[PATCH] D29929: [ELF] - Do not segfault when using -r and section groups.

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.

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 https://reviews.llvm.org/D29920 page.

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

Testcase based on one from @phosek (https://reviews.llvm.org/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", at 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().


https://reviews.llvm.org/D29929

Files:
  ELF/InputFiles.cpp
  test/ELF/relocation-group.test


Index: test/ELF/relocation-group.test
===================================================================
--- test/ELF/relocation-group.test
+++ test/ELF/relocation-group.test
@@ -0,0 +1,36 @@
+# 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
+
+--- !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
Index: ELF/InputFiles.cpp
===================================================================
--- ELF/InputFiles.cpp
+++ ELF/InputFiles.cpp
@@ -367,17 +367,20 @@
     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");


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