_vma
> +_bfd_elf_sframe_section_addend (bfd *output_bfd ATTRIBUTE_UNUSED,
> +			      struct bfd_link_info *info ATTRIBUTE_UNUSED,
> +			      asection *sec,
> +			      unsigned int reloc_index,
> +			      bfd_vma addend)
> +{
> +  struct sframe_dec_info *sfd_info;
> +
> +  if (sec->sec_info_type != SEC_INFO_TYPE_SFRAME)
> +    return addend;
> +
> +  sfd_info = (struct sframe_dec_info *) elf_section_data (sec)->sec_info;
> +  BFD_ASSERT (sfd_info && sfd_info->sfd_ctx);
> +
> +  return (addend - (sframe_decoder_get_hdr_size (sfd_info->sfd_ctx)
> +		    + reloc_index * sizeof (sframe_func_desc_entry)));

IIUC this is only valid as long as reloc_index == fde_index.

> +}
> +
>   /* Write out the .sframe section.  This must be called after
>      _bfd_elf_merge_section_sframe has been called on all input
>      .sframe sections.  */
> diff --git a/bfd/elflink.c b/bfd/elflink.c
> index 895fbb0206e..6bce2966359 100644
> --- a/bfd/elflink.c
> +++ b/bfd/elflink.c
> @@ -11939,7 +11939,9 @@ elf_link_input_bfd (struct elf_final_link_info *flinfo, bfd *input_bfd)
>   	      last_offset = o->output_offset;
>   	      if (!bfd_link_relocatable (flinfo->info))
>   		last_offset += o->output_section->vma;
> -	      for (next_erel = 0; irela < irelaend; irela++, next_erel++)
> +	      unsigned int num_reloc = 0;
> +	      for (next_erel = 0; irela < irelaend;
> +		   irela++, next_erel++, num_reloc++)

The introduction of num_reloc could be omitted.  See below.

>   		{
>   		  unsigned long r_symndx;
>   		  asection *sec;
> @@ -12070,10 +12072,24 @@ elf_link_input_bfd (struct elf_final_link_info *flinfo, bfd *input_bfd)
>   			    }
>   			}
>   
> -		      /* Adjust the addend according to where the
> -			 section winds up in the output section.  */
>   		      if (rela_normal)
> -			irela->r_addend += sec->output_offset;
> +			{
> +			  if (o->sec_info_type == SEC_INFO_TYPE_SFRAME)
> +			    {

			      unsigned int num_reloc = irela - internal_relocs;

> +			      bfd_vma addend
> +			       = _bfd_elf_sframe_section_addend (output_bfd,
> +							       flinfo->info, o,
> +							       num_reloc,
> +							       irela->r_addend);
> +			      /* Adjust the addend in the output RELA.  The
> +				 input SFrame section has already been
> +				 relocated.  */
> +			      irela->r_addend = addend + irela->r_offset;
> +			    }
> +			  /* Adjust the addend according to where the
> +			     section winds up in the output section.  */
> +			  irela->r_addend += sec->output_offset;
> +			}
>   		    }
>   		  else
>   		    {

Regards,
Jens
  

On 10.03.2025 14:16, Jens Remus wrote:
> On 08.03.2025 08:38, Indu Bhagat wrote:
>> With the fix in GAS, we now use a different PC-relative RELA for
>> updating the SFrame FDE function start address: The value is the offset
>> of the start PC of the function from the start of the SFrame section.
>>
>> When RELAs are output (e.g. for relocatable links), there is need to
>> adjust the r_addend.  This is because the r_addend values still have the
>> correct values for the _input_ SFrame section being linked / relocated.
>> The values must now be (before outputing the RELAs) with respect to the
>> _output_ SFrame section.
>>
>> PS: This patch should be merged with the previous commits before final
>> commit (Otherwise the tests will fail).  It is currently a separate
>> patch as I would like to check if this is OK to do.  If such a "addend
>> fixup" is risky or wrong, it seems we will need a new type of RELOC for
>> SFrame sections.
> 
>> diff --git a/bfd/elf-sframe.c b/bfd/elf-sframe.c
> 
>> @@ -574,6 +574,37 @@ _bfd_elf_sframe_section_offset (bfd *output_bfd ATTRIBUTE_UNUSED,
>>   	  + out_fde_idx * sizeof (sframe_func_desc_entry));
>>   }
>>   
>> +/* Get the "canonicalized" addend for the symbol reference corresponding to the
>> +   relocation at RELOC_INDEX.  E.g.,  for the following reloc for the SFrame
> 
> Nit: s/reloc/relocs/
> 
>> +   FDE function start address:
>> +	Offset          Type          Sym. Name + Addend
>> +	00000000001c  R_X86_64_PC32   .text + 1c
>> +	000000000030  R_X86_64_PC32   .text + 3b
>> +   The canonicalized addend are 0 and b respectively as the relocs are for
>> +   symbols (.text + 0) and (.text + b) respectively.
> 
> Maybe it would help to use the function symbols foo and bar in the example:
> 
>     relocation at RELOC_INDEX.  E.g.,  for the following annotated relocs for the SFrame
>     FDE function start address of FDE[0] and FDE[1] for functions foo and bar:
> 	Offset          Type          Sym. Name + Addend
> 	00000000001c  R_X86_64_PC32   .text + 1c   // FDE[0] for foo at .text + 0
> 	000000000030  R_X86_64_PC32   .text + 3b   // FDE[1] for bar at .text + b
>     With:
>       1c = 0 + sizeof(sframe_header) + 0 * sizeof(sframe_fde)
>       3b = b + sizeof(sframe_header) + 1 * sizeof(sframe_fde)
>     The canonicalized addend are 0 and b respectively as the relocs are for
>     symbols (.text + 0) and (.text + b) respectively.

Hmm, wait - there's a mix between .text and .sframe then, isn't there? That
would explain why custom handling is then necessary. Yet as said in the other
mail, custom handling should not be necessary. Not the least because other
ELF-consuming tools also need to work, not just what GNU binutils provides.

Jan
  

Hi Jan.

