[UNWIND] Handle non-existing FDE count / search table

[Bluerise]

Summary

On OpenBSD the binutils 2.17 linker in combination with clang++ 3.8.1 seems to produce an EH frame for statically linked binaries that does omit the fde count and the binary table. Unfortunately the code does not handle this scenario and will blow up. If we just return when it's omitted the fallback code will look through the full table and there the unwinding works.

[joerg]

The first chunk matches something I have in the NetBSD version, that one is fine. The rest looks like bugs in ld...

[Bluerise]

Not sure. binutils seems to only conditionally set fde_count.

[joerg]

Let me qualify that a bit. A FDE count of 0 can happen, if a DSO absolutely has no FDE entries at all. That's all the short cut that should be needed. A table entry format of DW_EH_PE_omit is only ever valid in combination with FDE count of 0, so that should be an assert at most. I'm trying to remember if table encodings other than sdata4+datarel are valid at all, but I can't find the specification right now.

[Bluerise]

Do you have a diff that you'd propose?

[emaste]

For future uploads can you please include context (git diff -U9999, svn diff --diff-cmd=diff -x -U999999 etc.)

src/AddressSpace.hpp

224–226

This is the one in NetBSD already (although as return 0). @joerg also has another case there:

  if (encoding == DW_EH_PE_omit)
    return 0;
  if (encoding == DW_EH_PE_aligned) {
    addr = (addr + sizeof(pint_t) - 1) & sizeof(pint_t);
    return getP(addr);
  }

// first get value

@joerg would you upstream these changes from NetBSD's copy?

src/AddressSpace.hpp

   const uint8_t *p = (uint8_t *)addr;
   pint_t result;
 
+  if (encoding == DW_EH_PE_omit) {
+    return (pint_t)NULL;
+  }
+
   // first get value
   switch (encoding & 0x0F) {
   case DW_EH_PE_ptr:

src/EHHeaderParser.hpp

   EHHeaderParser<A>::EHHeaderInfo hdrInfo;
   EHHeaderParser<A>::decodeEHHdr(addressSpace, ehHdrStart, ehHdrEnd, hdrInfo);
 
+  if (hdrInfo.fde_count == 0)
+    return false;
+
   size_t tableEntrySize = getTableEntrySize(hdrInfo.table_enc);
   pint_t tableEntry;
 

 
 template <typename A>
 size_t EHHeaderParser<A>::getTableEntrySize(uint8_t tableEnc) {
+  if (tableEnc == DW_EH_PE_omit) {
+    return 0;
+  }
+
   switch (tableEnc & 0x0f) {
   case DW_EH_PE_sdata2:
   case DW_EH_PE_udata2:

   case DW_EH_PE_sleb128:
   case DW_EH_PE_uleb128:
     _LIBUNWIND_ABORT("Can't binary search on variable length encoded data.");
-  case DW_EH_PE_omit:
-    return 0;
   default:
     _LIBUNWIND_ABORT("Unknown DWARF encoding for search table.");
   }