[ELF] - Do not segfault when using -r and section groups.

[grimar]

Summary

If we had SHT_GROUP sections, then when -r was used we might crash.
This is PR31952.

Issue happened because we emited relocation section though its target was discared
because was a member of duplicated group.
More details about how issue was revealed available in commens at D29920 page.

Fix was not to create SHT_REL[A] sections for -r when target was discarded.

Testcase based on one from @phosek (D29920).
Interesting that I was not able to create the test without yaml2obj.
But I think I could do that using GNU as.

If we have code below:

.section .text,"axG",@progbits,foo,comdat
.quad bar

Then output using GNU as is.

as test.s -o test.as
readelf -a test.as
COMDAT group section [    1] `.group' [foo] contains 1 sections:
   [Index]    Name
   [    5]   .text

But llvm-as puts .rela section to the group either:

llvm-mc -filetype=obj -triple=x86_64-pc-linux test.s -o test.o
readelf -a test.o
COMDAT group section [    3] `.group' [foo] contains 2 sections:
   [Index]    Name
   [    4]   .text
   [    5]   .rela.text

So GNU as does not put rela section to the group. That why issue happens I believe.
So looks objects created by llvm-as are not affected by issue. Because .rela.text from output above
is discarded as member of duplicated group earlier, when we hande SHT_GROUP in initializeSections().

[phosek]

Thanks for looking into this! Just to confirm your suspicion, this bug was found while using lld to link object files produced by G++ and GNU as.

lld/trunk/ELF/InputFiles.cpp

case SHT_ARM_ATTRIBUTES:
     // In a full implementation we would merge all attribute sections.
     if (In<ELFT>::ARMAttributes == nullptr) {
       In<ELFT>::ARMAttributes = make<InputSection<ELFT>>(this, &Sec, Name);
       return In<ELFT>::ARMAttributes;
     }
     return &InputSection<ELFT>::Discarded;
   case SHT_RELA:
   case SHT_REL: {
+    // Find the relocation target section and associate this
+    // section with it. Target can be discarded, for example
+    // if it is a duplicated member of SHT_GROUP section, we
+    // do not create or proccess relocatable sections then.
+    InputSectionBase<ELFT> *Target = getRelocTarget(Sec);
+    if (!Target)
+      return nullptr;
+
     // This section contains relocation information.
     // If -r is given, we do not interpret or apply relocation
     // but just copy relocation sections to output.
     if (Config->Relocatable)
       return make<InputSection<ELFT>>(this, &Sec, Name);
 
-    // Find the relocation target section and associate this
-    // section with it.
-    InputSectionBase<ELFT> *Target = getRelocTarget(Sec);
-    if (!Target)
-      return nullptr;
     if (Target->FirstRelocation)
       fatal(toString(this) +
             ": multiple relocation sections to one section are not supported");
     if (!isa<InputSection<ELFT>>(Target) && !isa<EhInputSection<ELFT>>(Target))
       fatal(toString(this) +
             ": relocations pointing to SHF_MERGE are not supported");
 
     size_t NumRelocations;

lld/trunk/test/ELF/relocation-group.test

+# RUN: yaml2obj %s -o %t.o
+# RUN: ld.lld %t.o %t.o -o %t -r
+# RUN: llvm-readobj -s %t.o | FileCheck %s
+
+# CHECK:     Name: .text.foo
+# CHECK:     Name: .rela.text.foo
+
+## YAML below corresponds to following asm code:
+## .section .text,"axG",@progbits,foo,comdat
+## .quad bar
+## gas 2.27 does not include .rela.text to group in that case:
+## COMDAT group section [    1] `.group' [foo] contains 1 sections:
+##   [Index]    Name
+##   [    5]   .text
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .group
+    Type:            SHT_GROUP
+    Link:            .symtab
+    Info:            foo
+    Members:
+      - SectionOrType:    GRP_COMDAT
+      - SectionOrType:    .text.foo
+  - Name:            .text.foo
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR, SHF_GROUP ]
+  - Name:            .rela.text.foo
+    Type:            SHT_RELA
+    Flags:           [ SHF_INFO_LINK ]
+    Link:            .symtab
+    Info:            .text.foo
+    Relocations:
+      - Offset:          0x0000000000000000
+        Symbol:          foo
+        Type:            R_X86_64_64
+Symbols:
+  Global:
+    - Name:            foo