> On 10.03.2025 14:16, Jens Remus wrote:
>> On 08.03.2025 08:38, Indu Bhagat wrote:
>>> With the fix in GAS, we now use a different PC-relative RELA for
>>> updating the SFrame FDE function start address: The value is the offset
>>> of the start PC of the function from the start of the SFrame section.
>>>
>>> When RELAs are output (e.g. for relocatable links), there is need to
>>> adjust the r_addend.  This is because the r_addend values still have the
>>> correct values for the _input_ SFrame section being linked / relocated.
>>> The values must now be (before outputing the RELAs) with respect to the
>>> _output_ SFrame section.
>>>
>>> PS: This patch should be merged with the previous commits before final
>>> commit (Otherwise the tests will fail).  It is currently a separate
>>> patch as I would like to check if this is OK to do.  If such a "addend
>>> fixup" is risky or wrong, it seems we will need a new type of RELOC for
>>> SFrame sections.
>> 
>>> diff --git a/bfd/elf-sframe.c b/bfd/elf-sframe.c
>> 
>>> @@ -574,6 +574,37 @@ _bfd_elf_sframe_section_offset (bfd *output_bfd ATTRIBUTE_UNUSED,
>>>   	  + out_fde_idx * sizeof (sframe_func_desc_entry));
>>>   }
>>>   
>>> +/* Get the "canonicalized" addend for the symbol reference corresponding to the
>>> +   relocation at RELOC_INDEX.  E.g.,  for the following reloc for the SFrame
>> 
>> Nit: s/reloc/relocs/
>> 
>>> +   FDE function start address:
>>> +	Offset          Type          Sym. Name + Addend
>>> +	00000000001c  R_X86_64_PC32   .text + 1c
>>> +	000000000030  R_X86_64_PC32   .text + 3b
>>> +   The canonicalized addend are 0 and b respectively as the relocs are for
>>> +   symbols (.text + 0) and (.text + b) respectively.
>> 
>> Maybe it would help to use the function symbols foo and bar in the example:
>> 
>>     relocation at RELOC_INDEX.  E.g.,  for the following annotated relocs for the SFrame
>>     FDE function start address of FDE[0] and FDE[1] for functions foo and bar:
>> 	Offset          Type          Sym. Name + Addend
>> 	00000000001c  R_X86_64_PC32   .text + 1c   // FDE[0] for foo at .text + 0
>> 	000000000030  R_X86_64_PC32   .text + 3b   // FDE[1] for bar at .text + b
>>     With:
>>       1c = 0 + sizeof(sframe_header) + 0 * sizeof(sframe_fde)
>>       3b = b + sizeof(sframe_header) + 1 * sizeof(sframe_fde)
>>     The canonicalized addend are 0 and b respectively as the relocs are for
>>     symbols (.text + 0) and (.text + b) respectively.
>
> Hmm, wait - there's a mix between .text and .sframe then, isn't there? That
> would explain why custom handling is then necessary. Yet as said in the other
> mail, custom handling should not be necessary. Not the least because other
> ELF-consuming tools also need to work, not just what GNU binutils provides.

[At a risk of being reiterative, but this also helps to my own
 understanding, because this is all very confusing :)]

I think there is nothing special about that "mix".  As far as I can tell
this is just one rather peculiar particular case of a PCrel reloc.
Suppose you have the typical:

  .text
  ...
    ldinsn OFFSET

  .data
  ...
  somedata:

And then you install a relocation for the instruction's offset, which is
PC-relative.  You use an expression like:

  ldinsn somedata

Then I would expect GAS to emit a PCrel with:

  .text.rela:
    P: offset_of_ldins_within_text
    S: .data
    A: offset_of_somedata_within_data

With resolving expression:

  S + A - P

to which the linker also adds .text, resulting in the expected:

  .data + offset_of_somedata_within_data - .text + offset_of_ldins_within_text

i.e. the distance between somedata and the instruction which field is to
be patched.

Now, for SFrame, the expression used wants to denote the distance
between the beginning of some given function and the beginning of the
.sframe section containing the FDE to patch.  GAS is given the
expression:

   .text + text_offset - .sframe

And the assembler smartly uses a PCrel relocation (it has no other
choice I suppose) such as:

  .sframe.rela:
    P: offset_of_fde_within_sframe
    S: .text
    A: offset_of_function_within_text + offset_of_fde_within_sframe
                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~

[I have been trying to find the code in GAS that actually does this,
 because it seems to me it must be some ad-hoc code, but couln't find
 it.]

Which is a little weird, because it has the additional
offset_of_fde_within_sframe, intended to basically annull the same value
in P in the resolving function:

   S + A - P

which is, with the linker-added .sframe:

   .text + (offset_of_funtion_within_text + offset_of_fde_within_sframe)
         - .sframe + offset_of_fde_within_sframe

which becomes:

   .text + offset_of_funtion_within_text - .sframe

which is the desired distance between the beginning of the function and
the beginning of the .sframe section.

I don't think ELF consuming tools that just apply relocations need any
special code to solve these relocs.

But due how the SFrame format works, as Jens explained, tools that merge
SFrame input sections into a relocatable output section need to
basically recreate the relocations for the FDEs in the output.  If this
merging consisted (like with ehframe) just in collating the input
sections together, then no special casing would be needed.  But merging
SFrame sections imply that the offset of the FDEs wrt the containing
output .sframe will likely be different.  This offset is embedded in the
input relocation's offset and addend, as shown above.  So the merging
tool has no other choice that to "remove" the old FDE offset from the
input relocation (from both addend and offset) and "insert" the new FDE
offset (in the resulting merged .sframe section in the output) in both
addend and offset.  Tools that merge SFrame sections must use
sframe-specific logic anyway, so I don't think this is a big deal...

The other changes in generic code in Indu's patch, as far as I can see,
are necessary because the linker assumes that the input section contents
are merged by stocking them (or a subset of them) in the merged output
section.  This is not so in SFrame, so thats why Indu's patch makes ld
to not increase that offset within the output section every iteration in
the loop, if the sections being merged are .sframe.  It is simply that
until now no format required this sort of merging, where I11,I12,I13 and
I21,I22,I23 areas in two input sections will be merged into for example
I11,I21,I12,I22,I13,I23 in the output section.
  

