Re: [PATCH] BaseTools X64: fold PLT relocations into simple relative references

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

> On 4 aug. 2016, at 21:03, Nicolas Owens <mischief@offblast.org> wrote:
> 
> ard,
> 
> i think you need to have R_X86_64_PLT32 case in WriteRelocations64.
> without that, i still hit the invalid relocation message.
> 

Good point. I will send out a v2 tomorrow


>> On 08/04/2016 01:45 AM, Ard Biesheuvel wrote:
>> For X64/GCC, we use position independent code with hidden visibility
>> to inform the compiler that symbols references are never resolved at
>> runtime, which removes the need for PLTs and GOTs. However, in some
>> cases GCC has been reported to still emit PLT based relocations, which
>> we need to handle in the ELF to PE/COFF perform by GenFw.
>> 
>> Unlike GOT based relocations, which are non-trivial to handle since the
>> indirections in the code can not be fixed up easily (although relocation
>> types exist for X64 that annotate relocation targets as suitable for
>> relaxation), PLT relocations simply point to jump targets, and we can
>> relax such relocations by resolving them using the symbol directly rather
>> than via a PLT entry that does nothing more than tail call the function
>> we already know it is going to call (since all symbol references are
>> resolved in the same module).
>> 
>> So handle R_X86_64_PLT32 as a R_X86_64_PC32 relocation.
>> 
>> Suggested-by: Steven Shi <steven.shi@intel.com>
>> Contributed-under: TianoCore Contribution Agreement 1.0
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>> BaseTools/Source/C/GenFw/Elf64Convert.c | 11 +++++++++++
>> 1 file changed, 11 insertions(+)
>> 
>> diff --git a/BaseTools/Source/C/GenFw/Elf64Convert.c b/BaseTools/Source/C/GenFw/Elf64Convert.c
>> index 944c94b8f8b4..7cbff0df0996 100644
>> --- a/BaseTools/Source/C/GenFw/Elf64Convert.c
>> +++ b/BaseTools/Source/C/GenFw/Elf64Convert.c
>> @@ -785,6 +785,17 @@ WriteSections64 (
>>             *(INT32 *)Targ = (INT32)((INT64)(*(INT32 *)Targ) - SymShdr->sh_addr + mCoffSectionsOffset[Sym->st_shndx]);
>>             VerboseMsg ("Relocation:  0x%08X", *(UINT32*)Targ);
>>             break;
>> +
>> +          case R_X86_64_PLT32:
>> +            //
>> +            // Treat R_X86_64_PLT32 relocations as R_X86_64_PC32: this is
>> +            // possible since we know all code symbol references resolve to
>> +            // definitions in the same module (UEFI has no shared libraries),
>> +            // and so there is never a reason to jump via a PLT entry,
>> +            // allowing us to resolve the reference using the symbol directly.
>> +            //
>> +            VerboseMsg ("Treating R_X86_64_PLT32 as R_X86_64_PC32 ...");
>> +            /* fall through */
>>           case R_X86_64_PC32:
>>             //
>>             // Relative relocation: Symbol - Ip + Addend
>>