[llvm-readelf] - Don't crash when e_shstrndx=SHN_XINDEX, but there is no section header.

[grimar]

Summary

Currently we crash when trying to print --sections and the SHN_XINDEX escape value
is used for the e_shstrndx field, but there is no section header at index 0 to
read the value from.

[MaskRay]

Thanks!

llvm/include/llvm/Object/ELF.h

521

Considering using the ELF spec wording:

... the actual index of the section name string table section is contained in the sh_link field of the section header at index 0.

[jhenderson]

LGTM, aside from the nit, I think my other comments are probably best left for another patch.

llvm/include/llvm/Object/ELF.h

526

Nit: either e_shstrndx == SHN_XINDEX or e_shstrndx is SHN_XINDEX

llvm/test/tools/llvm-readobj/ELF/many-sections.s

46

This being an error rather than a warning is a bit surprising, given that it would be trivial to treat the behaviour as no string table instead. I take it that's a different patch of code that would need changing though.

53–54

In a similar manner, the lack of a warning here is surprising.

llvm/include/llvm/Object/ELF.h

ELFFile<ELFT>::getRelocationSymbol(const Elf_Rel *Rel,
   return getEntry<Elf_Sym>(SymTab, Index);
 }
 
 template <class ELFT>
 Expected<StringRef>
 ELFFile<ELFT>::getSectionStringTable(Elf_Shdr_Range Sections,
                                      WarningHandler WarnHandler) const {
   uint32_t Index = getHeader()->e_shstrndx;
-  if (Index == ELF::SHN_XINDEX)
+  if (Index == ELF::SHN_XINDEX) {
+    // If the section name string table section index is greater than
+    // or equal to SHN_LORESERVE, then the actual index of the section name
+    // string table section is contained in the sh_link field of the section
+    // header at index 0.
+    if (Sections.empty())
+      return createError(
+          "e_shstrndx == SHN_XINDEX, but the section header table is empty");
+
     Index = Sections[0].sh_link;
+  }
 
   if (!Index) // no section string table.
     return "";
   if (Index >= Sections.size())
     return createError("section header string table index " + Twine(Index) +
                        " does not exist");
   return getStringTable(&Sections[Index], WarnHandler);
 }

llvm/test/tools/llvm-readobj/ELF/many-sections.s

- Type: SHT_NULL
     Link: .shstrtab
     Size: 0x3
 
 ## Another case is when e_shoff field set to zero, but not e_shstrndx.
 ## We want to show that this corrupt case is handled correctly.
 
 # RUN: yaml2obj --docnum=2 %s -o %t2
 
-# RUN: llvm-readelf --file-headers %t2 | FileCheck %s --check-prefix=GNU2
+# RUN: not llvm-readelf --file-headers --sections %t2 2>&1 | \
+# RUN:   FileCheck %s -DFILE=%t2 --check-prefix=GNU2
 # GNU2: Number of section headers:         0
 # GNU2: Section header string table index: 65535 (corrupt: out of range)
 
-# RUN: llvm-readobj --file-headers %t2 | FileCheck %s --check-prefix=LLVM2
-# LLVM2: SectionHeaderCount: 0
-# LLVM2: StringTableSectionIndex: 65535 (corrupt: out of range)
+# GNU2:       There are 0 section headers, starting at offset 0x0:
+# GNU2-EMPTY:
+# GNU2-NEXT:  Section Headers:
+# GNU2-NEXT:   [Nr] Name Type Address Off Size ES Flg Lk Inf Al
+# GNU2-NEXT:   error: '[[FILE]]': e_shstrndx == SHN_XINDEX, but the section header table is empty
+
+# RUN: llvm-readobj --file-headers --sections %t2 | \
+# RUN:   FileCheck %s --check-prefix=LLVM2 --implicit-check-not="warning:"
+# LLVM2:       SectionHeaderCount: 0
+# LLVM2:       StringTableSectionIndex: 65535 (corrupt: out of range)
+# LLVM2-NEXT: }
+# LLVM2-NEXT: Sections [
+# LLVM2-NEXT: ]
 
 --- !ELF
 FileHeader:
   Class:    ELFCLASS64
   Data:     ELFDATA2LSB
   Type:     ET_REL
   Machine:  EM_X86_64
   SHOff:    0
   SHNum:    0
   SHStrNdx: 0xffff ## SHN_XINDEX
 Sections:
   - Type: SHT_NULL
     Link: .shstrtab
     Size: 0x3