[llvm-objcopy] - Report SHT_GROUP sections with invalid alignment.

[grimar]

Summary

This patch fixes the reason of ubsan failure happened after landing the D59638 (I had to revert it).
(http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap-ubsan/builds/11760/steps/check-llvm%20ubsan/logs/stdio)

Problem is the following. Our implementation of GroupSection assumes that
its address is 4 bytes aligned when writes it:

template <class ELFT>
void ELFSectionWriter<ELFT>::visit(const GroupSection &Sec) {
  ELF::Elf32_Word *Buf =
      reinterpret_cast<ELF::Elf32_Word *>(Out.getBufferStart() + Sec.Offset);
...

But the test case for D59638 did not set AddressAlign in YAML. So address was
not aligned because Sec.Offset was odd. That triggered the issue.

Since such group sections should not be met in reality (I think), I would suggest to ban them.
This patch does that.

[rupprecht]

Since such group sections should not be met in reality (I think), I would suggest to ban them.
This patch does that.

Given the code snippet you included in ELFSectionWriter, it sounds like even if they *are* met in reality, llvm-objcopy is already broken. Having a specific error instead of a cryptic misaligned write crash seems more useful.

llvm/trunk/test/tools/llvm-objcopy/ELF/group-addr-misaligned.test

+# RUN: yaml2obj %s > %t
+# RUN: not llvm-objcopy %t %t2 2>&1 | FileCheck %s
+# CHECK: error: Invalid alignment 1 of group section .group.
+
+# In this test, we check that llvm-objcopy reports an error
+# for SHT_GROUP section with invalid alignment (not a multiple of 4). 
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+## It is not important for passing the test case to have this placeholder,
+## but having it would trigger ubsan failure when writing the group section
+## into a file if the error tested would not be reported by llvm-objcopy.
+  - Name:            .placeholder
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000001
+    Content:         "00"
+  - Name:            .group
+    Type:            SHT_GROUP
+    Link:            .symtab
+    AddressAlign:    0x0000000000000001
+    Info:            foo
+    Members:
+      - SectionOrType:   GRP_COMDAT
+      - SectionOrType:   .text.foo
+  - Name:            .text.foo
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR, SHF_GROUP ]
+    AddressAlign:    0x0000000000000001
+Symbols:
+  Local:
+    - Name:            foo
+      Section:         .group

llvm/trunk/tools/llvm-objcopy/ELF/Object.cpp

template <class ELFT> void ELFBuilder<ELFT>::readProgramHeaders() {
   for (auto &Child : Obj.segments())
     setParentSegment(Child);
   setParentSegment(ElfHdr);
   setParentSegment(PrHdr);
 }
 
 template <class ELFT>
 void ELFBuilder<ELFT>::initGroupSection(GroupSection *GroupSec) {
+  if (GroupSec->Align % sizeof(ELF::Elf32_Word) != 0)
+    error("Invalid alignment " + Twine(GroupSec->Align) + " of group section " +
+          GroupSec->Name);
   auto SecTable = Obj.sections();
   auto SymTab = SecTable.template getSectionOfType<SymbolTableSection>(
       GroupSec->Link,
       "Link field value " + Twine(GroupSec->Link) + " in section " +
           GroupSec->Name + " is invalid",
       "Link field value " + Twine(GroupSec->Link) + " in section " +
           GroupSec->Name + " is not a symbol table");
   auto Sym = SymTab->getSymbolByIndex(GroupSec->Info);