On 11.03.2025 10:26, Jose E. Marchesi wrote:
>> On 10.03.2025 14:16, Jens Remus wrote:
>>> On 08.03.2025 08:38, Indu Bhagat wrote:
>>>> With the fix in GAS, we now use a different PC-relative RELA for
>>>> updating the SFrame FDE function start address: The value is the offset
>>>> of the start PC of the function from the start of the SFrame section.
>>>>
>>>> When RELAs are output (e.g. for relocatable links), there is need to
>>>> adjust the r_addend.  This is because the r_addend values still have the
>>>> correct values for the _input_ SFrame section being linked / relocated.
>>>> The values must now be (before outputing the RELAs) with respect to the
>>>> _output_ SFrame section.
>>>>
>>>> PS: This patch should be merged with the previous commits before final
>>>> commit (Otherwise the tests will fail).  It is currently a separate
>>>> patch as I would like to check if this is OK to do.  If such a "addend
>>>> fixup" is risky or wrong, it seems we will need a new type of RELOC for
>>>> SFrame sections.
>>>
>>>> diff --git a/bfd/elf-sframe.c b/bfd/elf-sframe.c
>>>
>>>> @@ -574,6 +574,37 @@ _bfd_elf_sframe_section_offset (bfd *output_bfd ATTRIBUTE_UNUSED,
>>>>   	  + out_fde_idx * sizeof (sframe_func_desc_entry));
>>>>   }
>>>>   
>>>> +/* Get the "canonicalized" addend for the symbol reference corresponding to the
>>>> +   relocation at RELOC_INDEX.  E.g.,  for the following reloc for the SFrame
>>>
>>> Nit: s/reloc/relocs/
>>>
>>>> +   FDE function start address:
>>>> +	Offset          Type          Sym. Name + Addend
>>>> +	00000000001c  R_X86_64_PC32   .text + 1c
>>>> +	000000000030  R_X86_64_PC32   .text + 3b
>>>> +   The canonicalized addend are 0 and b respectively as the relocs are for
>>>> +   symbols (.text + 0) and (.text + b) respectively.
>>>
>>> Maybe it would help to use the function symbols foo and bar in the example:
>>>
>>>     relocation at RELOC_INDEX.  E.g.,  for the following annotated relocs for the SFrame
>>>     FDE function start address of FDE[0] and FDE[1] for functions foo and bar:
>>> 	Offset          Type          Sym. Name + Addend
>>> 	00000000001c  R_X86_64_PC32   .text + 1c   // FDE[0] for foo at .text + 0
>>> 	000000000030  R_X86_64_PC32   .text + 3b   // FDE[1] for bar at .text + b
>>>     With:
>>>       1c = 0 + sizeof(sframe_header) + 0 * sizeof(sframe_fde)
>>>       3b = b + sizeof(sframe_header) + 1 * sizeof(sframe_fde)
>>>     The canonicalized addend are 0 and b respectively as the relocs are for
>>>     symbols (.text + 0) and (.text + b) respectively.
>>
>> Hmm, wait - there's a mix between .text and .sframe then, isn't there? That
>> would explain why custom handling is then necessary. Yet as said in the other
>> mail, custom handling should not be necessary. Not the least because other
>> ELF-consuming tools also need to work, not just what GNU binutils provides.
> 
> [At a risk of being reiterative, but this also helps to my own
>  understanding, because this is all very confusing :)]
> 
> I think there is nothing special about that "mix".  As far as I can tell
> this is just one rather peculiar particular case of a PCrel reloc.
> Suppose you have the typical:
> 
>   .text
>   ...
>     ldinsn OFFSET
> 
>   .data
>   ...
>   somedata:
> 
> And then you install a relocation for the instruction's offset, which is
> PC-relative.  You use an expression like:
> 
>   ldinsn somedata
> 
> Then I would expect GAS to emit a PCrel with:
> 
>   .text.rela:
>     P: offset_of_ldins_within_text
>     S: .data
>     A: offset_of_somedata_within_data
> 
> With resolving expression:
> 
>   S + A - P
> 
> to which the linker also adds .text, resulting in the expected:
> 
>   .data + offset_of_somedata_within_data - .text + offset_of_ldins_within_text
> 
> i.e. the distance between somedata and the instruction which field is to
> be patched.
> 
> Now, for SFrame, the expression used wants to denote the distance
> between the beginning of some given function and the beginning of the
> .sframe section containing the FDE to patch.  GAS is given the
> expression:
> 
>    .text + text_offset - .sframe
> 
> And the assembler smartly uses a PCrel relocation (it has no other
> choice I suppose) such as:
> 
>   .sframe.rela:
>     P: offset_of_fde_within_sframe
>     S: .text
>     A: offset_of_function_within_text + offset_of_fde_within_sframe
>                                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~

I don't think this can work: You can't have offsets within two different
sections added together. When doing a relocatable link, _both_ offsets
may change. S+A has to describe the target of the relocation.

> [I have been trying to find the code in GAS that actually does this,
>  because it seems to me it must be some ad-hoc code, but couln't find
>  it.]
> 
> Which is a little weird, because it has the additional
> offset_of_fde_within_sframe, intended to basically annull the same value
> in P in the resolving function:
> 
>    S + A - P
> 
> which is, with the linker-added .sframe:
> 
>    .text + (offset_of_funtion_within_text + offset_of_fde_within_sframe)
>          - .sframe + offset_of_fde_within_sframe
> 
> which becomes:
> 
>    .text + offset_of_funtion_within_text - .sframe
> 
> which is the desired distance between the beginning of the function and
> the beginning of the .sframe section.
> 
> I don't think ELF consuming tools that just apply relocations need any
> special code to solve these relocs.
> 
> But due how the SFrame format works, as Jens explained, tools that merge
> SFrame input sections into a relocatable output section need to
> basically recreate the relocations for the FDEs in the output.

But that's what a random ELF consuming tool wouldn't do. I.e. relocations
here want expressing in a way that even e.g. GNU ld pre-dating the
introduction of .sframe can properly handle them, whether or not it is
passed the -r option.

Of course there may be situations where this is entirely impossible. But
I very much hope here it isn't one of those. (If it was, measures would
imo need taking to make sure unaware tools would diagnose occurrences
rather than silently doing the wrong thing.)

