[PATCH v2] BaseTools X64: fold PLT relocations into simple relative references

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

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 | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/BaseTools/Source/C/GenFw/Elf64Convert.c b/BaseTools/Source/C/GenFw/Elf64Convert.c
index 944c94b8f8b4..708c1a1d91a7 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
@@ -935,6 +946,7 @@ WriteRelocations64 (
             switch (ELF_R_TYPE(Rel->r_info)) {
             case R_X86_64_NONE:
             case R_X86_64_PC32:
+            case R_X86_64_PLT32:
               break;
             case R_X86_64_64:
               VerboseMsg ("EFI_IMAGE_REL_BASED_DIR64 Offset: 0x%08X", 
-- 
2.7.4