[ELF] Suppress "STT_SECTION symbol should be defined" on .eh_frame, .debug*, .zdebug* and .gcc_except_table

[MaskRay]

Summary

With -r or --emit-relocs, we warn STT_SECTION symbol should be defined
on relocations to discarded section symbol. This was added as an error
in rLLD319404, but was not so effective before D61583 (turned to a
warning).

Relocations from .eh_frame .debug* .zdebug* .gcc_except_table to
discarded .text are very common and somewhat expected. Don't warn/error
on them. As a reference, ld.bfd has a similar logic in
_bfd_elf_default_action_discarded() to allow these cases.

invalid-undef-section-symbol.test can be deleted as what it intended to
check is now covered by the updated comdat-discarded-reloc.s

[MaskRay]

In addition, allow .gcc_except_table

[ruiu]

LGTM

ELF/InputSection.cpp

446–447

There is a newline in a little bit weird place. Is this what clang-format formatted?

Maybe you should write this using two assignments rather than writing it as a single line.

[MaskRay]

Use two assignments:

uint32_t SecIdx = cast<Undefined>(Sym).DiscardedSecIdx;
Elf_Shdr_Impl<ELFT> Sec =
    CHECK(File->getObj().sections(), File)[SecIdx];

ELF/InputSection.cpp

if (Sym.Type == STT_SECTION) {
       // We combine multiple section symbols into only one per
       // section. This means we have to update the addend. That is
       // trivial for Elf_Rela, but for Elf_Rel we have to write to the
       // section data. We do that by adding to the Relocation vector.
 
       // .eh_frame is horribly special and can reference discarded sections. To
       // avoid having to parse and recreate .eh_frame, we just replace any
       // relocation in it pointing to discarded sections with R_*_NONE, which
-      // hopefully creates a frame that is ignored at runtime.
+      // hopefully creates a frame that is ignored at runtime. Also, don't warn
+      // on debug sections.
       auto *D = dyn_cast<Defined>(&Sym);
       if (!D) {
-        warn("STT_SECTION symbol should be defined");
+        if (!Sec->Name.startswith(".debug") &&
+            !Sec->Name.startswith(".zdebug") && Sec->Name != ".eh_frame")
+          warn("relocation refers to a discarded section\n>>> referenced by " +
+               getObjMsg(P->r_offset));
         P->setSymbolAndType(0, 0, false);
         continue;
       }
       SectionBase *Section = D->Section->Repl;
       if (!Section->isLive()) {
         P->setSymbolAndType(0, 0, false);
         continue;
       }
 
       int64_t Addend = getAddend<ELFT>(Rel);

test/ELF/Inputs/comdat-discarded-reloc.s

-.section .text.bar1,"aG",@progbits,group,comdat
+.global bar, _start
 
-.section .text.bar2
-.global bar
-bar:
- .quad .text.bar1
+.section .text.foo,"aG",@progbits,group,comdat
+
+.section .text
+_start:
+ .quad .text.foo
+ .quad bar

test/ELF/comdat-discarded-reloc.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/comdat-discarded-reloc.s -o %t2.o
-# RUN: ld.lld -gc-sections --noinhibit-exec %t.o %t2.o -o /dev/null
+# RUN: ld.lld -gc-sections --noinhibit-exec %t2.o %t.o -o /dev/null
+# RUN: ld.lld -r %t2.o %t.o -o %t 2>&1 | FileCheck --check-prefix=WARN %s
+# RUN: llvm-readobj -r %t | FileCheck --check-prefix=RELOC %s
 
 ## ELF spec doesn't allow a relocation to point to a deduplicated
 ## COMDAT section. Unfortunately this happens in practice (e.g. .eh_frame)
 ## Test case checks we do not crash.
 
-.global bar, _start
+# WARN: warning: relocation refers to a discarded section
+# WARN-NEXT: >>> referenced by {{.*}}.o:(.rela.text.bar2+0x0)
+# WARN-NOT: warning
 
-.section .text.foo,"aG",@progbits,group,comdat
+# RELOC:      .rela.eh_frame {
+# RELOC-NEXT:   R_X86_64_NONE
+# RELOC-NEXT: }
+# RELOC-NEXT: .rela.debug_foo {
+# RELOC-NEXT:   R_X86_64_NONE
+# RELOC-NEXT: }
 
-.section .text
-_start:
- .quad .text.foo
- .quad bar
+.section .text.bar1,"aG",@progbits,group,comdat
+
+## .text.bar2 has the SHF_ALLOC flag. Warn on its relocation to the discarded .text.bar1
+.section .text.bar2,"ax"
+.globl bar
+bar:
+ .quad .text.bar1
+
+## Don't warn on .eh_frame, .debug* or .zdebug*
+.section .eh_frame,"a"
+ .quad .text.bar1
+
+.section .debug_foo
+  .quad .text.bar1

test/ELF/invalid-undef-section-symbol.test

-# RUN: yaml2obj %s -o %t.o
-# RUN: not ld.lld -r --fatal-warnings %t.o -o /dev/null 2>&1 | FileCheck %s
-
-# We used to crash at this.
-# CHECK: STT_SECTION symbol should be defined
-
---- !ELF
-FileHeader:
-  Class:           ELFCLASS64
-  Data:            ELFDATA2LSB
-  Type:            ET_REL
-  Machine:         EM_X86_64
-Sections:
-  - Name:            .text
-    Type:            SHT_PROGBITS
-  - Name:            .rela.text
-    Type:            SHT_RELA
-    AddressAlign:    0x0000000000000008
-    Info:            .text
-    Relocations:
-      - Offset:          0x0000000000000000
-        Symbol:          .text
-        Type:            R_X86_64_NONE
-Symbols:
-  - Name:            .text
-    Type:            STT_SECTION