>  If this
> merging consisted (like with ehframe) just in collating the input
> sections together, then no special casing would be needed.  But merging
> SFrame sections imply that the offset of the FDEs wrt the containing
> output .sframe will likely be different.

Is this really different from .eh_frame?

>  This offset is embedded in the
> input relocation's offset and addend, as shown above.  So the merging
> tool has no other choice that to "remove" the old FDE offset from the
> input relocation (from both addend and offset) and "insert" the new FDE
> offset (in the resulting merged .sframe section in the output) in both
> addend and offset.  Tools that merge SFrame sections must use
> sframe-specific logic anyway, so I don't think this is a big deal...

As said above - as long as .sframe are ordinary PROGBITS sections, unaware
tools should be fine to use.

> The other changes in generic code in Indu's patch, as far as I can see,
> are necessary because the linker assumes that the input section contents
> are merged by stocking them (or a subset of them) in the merged output
> section.  This is not so in SFrame, so thats why Indu's patch makes ld
> to not increase that offset within the output section every iteration in
> the loop, if the sections being merged are .sframe.  It is simply that
> until now no format required this sort of merging, where I11,I12,I13 and
> I21,I22,I23 areas in two input sections will be merged into for example
> I11,I21,I12,I22,I13,I23 in the output section.

This part I fear I'm not following. Why would there be any re-ordering of
input "fragments"? What specifically do I11 etc denominate? Are sections
being taken apart and then - kind of randomly - re-assembled?

Jan
  

On 11.03.2025 10:26, Jose E. Marchesi wrote:
>> On 10.03.2025 14:16, Jens Remus wrote:
>>> On 08.03.2025 08:38, Indu Bhagat wrote:
>>>> With the fix in GAS, we now use a different PC-relative RELA for
>>>> updating the SFrame FDE function start address: The value is the offset
>>>> of the start PC of the function from the start of the SFrame section.
>>>>
>>>> When RELAs are output (e.g. for relocatable links), there is need to
>>>> adjust the r_addend.  This is because the r_addend values still have the
>>>> correct values for the _input_ SFrame section being linked / relocated.
>>>> The values must now be (before outputing the RELAs) with respect to the
>>>> _output_ SFrame section.
>>>>
>>>> PS: This patch should be merged with the previous commits before final
>>>> commit (Otherwise the tests will fail).  It is currently a separate
>>>> patch as I would like to check if this is OK to do.  If such a "addend
>>>> fixup" is risky or wrong, it seems we will need a new type of RELOC for
>>>> SFrame sections.
>>>
>>>> diff --git a/bfd/elf-sframe.c b/bfd/elf-sframe.c
>>>
>>>> @@ -574,6 +574,37 @@ _bfd_elf_sframe_section_offset (bfd *output_bfd ATTRIBUTE_UNUSED,
>>>>    	  + out_fde_idx * sizeof (sframe_func_desc_entry));
>>>>    }
>>>>    
>>>> +/* Get the "canonicalized" addend for the symbol reference corresponding to the
>>>> +   relocation at RELOC_INDEX.  E.g.,  for the following reloc for the SFrame
>>>
>>> Nit: s/reloc/relocs/
>>>
>>>> +   FDE function start address:
>>>> +	Offset          Type          Sym. Name + Addend
>>>> +	00000000001c  R_X86_64_PC32   .text + 1c
>>>> +	000000000030  R_X86_64_PC32   .text + 3b
>>>> +   The canonicalized addend are 0 and b respectively as the relocs are for
>>>> +   symbols (.text + 0) and (.text + b) respectively.
>>>
>>> Maybe it would help to use the function symbols foo and bar in the example:
>>>
>>>      relocation at RELOC_INDEX.  E.g.,  for the following annotated relocs for the SFrame
>>>      FDE function start address of FDE[0] and FDE[1] for functions foo and bar:
>>> 	Offset          Type          Sym. Name + Addend
>>> 	00000000001c  R_X86_64_PC32   .text + 1c   // FDE[0] for foo at .text + 0
>>> 	000000000030  R_X86_64_PC32   .text + 3b   // FDE[1] for bar at .text + b
>>>      With:
>>>        1c = 0 + sizeof(sframe_header) + 0 * sizeof(sframe_fde)
>>>        3b = b + sizeof(sframe_header) + 1 * sizeof(sframe_fde)
>>>      The canonicalized addend are 0 and b respectively as the relocs are for
>>>      symbols (.text + 0) and (.text + b) respectively.
>>
>> Hmm, wait - there's a mix between .text and .sframe then, isn't there? That
>> would explain why custom handling is then necessary. Yet as said in the other
>> mail, custom handling should not be necessary. Not the least because other
>> ELF-consuming tools also need to work, not just what GNU binutils provides.
> 
> [At a risk of being reiterative, but this also helps to my own
>   understanding, because this is all very confusing :)]
> 
> I think there is nothing special about that "mix".  As far as I can tell
> this is just one rather peculiar particular case of a PCrel reloc.
> Suppose you have the typical:
> 
>    .text
>    ...
>      ldinsn OFFSET
> 
>    .data
>    ...
>    somedata:
> 
> And then you install a relocation for the instruction's offset, which is
> PC-relative.  You use an expression like:
> 
>    ldinsn somedata
> 
> Then I would expect GAS to emit a PCrel with:
> 
>    .text.rela:
>      P: offset_of_ldins_within_text
>      S: .data
>      A: offset_of_somedata_within_data
> 
> With resolving expression:
> 
>    S + A - P
> 
> to which the linker also adds .text, resulting in the expected:
> 
>    .data + offset_of_somedata_within_data - .text + offset_of_ldins_within_text
> 
> i.e. the distance between somedata and the instruction which field is to
> be patched.

Isn't above in real world use cases actually already like what you
describe below for SFrame, in regard that the addend often consists of
the sum of the target offset and some source offset?

On s390x PC-relative addressing is relative to the instruction, so that:

    .text.rela:
      P: offset_of_ldins_within_text
      S: .data
      A: offset_of_somedata_within_data + offset_of_field_within_ldins

$ echo "brasl %r14,func@PLT" | as && objdump -dr a.out
...
    0:   c0 e5 00 00 00 00       brasl   %r14,0x0
                         2: R_390_PLT32DBL       func+0x2
                                                     ~~~~
The +0x2 is the offset_of_field_within_ldins, which is very similar
the offset_of_fde_within_sframe in your below SFrame explanation.

