[Object/ELF] - Check that e_shnum is null when e_shoff is.

[grimar]

Summary

Spec says (http://www.sco.com/developers/gabi/1998-04-29/ch4.eheader.html) :
e_shnum
This member holds the number of entries in the section header table. Thus the product of e_shentsize and e_shnum gives the section header table's size in bytes. If a file has no section header table, e_shnum holds the value zero.

Revealed using "id_000037,sig_11,src_000015,op_havoc,rep_8" from PR30540

That was the reason of crash in lld on incorrect input file.
Binary reduced using afl-min.

[grimar]

Ping.

[• rafael]

LGTM, but please also include a lld test as we might want to me a bit more lenient in here in the future.

[grimar]

In D25090#571020, @rafael wrote:

LGTM, but please also include a lld test as we might want to me a bit more lenient in here in the future.

r284375

Also I had to modify 2 precompiled inputs to commit this finally. Because it broke buildbots:
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/builds/21736/steps/test/logs/stdio

So I just zeroed the e_shnum fields in hex editor to let them pass this new check (https://reviews.llvm.org/rL284374).

llvm/trunk/include/llvm/Object/ELF.h

ELFFile<ELFT>::ELFFile(StringRef Object, std::error_code &EC)
   if (sizeof(Elf_Ehdr) > FileSize) {
     // File too short!
     EC = object_error::parse_failed;
     return;
   }
 
   Header = reinterpret_cast<const Elf_Ehdr *>(base());
 
-  if (Header->e_shoff == 0)
+  if (Header->e_shoff == 0) {
+    if (Header->e_shnum != 0)
+      report_fatal_error(
+          "e_shnum should be zero if a file has no section header table");
     return;
+  }
 
   const uint64_t SectionTableOffset = Header->e_shoff;
 
   if (SectionTableOffset + sizeof(Elf_Shdr) > FileSize) {
     // Section header table goes past end of file!
     EC = object_error::parse_failed;
     return;
   }

llvm/trunk/test/Object/invalid.test

 
 RUN: not llvm-readobj -t %p/Inputs/invalid-symbol-table-size.elf 2>&1 | FileCheck --check-prefix=INVALID-SYMTAB-SIZE %s
 INVALID-SYMTAB-SIZE: Invalid data was encountered while parsing the file
 
 
 RUN: not llvm-readobj -t %p/Inputs/invalid-xindex-size.elf 2>&1 | FileCheck --check-prefix=INVALID-XINDEX-SIZE %s
 INVALID-XINDEX-SIZE: Invalid data was encountered while parsing the file.
 
+RUN: not llvm-readobj -t %p/Inputs/invalid-e_shnum.elf 2>&1 | FileCheck --check-prefix=INVALID-SH-NUM %s
+INVALID-SH-NUM: e_shnum should be zero if a file has no section header table
+
 RUN: not llvm-readobj -t %p/Inputs/invalid-ext-symtab-index.elf-x86-64 2>&1 | \
 RUN:   FileCheck --check-prefix=INVALID-EXT-SYMTAB-INDEX %s
 INVALID-EXT-SYMTAB-INDEX: Invalid symbol table index
 
 RUN: not llvm-readobj -r %p/Inputs/invalid-relocation-sec-sh_offset.elf-i386 2>&1 | \
 RUN:   FileCheck --check-prefix=INVALID-RELOC-SH-OFFSET %s
 RUN: not llvm-readobj -r %p/Inputs/invalid-relocation-sec-sh_offset.elf-x86-64 2>&1 | \
 RUN:   FileCheck --check-prefix=INVALID-RELOC-SH-OFFSET %s
 INVALID-RELOC-SH-OFFSET: Invalid data was encountered while parsing the file