Re: [PATCH][x86_64] Convert indirect call via GOT to direct when possible

[Sriraman Tallam <tmsriram at google dot com>]

On Sat, May 28, 2016 at 10:44 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, May 27, 2016 at 3:14 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>> On Fri, May 20, 2016 at 1:32 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Fri, May 20, 2016 at 1:27 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>>>> Hi,
>>>>
>>>>    GCC has option -fno-plt which converts all extern calls to indirect
>>>> calls via GOT to prevent the linker for generating any PLT stubs.
>>>> However, if the function ends up defined in the executable this patch
>>>> will convert those indirect calls/jumps to direct.  Since the indirect
>>>> calls are one byte longer, an extra nop is needed at the beginning.
>>>>
>>>> Here is a simple example:
>>>>
>>>> main.c
>>>> ---------
>>>> extern int foo();
>>>> int main() {
>>>>   return foo();
>>>> }
>>>>
>>>> deffoo.c
>>>> -----------
>>>> int foo() {
>>>>   return 0;
>>>> }
>>>>
>>>> $ gcc -fno-plt main.c deffoo.c
>>>> $objdump -d a.out
>>>>
>>>> 0000000000400626 <main>:
>>>>   ...
>>>>   40062a:       ff 15 28 14 00 00       callq  *0x1428(%rip)        #
>>>> 401a58 <_DYNAMIC+0x1d8>
>>>>
>>>> The call is indirect even though foo is defined in the executable.
>>>>
>>>> With this patch,
>>>> 0000000000400606 <main>:
>>>>    ....
>>>>    40060a:       90                      nop
>>>>   40060b:       e8 03 00 00 00          callq  400613 <foo>
>>>>
>>>> The call is now direct with an extra nop.
>>>>
>>>>
>>>
>>> Please try ld, which uses 0x67 prefix (addr32) instead of nop.
>>> Also for
>>>
>>> jmp *foo#GOTPCREL(%rip)
>>>
>>>  ld converts it to
>>>
>>> jmp foo
>>> nop
>>
>> I have modified the patch to keep it consistent with what ld produces.
>>
>> Please take another look.
>>
>> * x86_64.cc (can_convert_callq_to_direct): New function.
>> Target_x86_64<size>::Scan::global: Check if an indirect call via
>> GOT can be converted to direct.
>> Target_x86_64<size>::Relocate::relocate: Change any indirect call
>> via GOT that can be converted.
>> * testsuite/Makefile.am (x86_64_indirect_call_to_direct.sh): New test.
>> * testsuite/Makefile.in: Regenerate.
>> * testsuite/x86_64_indirect_call_to_direct1.s: New file.
>> * testsuite/x86_64_indirect_jump_to_direct1.s: New file.
>>
>
> Do you need to check R_X86_64_REX_GOTPCRELX for branch?

Ok, patch changed to not check for this and refactored a bit.

Thanks
Sri

>
> --
> H.J.

Attachment: convert_indirect_call_patch.txt
Description: Text document