On x86-64 PC-relative addressing is relative to the next instruction,
which makes it a bit less apparent:

$ echo "call func@PLT" | as && objdump -dr a.out
...
    0:   e8 00 00 00 00          call   0x5
                         1: R_X86_64_PLT32       func-0x4
                                                     ~~~~

The -0x4 is the offset_of_field_from_insn_following_ldins.

> Now, for SFrame, the expression used wants to denote the distance
> between the beginning of some given function and the beginning of the
> .sframe section containing the FDE to patch.  GAS is given the
> expression:
> 
>     .text + text_offset - .sframe
> 
> And the assembler smartly uses a PCrel relocation (it has no other
> choice I suppose) such as:
> 
>    .sframe.rela:
>      P: offset_of_fde_within_sframe
>      S: .text
>      A: offset_of_function_within_text + offset_of_fde_within_sframe
>                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> [I have been trying to find the code in GAS that actually does this,
>   because it seems to me it must be some ad-hoc code, but couln't find
>   it.]
> 
> Which is a little weird, because it has the additional
> offset_of_fde_within_sframe, intended to basically annull the same value
> in P in the resolving function:
> 
>     S + A - P
> 
> which is, with the linker-added .sframe:
> 
>     .text + (offset_of_funtion_within_text + offset_of_fde_within_sframe)
>           - .sframe + offset_of_fde_within_sframe
> 
> which becomes:
> 
>     .text + offset_of_funtion_within_text - .sframe
> 
> which is the desired distance between the beginning of the function and
> the beginning of the .sframe section.
> 
> I don't think ELF consuming tools that just apply relocations need any
> special code to solve these relocs.

I agree.  See the s390x and x86-64 examples above, which do not require
any special processing.

> But due how the SFrame format works, as Jens explained, tools that merge
> SFrame input sections into a relocatable output section need to
> basically recreate the relocations for the FDEs in the output.  If this
> merging consisted (like with ehframe) just in collating the input
> sections together, then no special casing would be needed.  But merging
> SFrame sections imply that the offset of the FDEs wrt the containing
> output .sframe will likely be different.  This offset is embedded in the
> input relocation's offset and addend, as shown above.  So the merging
> tool has no other choice that to "remove" the old FDE offset from the
> input relocation (from both addend and offset) and "insert" the new FDE
> offset (in the resulting merged .sframe section in the output) in both
> addend and offset.  Tools that merge SFrame sections must use
> sframe-specific logic anyway, so I don't think this is a big deal...
> 
> The other changes in generic code in Indu's patch, as far as I can see,
> are necessary because the linker assumes that the input section contents
> are merged by stocking them (or a subset of them) in the merged output
> section.  This is not so in SFrame, so thats why Indu's patch makes ld
> to not increase that offset within the output section every iteration in
> the loop, if the sections being merged are .sframe.  It is simply that
> until now no format required this sort of merging, where I11,I12,I13 and
> I21,I22,I23 areas in two input sections will be merged into for example
> I11,I21,I12,I22,I13,I23 in the output section.

Very good explanation!  This is the main difference, why SFrame requires
special processing - but only for merging of .sframe sections.

Regards,
Jens
  

On 3/11/25 5:22 AM, Jan Beulich wrote:
> On 11.03.2025 10:26, Jose E. Marchesi wrote:
>>> On 10.03.2025 14:16, Jens Remus wrote:
>>>> On 08.03.2025 08:38, Indu Bhagat wrote:
>>>>> With the fix in GAS, we now use a different PC-relative RELA for
>>>>> updating the SFrame FDE function start address: The value is the offset
>>>>> of the start PC of the function from the start of the SFrame section.
>>>>>
>>>>> When RELAs are output (e.g. for relocatable links), there is need to
>>>>> adjust the r_addend.  This is because the r_addend values still have the
>>>>> correct values for the _input_ SFrame section being linked / relocated.
>>>>> The values must now be (before outputing the RELAs) with respect to the
>>>>> _output_ SFrame section.
>>>>>
>>>>> PS: This patch should be merged with the previous commits before final
>>>>> commit (Otherwise the tests will fail).  It is currently a separate
>>>>> patch as I would like to check if this is OK to do.  If such a "addend
>>>>> fixup" is risky or wrong, it seems we will need a new type of RELOC for
>>>>> SFrame sections.
>>>>
>>>>> diff --git a/bfd/elf-sframe.c b/bfd/elf-sframe.c
>>>>
>>>>> @@ -574,6 +574,37 @@ _bfd_elf_sframe_section_offset (bfd *output_bfd ATTRIBUTE_UNUSED,
>>>>>    	  + out_fde_idx * sizeof (sframe_func_desc_entry));
>>>>>    }
>>>>>    
>>>>> +/* Get the "canonicalized" addend for the symbol reference corresponding to the
>>>>> +   relocation at RELOC_INDEX.  E.g.,  for the following reloc for the SFrame
>>>>
>>>> Nit: s/reloc/relocs/
>>>>
>>>>> +   FDE function start address:
>>>>> +	Offset          Type          Sym. Name + Addend
>>>>> +	00000000001c  R_X86_64_PC32   .text + 1c
>>>>> +	000000000030  R_X86_64_PC32   .text + 3b
>>>>> +   The canonicalized addend are 0 and b respectively as the relocs are for
>>>>> +   symbols (.text + 0) and (.text + b) respectively.
>>>>
>>>> Maybe it would help to use the function symbols foo and bar in the example:
>>>>
>>>>      relocation at RELOC_INDEX.  E.g.,  for the following annotated relocs for the SFrame
>>>>      FDE function start address of FDE[0] and FDE[1] for functions foo and bar:
>>>> 	Offset          Type          Sym. Name + Addend
>>>> 	00000000001c  R_X86_64_PC32   .text + 1c   // FDE[0] for foo at .text + 0
>>>> 	000000000030  R_X86_64_PC32   .text + 3b   // FDE[1] for bar at .text + b
>>>>      With:
>>>>        1c = 0 + sizeof(sframe_header) + 0 * sizeof(sframe_fde)
>>>>        3b = b + sizeof(sframe_header) + 1 * sizeof(sframe_fde)
>>>>      The canonicalized addend are 0 and b respectively as the relocs are for
>>>>      symbols (.text + 0) and (.text + b) respectively.
>>>
>>> Hmm, wait - there's a mix between .text and .sframe then, isn't there? That
>>> would explain why custom handling is then necessary. Yet as said in the other
>>> mail, custom handling should not be necessary. Not the least because other
>>> ELF-consuming tools also need to work, not just what GNU binutils provides.
>>
>> [At a risk of being reiterative, but this also helps to my own
>>   understanding, because this is all very confusing :)]
>>
>> I think there is nothing special about that "mix".  As far as I can tell
>> this is just one rather peculiar particular case of a PCrel reloc.
>> Suppose you have the typical:
>>
>>    .text
>>    ...
>>      ldinsn OFFSET
>>
>>    .data
>>    ...
>>    somedata:
>>
>> And then you install a relocation for the instruction's offset, which is
>> PC-relative.  You use an expression like:
>>
>>    ldinsn somedata
>>
>> Then I would expect GAS to emit a PCrel with:
>>
>>    .text.rela:
>>      P: offset_of_ldins_within_text
>>      S: .data
>>      A: offset_of_somedata_within_data
>>
>> With resolving expression:
>>
>>    S + A - P
>>
>> to which the linker also adds .text, resulting in the expected:
>>
>>    .data + offset_of_somedata_within_data - .text + offset_of_ldins_within_text
>>
>> i.e. the distance between somedata and the instruction which field is to
>> be patched.
>>
>> Now, for SFrame, the expression used wants to denote the distance
>> between the beginning of some given function and the beginning of the
>> .sframe section containing the FDE to patch.  GAS is given the
>> expression:
>>
>>     .text + text_offset - .sframe
>>
>> And the assembler smartly uses a PCrel relocation (it has no other
>> choice I suppose) such as:
>>
>>    .sframe.rela:
>>      P: offset_of_fde_within_sframe
>>      S: .text
>>      A: offset_of_function_within_text + offset_of_fde_within_sframe
>>                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> I don't think this can work: You can't have offsets within two different
> sections added together. When doing a relocatable link, _both_ offsets
> may change. S+A has to describe the target of the relocation.
> 
>> [I have been trying to find the code in GAS that actually does this,
>>   because it seems to me it must be some ad-hoc code, but couln't find
>>   it.]
>>
>> Which is a little weird, because it has the additional
>> offset_of_fde_within_sframe, intended to basically annull the same value
>> in P in the resolving function:
>>
>>     S + A - P
>>
>> which is, with the linker-added .sframe:
>>
>>     .text + (offset_of_funtion_within_text + offset_of_fde_within_sframe)
>>           - .sframe + offset_of_fde_within_sframe
>>
>> which becomes:
>>
>>     .text + offset_of_funtion_within_text - .sframe
>>
>> which is the desired distance between the beginning of the function and
>> the beginning of the .sframe section.
>>
>> I don't think ELF consuming tools that just apply relocations need any
>> special code to solve these relocs.
>>
>> But due how the SFrame format works, as Jens explained, tools that merge
>> SFrame input sections into a relocatable output section need to
>> basically recreate the relocations for the FDEs in the output.
> 
> But that's what a random ELF consuming tool wouldn't do. I.e. relocations
> here want expressing in a way that even e.g. GNU ld pre-dating the
> introduction of .sframe can properly handle them, whether or not it is
> passed the -r option.
> 
> Of course there may be situations where this is entirely impossible. But
> I very much hope here it isn't one of those. (If it was, measures would
> imo need taking to make sure unaware tools would diagnose occurrences
> rather than silently doing the wrong thing.)
> 
>>   If this
>> merging consisted (like with ehframe) just in collating the input
>> sections together, then no special casing would be needed.  But merging
>> SFrame sections imply that the offset of the FDEs wrt the containing
>> output .sframe will likely be different.
> 
> Is this really different from .eh_frame?
> 

Yes, SFrame merging is different from .eh_frame.

See some notes at the end of this email as the answer is related to your 
question there...

>>   This offset is embedded in the
>> input relocation's offset and addend, as shown above.  So the merging
>> tool has no other choice that to "remove" the old FDE offset from the
>> input relocation (from both addend and offset) and "insert" the new FDE
>> offset (in the resulting merged .sframe section in the output) in both
>> addend and offset.  Tools that merge SFrame sections must use
>> sframe-specific logic anyway, so I don't think this is a big deal...
> 
> As said above - as long as .sframe are ordinary PROGBITS sections, unaware
> tools should be fine to use.
> 
>> The other changes in generic code in Indu's patch, as far as I can see,
>> are necessary because the linker assumes that the input section contents
>> are merged by stocking them (or a subset of them) in the merged output
>> section.  This is not so in SFrame, so thats why Indu's patch makes ld
>> to not increase that offset within the output section every iteration in
>> the loop, if the sections being merged are .sframe.  It is simply that
>> until now no format required this sort of merging, where I11,I12,I13 and
>> I21,I22,I23 areas in two input sections will be merged into for example
>> I11,I21,I12,I22,I13,I23 in the output section.
> 
> This part I fear I'm not following. Why would there be any re-ordering of
> input "fragments"? What specifically do I11 etc denominate? Are sections
> being taken apart and then - kind of randomly - re-assembled?
> 

Reusing the figure used earlier in another thread, I have renamed FDE 
entries to show what I11, I12 etc denominate.  Basically I1X are FDEs 
from input 1, I2x are FDEs from input 2...



