[ELF] - Implemented support for R_386_PC8/R_386_8 relocations.

[grimar]

Summary

These relocations are used in linux kernel.

[davide]

I think we can either implement or error out on these relocations, I personally don't care as implementing this relocations is literally 3 lines of code, but wait for a second opinion.
What I'm most worried about is that we should never produce a broken output given valid input, we should emit a diagnostic and exit.

[ruiu]

Somewhat orthogonal to this patch, but I think we need to modify X86TargetInfo::relocateOne so that we explicitly list relocation types that we understand and error out other relocations.

[grimar]

D28516 was landed instead.

[grimar]

Reported to be used in linux kernel. Will rebase in a minute.

[grimar]

• Rebased.

[grimar]

• Addressed review comments.

[ruiu]

ELF/Target.cpp

493

No such thing like a one byte little endian value as the endianness is inherently a concept when you have more than one byte, so don't use read<>. That's why we don't have read8{be,le}. Just dereference the uint8_t pointer.

[grimar]

• Addressed review comments.

ELF/Target.cpp

493

Yep. Not sure why I wrote in that way.

[ruiu]

lld/trunk/ELF/Target.cpp

518 ↗

(On Diff #85738)

Do you need a cast?

ELF/Target.cpp

X86TargetInfo::X86TargetInfo() {
   GotPltEntrySize = 4;
   PltEntrySize = 16;
   PltHeaderSize = 16;
   TlsGdRelaxSkip = 2;
 }
 
 RelExpr X86TargetInfo::getRelExpr(uint32_t Type, const SymbolBody &S) const {
   switch (Type) {
+  case R_386_8:
   case R_386_16:
   case R_386_32:
   case R_386_TLS_LDO_32:
     return R_ABS;
   case R_386_TLS_GD:
     return R_TLSGD;
   case R_386_TLS_LDM:
     return R_TLSLD;
   case R_386_PLT32:
     return R_PLT_PC;
+  case R_386_PC8:
   case R_386_PC16:
   case R_386_PC32:
     return R_PC;
   case R_386_GOTPC:
     return R_GOTONLY_PC_FROM_END;
   case R_386_TLS_IE:
     return R_GOT;
   case R_386_GOT32:
void X86TargetInfo::writePlt(uint8_t *Buf, uint64_t GotEntryAddr,
   write32le(Buf + 12, -Index * PltEntrySize - PltHeaderSize - 16);
 }
 
 uint64_t X86TargetInfo::getImplicitAddend(const uint8_t *Buf,
                                           uint32_t Type) const {
   switch (Type) {
   default:
     return 0;
+  case R_386_8:
+  case R_386_PC8:
+    return *Buf;
   case R_386_16:
   case R_386_PC16:
     return read16le(Buf);
   case R_386_32:
   case R_386_GOT32:
   case R_386_GOT32X:
   case R_386_GOTOFF:
   case R_386_GOTPC:
   case R_386_PC32:
   case R_386_PLT32:
   case R_386_TLS_LE:
     return read32le(Buf);
   }
 }
 
 void X86TargetInfo::relocateOne(uint8_t *Loc, uint32_t Type,
                                 uint64_t Val) const {
   checkInt<32>(Loc, Val, Type);
 
-  // R_386_PC16 and R_386_16 are not part of the current i386 psABI. They are
-  // used by 16-bit x86 objects, like boot loaders.
+  // R_386_PC16/R_386_16/R_386_PC8/R_386_8 are not part of the current i386
+  // psABI. They are used by 16-bit x86 objects, like boot loaders.
+  if (Type == R_386_8 || Type == R_386_PC8) {
+    *Loc = (uint8_t)Val;
+    return;
+  }
   if (Type == R_386_16 || Type == R_386_PC16) {
     write16le(Loc, Val);
     return;
   }
   write32le(Loc, Val);
 }
 
 void X86TargetInfo::relaxTlsGdToLe(uint8_t *Loc, uint32_t Type,

test/ELF/Inputs/i386-pc8.s

+.global und
+und:

test/ELF/i386-pc8.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=i386-pc-linux-gnu %s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=i386-pc-linux-gnu %S/Inputs/i386-pc8.s -o %t2.o
+# RUN: ld.lld %t1.o %t2.o -o %t.out
+# RUN: llvm-objdump -s -section=.text %t.out | FileCheck %s
+
+# CHECK:      Contents of section .text:
+# CHECK-NEXT:  11000 15253748
+
+.byte und-.+0x11
+.byte und-.+0x22
+.byte und+0x33
+.byte und+0x44

test/ELF/unknown-reloc.s

-# REQUIRES: x86
-# RUN: llvm-mc -filetype=obj -triple=i386-pc-linux-gnu %s -o %t1.o
-# RUN: llvm-mc -filetype=obj -triple=i386-pc-linux-gnu %S/Inputs/unknown-reloc.s -o %t2.o
-# RUN: not ld.lld %t1.o %t2.o -o %t.out 2>&1 | FileCheck %s
-
-# CHECK: do not know how to handle relocation 'R_386_PC8' (23)
-# CHECK: do not know how to handle relocation 'R_386_8' (22)
-
-.text
-.global foo
-foo:
-
-.byte und-foo
-.byte foo