[ELF] Use the Repl point to avoid the segfault when using ICF

[phosek]

Summary

This addresses PR38918.

[phosek]

I haven't managed to come up with a minimal reproducer yet, but this addresses the issue when using the reproducer attached in https://bugs.llvm.org/show_bug.cgi?id=38918.

[phosek]

Now also includes a reproducer.

[phosek]

So after looking a bit more into the output of the reproducer, I realized this is not the correct solution because it leads to relocations being duplicated. Instead, relocations for the folded section shouldn't be emitted at all so this's a deeper rooted issue.

[mcgrathr]

The test will need to verify that the emitted relocs are correct too.

[ruiu]

lld/test/ELF/icf18.s

1

This is not an assembly although the file extension is .s. Can you write this in assembly?

[ruiu]

LGTM

lld/ELF/InputSection.cpp

if (Sym.Type == STT_SECTION) {
         // not calculate relocations in this mode and loses
         // individual "gp" values used by each input object file.
         // As a workaround we add the "gp" value to the relocation
         // addend and save it back to the file.
         Addend += Sec->getFile<ELFT>()->MipsGp0;
       }
 
       if (RelTy::IsRela)
-        P->r_addend = Sym.getVA(Addend) - Section->getOutputSection()->Addr;
+        P->r_addend = Sym.getVA(Addend) - Section->Repl->getOutputSection()->Addr;
       else if (Config->Relocatable)
         Sec->Relocations.push_back({R_ABS, Type, Rel.r_offset, Addend, &Sym});
     }
   }
 }
 
 // The ARM and AArch64 ABI handle pc-relative relocations to undefined weak
 // references specially. The general rule is that the value of the symbol in

lld/test/ELF/icf18.s

+# RUN: yaml2obj %s -o %t.o
+# RUN: ld.lld --icf=all --emit-relocs %t.o -o /dev/null --print-icf-sections 2>&1 | FileCheck %s
+
+# Checks that ICF with --emit-relocs doesn't crash.
+
+# CHECK: removing identical section
+# CHECK: (.text.bar)
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  OSABI:           ELFOSABI_FREEBSD
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .text.foo
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    Content:         "FFFFFFFFFFFFFFFF"
+  - Name:            .text.bar
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    Content:         "FFFFFFFFFFFFFFFF"
+  - Name:            .text.foo_test
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+  - Name:            .rela.text.foo_test
+    Type:            SHT_RELA
+    Info:            .text.foo_test
+    Relocations:
+      - Offset:          0x0000000000000000
+        Symbol:          .text.foo
+        Type:            R_X86_64_PLT32
+      - Offset:          0x0000000000000000
+        Symbol:          .text.bar
+        Type:            R_X86_64_PLT32
+Symbols:
+  Local:
+    - Name:          .text.foo
+      Type:          STT_SECTION
+      Section:       .text.foo
+    - Name:          .text.bar
+      Type:          STT_SECTION
+      Section:       .text.bar
+  Global:
+    - Name:          _start
+      Type:          STT_FUNC
+      Section:       .text.foo_test