Re: [PATCH v2 2/7] BaseTools-GenFw:Add new x86_64 Elf relocation types for PIC/PIE code

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 1 August 2016 at 09:54, Shi, Steven <steven.shi@intel.com> wrote:
>> On 1 August 2016 at 09:19, Shi, Steven <steven.shi@intel.com<mailto:steven.shi@intel.com>> wrote:
>> >> >>
>> >> >> The fact that it works does not make it safe. Having multiple fixups
>> >> >> for the same symbol in the .reloc section is a problem, and so is
>> >> >> reapplying GOTPCRELX to places where the original instruction has
>> been
>> >> >> replaced by the linker.
>> >> >>
>> >> > [Steven]: I still don't understand why there will be multiple fixups for the
>> >> same symbol in the .reloc section?
>> >> >
>> >>
>> >> Remember this example
>> >>
>> >> >> > int n;
>> >> >> > int f () { return n; }
>> >> >> > int g () { return n; }
>> >> >> > int h () { return n; }
>> >>
>> >> If every 'return n' results in a GOTPCREL relocation, how are you
>> >> going to make sure that the GOT entry for 'n' is only fixed up a
>> >> single time?
>> >
>> > [Steven]: the 'return n' will not result in relocation, but the 'int n' will result
>> in the relocation in GOT. The three 'return n' will point to the same 'int n'
>> relocation item. So, we need only fixup 'int n' once, all three 'return n' will
>> use the correct global 'n' value.
>>
>> Every 'return n' will result in a GOTPCREL relocation against n. And
>> your code emits a relocation for the GOT entry every time.
>>
> [Steven]: I don't think so. please give a real case and offer its source code to prove " Every 'return n' will result in a GOTPCREL relocation against n ".
>

Compiling the code above using

gcc -c -O -fpic /tmp/pie.c -o pie.o

and dumping it using

readelf -r pie.o

gives me

Relocation section '.rela.text' at offset 0x250 contains 3 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000003  000a0000002a R_X86_64_REX_GOTP 0000000000000004 n - 4
00000000000d  000a0000002a R_X86_64_REX_GOTP 0000000000000004 n - 4
000000000017  000a0000002a R_X86_64_REX_GOTP 0000000000000004 n - 4
...

-- 
Ard.