[lld] Handle empty CIE augmentation field

[emaste]

Summary

Search for the CIE augmentation null terminator from the first byte of the field. If the field is empty the terminator is the first byte.

While testing linking FreeBSD/mips64 with lld I encountered CIE code alignment must be 1, with the root cause being the empty Augmentation string:

From readelf --debug-dump lib/libc/cancelpoints_sem_new.So:

00000000 0000000c ffffffff CIE
  Version:               1
  Augmentation:          ""
  Code alignment factor: 1
  Data alignment factor: -4
  Return address column: 31

  DW_CFA_def_cfa: r29 ofs 0

Unfortunately I don't have a good test for this yet.

[grimar]

That is LGTM.

[atanasyan]

LGTM

What's problem with the test case? Is it hard to reproduce the bug or create input file which has an empty augmentation string?

[grimar]

I guess creating input file from asm with empty augmentation string is the problem.

[atanasyan]

In D19928#422002, @grimar wrote:

I guess creating input file from asm with empty augmentation string is the problem.

In that case we can use existing binary. It is worse than asm code but better than no test case at all.

[emaste]

Rebase after rL270526

[emaste]

I don't have a trivial/small binary that demonstrates the problem yet. Should I commit with a larger object, commit without a test, or wait for a smaller reproduction case?

lld/trunk/ELF/EhFrame.cpp

template <class ELFT> uint8_t getFdeEncoding(ArrayRef<uint8_t> D) {
   if (D.size() < 8)
     fatal("CIE too small");
   D = D.slice(8);
 
   uint8_t Version = readByte(D);
   if (Version != 1 && Version != 3)
     fatal("FDE version 1 or 3 expected, but got " + Twine((unsigned)Version));
 
-  const unsigned char *AugEnd = std::find(D.begin() + 1, D.end(), '\0');
+  const unsigned char *AugEnd = std::find(D.begin(), D.end(), '\0');
   if (AugEnd == D.end())
     fatal("corrupted CIE");
   StringRef Aug(reinterpret_cast<const char *>(D.begin()), AugEnd - D.begin());
   D = D.slice(Aug.size() + 1);
 
   // Code alignment factor should always be 1 for .eh_frame.
   if (readByte(D) != 1)
     fatal("CIE code alignment must be 1");

lld/trunk/test/ELF/no-augmentation.test

+// RUN: llvm-mc -filetype=obj -triple=mips64-unknown-freebsd %s -o %t.o
+// RUN: ld.lld --eh-frame-hdr %t.o %p/Inputs/no-augmentation.o -o %t \
+// RUN:   | FileCheck -allow-empty %s
+
+// REQUIRES: mips
+
+// CHECK-NOT: corrupted or unsupported CIE information
+
+.global __start
+__start: