mbox series

[v8,0/5] New attribute "counted_by" to annotate bounds for C99 FAM(PR108896)

Message ID 20240329160703.4012941-1-qing.zhao@oracle.com
Headers
Series New attribute "counted_by" to annotate bounds for C99 FAM(PR108896) |

Message

Qing Zhao March 29, 2024, 4:06 p.m. UTC 
  Hi,

This is the 8th version of the patch.

compare with the 7th version, the difference are:

updates per Joseph's comments:

1. Wording changes in diagnostics;
   "non flexible" to "non-flexible";
   Diagnostics starts with a lowercase letter;
2. Documentation changes:
   "named ``@var{count}'' to ``@var{count}'';
   use present tense in the documentation;
3. Checking "INTEGRAL_TYPE_P" instead of just INTEGER_TYPE for integer types.
   Add testcases for _Bool/enum/_BitInt count fields. 
4. Add handling for multiple counted_by attributes on the same field:
   Allow duplicates if they name the same field;
   Error when they name different fields.
   Add testcase for this.
5. Updates for comments style.


The 7th version is at:
https://gcc.gnu.org/pipermail/gcc-patches/2024-March/648087.html
https://gcc.gnu.org/pipermail/gcc-patches/2024-March/648088.html
https://gcc.gnu.org/pipermail/gcc-patches/2024-March/648089.html
https://gcc.gnu.org/pipermail/gcc-patches/2024-March/648090.html
https://gcc.gnu.org/pipermail/gcc-patches/2024-March/648091.html

It based on the following original proposal:

https://gcc.gnu.org/pipermail/gcc-patches/2023-November/635884.html
Represent the missing dependence for the "counted_by" attribute and its consumers

******The summary of the proposal is:

* Add a new internal function ".ACCESS_WITH_SIZE" to carry the size information for every reference to a FAM field;
* In C FE, Replace every reference to a FAM field whose TYPE has the "counted_by" attribute with the new internal function ".ACCESS_WITH_SIZE";
* In every consumer of the size information, for example, BDOS or array bound sanitizer, query the size information or ACCESS_MODE information from the new internal function;
* When expansing to RTL, replace the internal function with the actual reference to the FAM field;
* Some adjustment to ipa alias analysis, and other SSA passes to mitigate the impact to the optimizer and code generation.


******The new internal function

  .ACCESS_WITH_SIZE (REF_TO_OBJ, REF_TO_SIZE, CLASS_OF_SIZE, TYPE_OF_SIZE, ACCESS_MODE, TYPE_OF_REF)

INTERNAL_FN (ACCESS_WITH_SIZE, ECF_LEAF | ECF_NOTHROW, NULL)

which returns the "REF_TO_OBJ" same as the 1st argument;

Both the return type and the type of the first argument of this function have been converted from the incomplete array type to the corresponding pointer type.

The call to .ACCESS_WITH_SIZE is wrapped with an INDIRECT_REF, whose type is the original imcomplete array type.

Please see the following link for why:
https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638793.html
https://gcc.gnu.org/pipermail/gcc-patches/2023-December/639605.html

1st argument "REF_TO_OBJ": The reference to the object;
2nd argument "REF_TO_SIZE": The reference to the size of the object,
3rd argument "CLASS_OF_SIZE": The size referenced by the REF_TO_SIZE represents
   0: the number of bytes;
   1: the number of the elements of the object type;
4th argument "TYPE_OF_SIZE": A constant 0 with the TYPE of the object
  refed by REF_TO_SIZE
5th argument "ACCESS_MODE":
  -1: Unknown access semantics
   0: none
   1: read_only
   2: write_only
   3: read_write
6th argument "TYPE_OF_REF": A constant 0 with the pointer TYPE to
  to the original flexible array type.

****** The Patch sets included:

1. Provide counted_by attribute to flexible array member field;
      which includes:
      * "counted_by" attribute documentation;
      * C FE handling of the new attribute;
        syntax checking, error reporting;
      * testing cases;

2. Convert "counted_by" attribute to/from .ACCESS_WITH_SIZE.
      which includes:
      * The definition of the new internal function .ACCESS_WITH_SIZE in internal-fn.def.
      * C FE converts every reference to a FAM with "counted_by" attribute to a call to the internal function .ACCESS_WITH_SIZE.
        (build_component_ref in c_typeck.cc)
        This includes the case when the object is statically allocated and initialized.
        In order to make this working, we should update initializer_constant_valid_p_1 and output_constant in varasm.cc to include calls to .ACCESS_WITH_SIZE.

        However, for the reference inside "offsetof", ignore the "counted_by" attribute since it's not useful at all. (c_parser_postfix_expression in c/c-parser.cc)
	In addtion to "offsetof", for the reference inside operator "typeof" and
  "alignof", we ignore counted_by attribute too.
  	When building ADDR_EXPR for the .ACCESS_WITH_SIZE in C FE,
  replace the call with its first argument.

