[ELF] - Ignore non-absolute R_386_GOTPC relocation in debug sections.

[grimar]

Summary

This is PR34852.

Looks gcc has a bug and produce R_386_GOTPC relocation in '.rel.debug_info':
Only i386 target and all gcc versions atm are affected it seems.
ld.bfd behavior is to silently resolve the relocation. Given the code we have, I think we can try to ignore it in LLD.

[ruiu]

Where is the bug for gcc, and what was a response from gcc develoeprs?

[grimar]

In D38625#890908, @ruiu wrote:

Where is the bug for gcc, and what was a response from gcc develoeprs?

I did not find bug report page or something like that, but gcc-5.4 produce section that is not readable by
dwarfdump tool for me:
dwarfdump ERROR: dwarf_get_loclist_from_expr_a: DW_DLE_LOC_EXPR_BAD (128)

And this section has PC relocation, while other compilers or versions of gcc does not do that and produce
correctly readable section. I assume it is a bug of gcc.

[ruiu]

Please report a bug to gcc first.

[grimar]

• Addressed review comments, updated comment.

[grimar]

Accepted by Rafael in mail list.
Rui, are you OK to land this one ?

[ruiu]

ELF/InputSection.cpp

686–690

// GCC 8.0 or earlier have a bug that it emits R_386_GOTPC relocations against _GLOBAL_OFFSET_TABLE for .debug_info. The bug seems to have been fixed in 2017 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82630), but we need to keep this bug-compatible code for a while.

Also, you should be more explicit on what you are doing. Specifically, you should avoid calling getRelExpr.

[grimar]

ELF/InputSection.cpp

686–690

Maximal explicit way I think would be to check exact target specific relocation and a symbol name here.
Do you prefer to check for R_386_GOTPC or both R_386_GOTPC and _GLOBAL_OFFSET_TABLE_ symbol name here ?

[ruiu]

ELF/InputSection.cpp

686–690

Config->EMachine == EM_386 && Type == R_386_GOTPC should suffice.

[grimar]

• Addressed review comments.

[ruiu]

LGTM

test/ELF/i386-debug-noabs.test

8–9

This is for https://bugs.llvm.org//show_bug.cgi?id=34852. GCC 8.0 or earlier have a bug which creates non-absolute R_386_GOTPC relocations in non-allocated sections. It is illegal, but we want to make sure that lld skips them instead of reporting errors.

ELF/InputSection.cpp

for (const RelTy &Rel : Rels) {
     if (!RelTy::IsRela)
       Addend += Target->getImplicitAddend(BufLoc, Type);
 
     SymbolBody &Sym = this->getFile<ELFT>()->getRelocTargetSym(Rel);
     RelExpr Expr = Target->getRelExpr(Type, Sym, *File, BufLoc);
     if (Expr == R_NONE)
       continue;
     if (Expr != R_ABS) {
+      // gcc-5.4 has a bug which creates R_386_GOTPC relocation against
+      // _GLOBAL_OFFSET_TABLE_ in .debug_info. We ignore such relocation.
+      // TODO: remove this hack once we stop supporting gcc-5.4.
+      if (Config->EMachine == EM_386 && Expr == R_GOTONLY_PC_FROM_END &&
+          Sym.getName() == "_GLOBAL_OFFSET_TABLE_")
+        continue;
       error(this->getLocation<ELFT>(Offset) + ": has non-ABS reloc");
       return;
     }
 
     uint64_t AddrLoc = getParent()->Addr + Offset;
     uint64_t SymVA = 0;
     if (!Sym.isTls() || Out::TlsPhdr)
       SymVA = SignExtend64<sizeof(typename ELFT::uint) * 8>(

test/ELF/i386-debug-noabs.test

+# REQUIRES: x86
+
+# RUN: yaml2obj %s -o %t.o
+# RUN: ld.lld %t.o -o %t.exe
+
+## It is for PR34852 (https://bugs.llvm.org//show_bug.cgi?id=34852)
+## gcc-5.4 has a bug which creates non-absolute R_386_GOTPC relocation
+## against _GLOBAL_OFFSET_TABLE_ when producing objects for i386 target.
+## Here we check that LLD is able to use such objects.
+
+--- !ELF
+FileHeader:      
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_386
+Sections:        
+  - Name:            .debug_info
+    Type:            SHT_PROGBITS
+    AddressAlign:    0x0000000000000001
+    Content:         0000000000000000
+  - Name:            .rel.debug_info
+    Type:            SHT_REL
+    Link:            .symtab
+    AddressAlign:    0x0000000000000004
+    Info:            .debug_info
+    Relocations:     
+      - Offset:          0x000000000000041F
+        Symbol:          _GLOBAL_OFFSET_TABLE_
+        Type:            R_386_GOTPC
+Symbols:
+  Global:
+    - Name:            _GLOBAL_OFFSET_TABLE_