[07/14,v6] Prevent the rtld mapfile computation from dragging in __stack_chk_fail*.

  From: Nick Alcock <nick.alcock@oracle.com>

The previous commit prevented rtld itself from being built with
-fstack-protector, but this is not quite enough.  We identify which
objects belong in rtld via a test link and analysis of the resulting
mapfile.  That link is necessarily done against objects that are
stack-protected, so drags in __stack_chk_fail_local, __stack_chk_fail,
and all the libc and libio code they use.

To stop this happening, use --defsym in the test librtld.map-production
link to force the linker to predefine these two symbols (to 0, but it
could be to anything).  (In a real link, this would of course be
catastrophic, but these object files are never used for anything else.)

v2: New.
v6: Dummy out stack_chk_fail_local too.

	* elf/Makefile (dummy-stack-chk-fail): New.
	($(objpfx)librtld.map): Use it.
---
 elf/Makefile | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)
  

Nix <nix@esperi.org.uk> writes:

>  $(objpfx)librtld.map: $(objpfx)dl-allobjs.os $(common-objpfx)libc_pic.a
>  	@-rm -f $@T
> -	$(reloc-link) -o $@.o '-Wl,-(' $^ -lgcc '-Wl,-)' -Wl,-Map,$@T
> +	$(reloc-link) -o $@.o $(dummy-stack-chk-fail) '-Wl,-(' $^ -lgcc '-Wl,-)' -Wl,-Map,$@T
>  	rm -f $@.o
>  	mv -f $@T $@

Please fold long lines.

Andreas.
  

On 7 Jun 2016, Andreas Schwab spake thusly:

> Nix <nix@esperi.org.uk> writes:
>
>>  $(objpfx)librtld.map: $(objpfx)dl-allobjs.os $(common-objpfx)libc_pic.a
>>  	@-rm -f $@T
>> -	$(reloc-link) -o $@.o '-Wl,-(' $^ -lgcc '-Wl,-)' -Wl,-Map,$@T
>> +	$(reloc-link) -o $@.o $(dummy-stack-chk-fail) '-Wl,-(' $^ -lgcc '-Wl,-)' -Wl,-Map,$@T
>>  	rm -f $@.o
>>  	mv -f $@T $@
>
> Please fold long lines.

Oops! Will be fixed in next posting. This and one comment in the same
patch (also fixed) are the only places where I overflow lines that are
fixable (configure.ac is more or less not fixable, AIUI).
  

On 06/07/2016 05:03 AM, Nix wrote:
> This and one comment in the same
> patch (also fixed) are the only places where I overflow lines that are
> fixable (configure.ac is more or less not fixable, AIUI).

Why is configure.ac not fixable? E.g., in this patch:

+	      AC_HELP_STRING([--enable-stack-protector=@<:@yes|no|all|strong@:>@],
+			     [Use -fstack-protector[-all|-strong] to detect glibc buffer overflows]),


The commentary can contain folded lines without problem (the extra indenting is removed), and the first line can be moved down, e.g.,

   AC_HELP_STRING(
     [--enable-stack-protector=@<:@yes|no|all|strong@:>@],
     [Use -fstack-protector[-all|-strong] to detect glibc
      buffer overflows]),
  

On 06/07/2016 01:06 PM, Nix wrote:
> 	* elf/Makefile (dummy-stack-chk-fail): New.
> 	($(objpfx)librtld.map): Use it.

This looks good to me (except for the long lines already pointed out).

Thanks,
Florian