     ---------------------              ---------------------
     |   SFrame Header   |              |   SFrame Header   |
     ---------------------              ---------------------
     |   SFrame FDE I11  |  (Linking)   |   SFrame FDE I11  |
     |   SFrame FDE I12  |              |   SFrame FDE I12  |
     |      ...          |      |------>|      ...          |
     |   SFrame FDE I1n  |      |       |   SFrame FDE I1n  |
     ---------------------      |       |   SFrame FDE I21  |
     |       ...         | ------       |   SFrame FDE I22  |
     |SFrame FREs (Sec 1)|      |       |      ...          |
     |(Frame Row Entries)|      |       |   SFrame FDE I2n  |
     |      ...          |      |       |-------------------|
     |      ...          |      |       |      ...          |
     ---------------------      |       |SFrame FREs (Sec 1)|
                                |       |SFrame FREs (Sec 2)|
                                |       |(Frame Row Entries)|
                                |       |      ...          |
     ---------------------      |       ---------------------
     |   SFrame Header   |      |
     ---------------------      |
     |   SFrame FDE I21  |      |
     |   SFrame FDE I22  |      |
     |      ...          | ------
     |   SFrame FDE I2n  |
     ---------------------
     |       ...         |
     |SFrame FREs (Sec 2)|
     |(Frame Row Entries)|
     |      ...          |
     |      ...          |
     ---------------------

Note how the SFrame FDEs in the output section are placed at the 
beginning of the output section.

This is in contrast to EH_Frame where the CIE/FDE _as_a_single_unit_is 
simply placed at an offset in the output section.  In SFrame the stack 
trace data is split, with there being ONE SFrame FDE + N SFrame FREs per 
function.

The first member of the SFrame FDE is the function start address, which 
has the relocation.
  