      * Convert every call to .ACCESS_WITH_SIZE to its first argument.
        (expand_ACCESS_WITH_SIZE in internal-fn.cc)
      * adjust alias analysis to exclude the new internal from clobbering anything.
        (ref_maybe_used_by_call_p_1 and call_may_clobber_ref_p_1 in tree-ssa-alias.cc)
      * adjust dead code elimination to eliminate the call to .ACCESS_WITH_SIZE when
        it's LHS is eliminated as dead code.
        (eliminate_unnecessary_stmts in tree-ssa-dce.cc)
      * Provide the utility routines to check the call is .ACCESS_WITH_SIZE and
        get the reference from the call to .ACCESS_WITH_SIZE.
        (is_access_with_size_p and get_ref_from_access_with_size in tree.cc)
      * testing cases. (for offsetof, static initialization, generation of calls to
        .ACCESS_WITH_SIZE, code runs correctly after calls to .ACCESS_WITH_SIZE are
        converted back)

3. Use the .ACCESS_WITH_SIZE in builtin object size (sub-object only)
      which includes:
      * use the size info of the .ACCESS_WITH_SIZE for sub-object.
      * when the size is a negative integer, treat it as zero.
      * testing cases. 

4 Use the .ACCESS_WITH_SIZE in bound sanitizer
      which includes:
      * Instrument array_ref with a call to .ACCESS_WITH_SIZE for bound sanitizer.
      * when the size is a negative integer, treat it as zero.
      * testing cases. 

5. Add the 6th argument to .ACCESS_WITH_SIZE to carry the TYPE of the flexible array.
      which includes:
      * Add the 6th argument to .ACCESS_WITH_SIZE.
      * use the type of the 6th argument of the routine in tree-object-size.cc
      * testing case.

******Remaining works: 

6  Improve __bdos to use the counted_by info in whole-object size for the structure with FAM.
7  Emit warnings when the user breaks the requirments for the new counted_by attribute
   compilation time: -Wcounted-by
   run time: -fsanitizer=counted-by
      * The initialization to the size field should be done before the first reference to the FAM field.
      * the array has at least # of elements specified by the size field all the time during the program.

I have bootstrapped and regression tested on both x86 and aarch64, no issue.

Let me know your comments.

thanks.

Qing

Comments

Tom Tromey March 29, 2024, 6:09 p.m. UTC | #1 
>>>>> Qing Zhao <qing.zhao@oracle.com> writes:

> This is the 8th version of the patch.

> compare with the 7th version, the difference are:

[...]

Hi.  I was curious to know if the information supplied by this attribute
shows up in the DWARF.  It would be good if it did, because that would
let gdb correctly print these arrays without user intervention.

Tom
Kees Cook March 29, 2024, 7:16 p.m. UTC | #2 
On Fri, Mar 29, 2024 at 12:09:15PM -0600, Tom Tromey wrote:
> >>>>> Qing Zhao <qing.zhao@oracle.com> writes:
> 
> > This is the 8th version of the patch.
> 
> > compare with the 7th version, the difference are:
> 
> [...]
> 
> Hi.  I was curious to know if the information supplied by this attribute
> shows up in the DWARF.  It would be good if it did, because that would
> let gdb correctly print these arrays without user intervention.

Does DWARF have such an annotation? Regardless, I think this could be a
future patch to not hold up landing the initial feature.
Qing Zhao March 29, 2024, 7:58 p.m. UTC | #3 
Hi,  Tom,

Thanks a lot for the comments. 

It’s good to hear that this new attribute might be able to be used to help gdb. 

We might spend some time to study to use this information in other consumers, for example, gdb, in the future, if necessary and possible.  If you have good examples to show the importance of using such information in gdb, please let me know. I’m glad to study a little more. 

At this time, I agree with Kees, it’s better for the initial patches of the “counted-by” support to focus on the the attribute itself and the immediate security consumers, such as array bound sanitizer and dynamic object size, etc. 

So, let’s delay the possible support to gdb in a later patch. 

Does this sound reasonable to you?

Qing



> On Mar 29, 2024, at 15:16, Kees Cook <keescook@chromium.org> wrote:
> 
> On Fri, Mar 29, 2024 at 12:09:15PM -0600, Tom Tromey wrote:
>>>>>>> Qing Zhao <qing.zhao@oracle.com> writes:
>> 
>>> This is the 8th version of the patch.
>> 
>>> compare with the 7th version, the difference are:
>> 
>> [...]
>> 
>> Hi.  I was curious to know if the information supplied by this attribute
>> shows up in the DWARF.  It would be good if it did, because that would
>> let gdb correctly print these arrays without user intervention.
> 
> Does DWARF have such an annotation? Regardless, I think this could be a
> future patch to not hold up landing the initial feature.
> 
> -- 
> Kees Cook
Tom Tromey March 30, 2024, 12:15 a.m. UTC | #4 
Kees> Does DWARF have such an annotation? Regardless, I think this could be a
Kees> future patch to not hold up landing the initial feature.

Sure, the compiler can emit the array length (and structure size) as a
DWARF expression using the length.

Tom
Tom Tromey March 30, 2024, 12:16 a.m. UTC | #5 
> So, let’s delay the possible support to gdb in a later patch. 

> Does this sound reasonable to you?

It's not really up to me, but sure.  I was just curious if it perhaps
already worked, but not enough to apply the patches and find out.

Tom
Kees Cook March 30, 2024, 1:57 p.m. UTC | #6 
On Fri, Mar 29, 2024 at 04:06:58PM +0000, Qing Zhao wrote:
> This is the 8th version of the patch.

Thanks for the updated version!

I've done a full Linux kernel build and run through my behavioral
regression test suite. Everything is working as expected.

-Kees