Accept sh_entsize = 0.

[ruiu]

Summary

This surfaced again with Rust. As per bug 30435, rustc creates a
mergeable section with a sh_entsize. It bit us before, too.
So I think we should relax the input check rather than being too picky.

[emaste]

I'm indifferent about moving the multiple of sh_entsize test before SHF_MERGE. LGTM either way.

ELF/InputFiles.cpp

bool elf::ObjectFile<ELFT>::shouldMerge(const Elf_Shdr &Sec) {
 
   // A mergeable section with size 0 is useless because they don't have
   // any data to merge. A mergeable string section with size 0 can be
   // argued as invalid because it doesn't end with a null character.
   // We'll avoid a mess by handling them as if they were non-mergeable.
   if (Sec.sh_size == 0)
     return false;
 
+  // Check for sh_entsize. The ELF spec is not clear about the zero
+  // sh_entsize. It says that "the member [sh_entsize] contains 0 if
+  // the section does not hold a table of fixed-size entries". We know
+  // that Rust 1.13 produces a string mergeable section with a zero
+  // sh_entsize. Here we just accept it rather than being picky about it.
+  uintX_t EntSize = Sec.sh_entsize;
+  if (EntSize == 0)
+    return false;
+  if (Sec.sh_size % EntSize)
+    fatal(getFilename(this) +
+          ": SHF_MERGE section size must be a multiple of sh_entsize");
+
   uintX_t Flags = Sec.sh_flags;
   if (!(Flags & SHF_MERGE))
     return false;
   if (Flags & SHF_WRITE)
     fatal(getFilename(this) + ": writable SHF_MERGE section is not supported");
-  uintX_t EntSize = Sec.sh_entsize;
-  if (!EntSize || Sec.sh_size % EntSize)
-    fatal(getFilename(this) +
-          ": SHF_MERGE section size must be a multiple of sh_entsize");
 
   // Don't try to merge if the alignment is larger than the sh_entsize and this
   // is not SHF_STRINGS.
   //
   // Since this is not a SHF_STRINGS, we would need to pad after every entity.
   // It would be equivalent for the producer of the .o to just set a larger
   // sh_entsize.
   if (Flags & SHF_STRINGS)

test/ELF/invalid-elf.test

 # RUN: not ld.lld %p/Inputs/invalid-section-index.elf -o %t2 2>&1 | \
 # RUN:   FileCheck --check-prefix=INVALID-SECTION-INDEX-LLD %s
 # INVALID-SECTION-INDEX-LLD: invalid section index
 
 # RUN: not ld.lld %p/Inputs/invalid-shstrndx.so -o %t2 2>&1 | \
 # RUN:   FileCheck --check-prefix=INVALID-SECTION-INDEX %s
 # INVALID-SECTION-INDEX: Invalid section index
 
-# RUN: not ld.lld %p/Inputs/invalid-shentsize-zero.elf -o %t2 2>&1 | \
-# RUN:   FileCheck --check-prefix=INVALID-SHENTSIZE-ZERO %s
-# INVALID-SHENTSIZE-ZERO: SHF_MERGE section size must be a multiple of sh_entsize
-
 # RUN: not ld.lld %p/Inputs/invalid-multiple-eh-relocs.elf -o %t2 2>&1 | \
 # RUN:   FileCheck --check-prefix=INVALID-EH-RELOCS %s
 # INVALID-EH-RELOCS: multiple relocation sections to .eh_frame are not supported
 
 .long foo

test/ELF/merge-invalid-size.s

 // REQUIRES: x86
 // RUN: llvm-mc %s -o %t.o -filetype=obj -triple=x86_64-pc-linux
 // RUN: not ld.lld %t.o -o %t.so 2>&1 | FileCheck %s
 // CHECK: SHF_MERGE section size must be a multiple of sh_entsize
 
-        .section  .foo,"aM",@progbits,4
-        .short 42
+// Test that we accept a zero sh_entsize.
+// RUN: ld.lld %p/Inputs/invalid-shentsize-zero.elf -o %t2
+
+.section .foo,"aM",@progbits,4