On 14.03.2025 05:48, Indu Bhagat wrote:
> On 3/11/25 5:22 AM, Jan Beulich wrote:
>> On 11.03.2025 10:26, Jose E. Marchesi wrote:
>>>> On 10.03.2025 14:16, Jens Remus wrote:
>>>>> On 08.03.2025 08:38, Indu Bhagat wrote:
>>>>>> With the fix in GAS, we now use a different PC-relative RELA for
>>>>>> updating the SFrame FDE function start address: The value is the offset
>>>>>> of the start PC of the function from the start of the SFrame section.
>>>>>>
>>>>>> When RELAs are output (e.g. for relocatable links), there is need to
>>>>>> adjust the r_addend.  This is because the r_addend values still have the
>>>>>> correct values for the _input_ SFrame section being linked / relocated.
>>>>>> The values must now be (before outputing the RELAs) with respect to the
>>>>>> _output_ SFrame section.
>>>>>>
>>>>>> PS: This patch should be merged with the previous commits before final
>>>>>> commit (Otherwise the tests will fail).  It is currently a separate
>>>>>> patch as I would like to check if this is OK to do.  If such a "addend
>>>>>> fixup" is risky or wrong, it seems we will need a new type of RELOC for
>>>>>> SFrame sections.
>>>>>
>>>>>> diff --git a/bfd/elf-sframe.c b/bfd/elf-sframe.c
>>>>>
>>>>>> @@ -574,6 +574,37 @@ _bfd_elf_sframe_section_offset (bfd *output_bfd ATTRIBUTE_UNUSED,
>>>>>>    	  + out_fde_idx * sizeof (sframe_func_desc_entry));
>>>>>>    }
>>>>>>    
>>>>>> +/* Get the "canonicalized" addend for the symbol reference corresponding to the
>>>>>> +   relocation at RELOC_INDEX.  E.g.,  for the following reloc for the SFrame
>>>>>
>>>>> Nit: s/reloc/relocs/
>>>>>
>>>>>> +   FDE function start address:
>>>>>> +	Offset          Type          Sym. Name + Addend
>>>>>> +	00000000001c  R_X86_64_PC32   .text + 1c
>>>>>> +	000000000030  R_X86_64_PC32   .text + 3b
>>>>>> +   The canonicalized addend are 0 and b respectively as the relocs are for
>>>>>> +   symbols (.text + 0) and (.text + b) respectively.
>>>>>
>>>>> Maybe it would help to use the function symbols foo and bar in the example:
>>>>>
>>>>>      relocation at RELOC_INDEX.  E.g.,  for the following annotated relocs for the SFrame
>>>>>      FDE function start address of FDE[0] and FDE[1] for functions foo and bar:
>>>>> 	Offset          Type          Sym. Name + Addend
>>>>> 	00000000001c  R_X86_64_PC32   .text + 1c   // FDE[0] for foo at .text + 0
>>>>> 	000000000030  R_X86_64_PC32   .text + 3b   // FDE[1] for bar at .text + b
>>>>>      With:
>>>>>        1c = 0 + sizeof(sframe_header) + 0 * sizeof(sframe_fde)
>>>>>        3b = b + sizeof(sframe_header) + 1 * sizeof(sframe_fde)
>>>>>      The canonicalized addend are 0 and b respectively as the relocs are for
>>>>>      symbols (.text + 0) and (.text + b) respectively.
>>>>
>>>> Hmm, wait - there's a mix between .text and .sframe then, isn't there? That
>>>> would explain why custom handling is then necessary. Yet as said in the other
>>>> mail, custom handling should not be necessary. Not the least because other
>>>> ELF-consuming tools also need to work, not just what GNU binutils provides.
>>>
>>> [At a risk of being reiterative, but this also helps to my own
>>>   understanding, because this is all very confusing :)]
>>>
>>> I think there is nothing special about that "mix".  As far as I can tell
>>> this is just one rather peculiar particular case of a PCrel reloc.
>>> Suppose you have the typical:
>>>
>>>    .text
>>>    ...
>>>      ldinsn OFFSET
>>>
>>>    .data
>>>    ...
>>>    somedata:
>>>
>>> And then you install a relocation for the instruction's offset, which is
>>> PC-relative.  You use an expression like:
>>>
>>>    ldinsn somedata
>>>
>>> Then I would expect GAS to emit a PCrel with:
>>>
>>>    .text.rela:
>>>      P: offset_of_ldins_within_text
>>>      S: .data
>>>      A: offset_of_somedata_within_data
>>>
>>> With resolving expression:
>>>
>>>    S + A - P
>>>
>>> to which the linker also adds .text, resulting in the expected:
>>>
>>>    .data + offset_of_somedata_within_data - .text + offset_of_ldins_within_text
>>>
>>> i.e. the distance between somedata and the instruction which field is to
>>> be patched.
>>>
>>> Now, for SFrame, the expression used wants to denote the distance
>>> between the beginning of some given function and the beginning of the
>>> .sframe section containing the FDE to patch.  GAS is given the
>>> expression:
>>>
>>>     .text + text_offset - .sframe
>>>
>>> And the assembler smartly uses a PCrel relocation (it has no other
>>> choice I suppose) such as:
>>>
>>>    .sframe.rela:
>>>      P: offset_of_fde_within_sframe
>>>      S: .text
>>>      A: offset_of_function_within_text + offset_of_fde_within_sframe
>>>                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> I don't think this can work: You can't have offsets within two different
>> sections added together. When doing a relocatable link, _both_ offsets
>> may change. S+A has to describe the target of the relocation.
>>
>>> [I have been trying to find the code in GAS that actually does this,
>>>   because it seems to me it must be some ad-hoc code, but couln't find
>>>   it.]
>>>
>>> Which is a little weird, because it has the additional
>>> offset_of_fde_within_sframe, intended to basically annull the same value
>>> in P in the resolving function:
>>>
>>>     S + A - P
>>>
>>> which is, with the linker-added .sframe:
>>>
>>>     .text + (offset_of_funtion_within_text + offset_of_fde_within_sframe)
>>>           - .sframe + offset_of_fde_within_sframe
>>>
>>> which becomes:
>>>
>>>     .text + offset_of_funtion_within_text - .sframe
>>>
>>> which is the desired distance between the beginning of the function and
>>> the beginning of the .sframe section.
>>>
>>> I don't think ELF consuming tools that just apply relocations need any
>>> special code to solve these relocs.
>>>
>>> But due how the SFrame format works, as Jens explained, tools that merge
>>> SFrame input sections into a relocatable output section need to
>>> basically recreate the relocations for the FDEs in the output.
>>
>> But that's what a random ELF consuming tool wouldn't do. I.e. relocations
>> here want expressing in a way that even e.g. GNU ld pre-dating the
>> introduction of .sframe can properly handle them, whether or not it is
>> passed the -r option.
>>
>> Of course there may be situations where this is entirely impossible. But
>> I very much hope here it isn't one of those. (If it was, measures would
>> imo need taking to make sure unaware tools would diagnose occurrences
>> rather than silently doing the wrong thing.)
>>
>>>   If this
>>> merging consisted (like with ehframe) just in collating the input
>>> sections together, then no special casing would be needed.  But merging
>>> SFrame sections imply that the offset of the FDEs wrt the containing
>>> output .sframe will likely be different.
>>
>> Is this really different from .eh_frame?
>>
> 
> Yes, SFrame merging is different from .eh_frame.
> 
> See some notes at the end of this email as the answer is related to your 
> question there...
> 
>>>   This offset is embedded in the
>>> input relocation's offset and addend, as shown above.  So the merging
>>> tool has no other choice that to "remove" the old FDE offset from the
>>> input relocation (from both addend and offset) and "insert" the new FDE
>>> offset (in the resulting merged .sframe section in the output) in both
>>> addend and offset.  Tools that merge SFrame sections must use
>>> sframe-specific logic anyway, so I don't think this is a big deal...
>>
>> As said above - as long as .sframe are ordinary PROGBITS sections, unaware
>> tools should be fine to use.
>>
>>> The other changes in generic code in Indu's patch, as far as I can see,
>>> are necessary because the linker assumes that the input section contents
>>> are merged by stocking them (or a subset of them) in the merged output
>>> section.  This is not so in SFrame, so thats why Indu's patch makes ld
>>> to not increase that offset within the output section every iteration in
>>> the loop, if the sections being merged are .sframe.  It is simply that
>>> until now no format required this sort of merging, where I11,I12,I13 and
>>> I21,I22,I23 areas in two input sections will be merged into for example
>>> I11,I21,I12,I22,I13,I23 in the output section.
>>
>> This part I fear I'm not following. Why would there be any re-ordering of
>> input "fragments"? What specifically do I11 etc denominate? Are sections
>> being taken apart and then - kind of randomly - re-assembled?
>>
> 
> Reusing the figure used earlier in another thread, I have renamed FDE 
> entries to show what I11, I12 etc denominate.  Basically I1X are FDEs 
> from input 1, I2x are FDEs from input 2...
> 
> 
> 
>      ---------------------              ---------------------
>      |   SFrame Header   |              |   SFrame Header   |
>      ---------------------              ---------------------
>      |   SFrame FDE I11  |  (Linking)   |   SFrame FDE I11  |
>      |   SFrame FDE I12  |              |   SFrame FDE I12  |
>      |      ...          |      |------>|      ...          |
>      |   SFrame FDE I1n  |      |       |   SFrame FDE I1n  |
>      ---------------------      |       |   SFrame FDE I21  |
>      |       ...         | ------       |   SFrame FDE I22  |
>      |SFrame FREs (Sec 1)|      |       |      ...          |
>      |(Frame Row Entries)|      |       |   SFrame FDE I2n  |
>      |      ...          |      |       |-------------------|
>      |      ...          |      |       |      ...          |
>      ---------------------      |       |SFrame FREs (Sec 1)|
>                                 |       |SFrame FREs (Sec 2)|
>                                 |       |(Frame Row Entries)|
>                                 |       |      ...          |
>      ---------------------      |       ---------------------
>      |   SFrame Header   |      |
>      ---------------------      |
>      |   SFrame FDE I21  |      |
>      |   SFrame FDE I22  |      |
>      |      ...          | ------
>      |   SFrame FDE I2n  |
>      ---------------------
>      |       ...         |
>      |SFrame FREs (Sec 2)|
>      |(Frame Row Entries)|
>      |      ...          |
>      |      ...          |
>      ---------------------
> 
> Note how the SFrame FDEs in the output section are placed at the 
> beginning of the output section.
> 
> This is in contrast to EH_Frame where the CIE/FDE _as_a_single_unit_is 
> simply placed at an offset in the output section.  In SFrame the stack 
> trace data is split, with there being ONE SFrame FDE + N SFrame FREs per 
> function.

Hmm, this is pretty odd imo. There should have been two section kinds (or
maybe even three, seeing that the SFrame headers end up fully folded in
the diagram above), which would be combined only when linking final
binaries. I now regret that I didn't look more closely at all of this when
it was first introduced.

Jan