[libunwind] Don't abort if encoutering invalid .eh_frame_hdr

[phosek]

Summary

Recent Linux kernel release has introduced a bug as part of the ORC
rollout where the vDSO has a valid .eh_frame section, but it's missing
the .eh_frame_hdr section and GNU_EH_FRAME segment has zero size. This
causes libunwind to abort which breaks programs that use libunwind.

The other unwinder implementation (libgcc, non-gnu) instead silently
bail out unless being compiled as debug. This change modifies libunwind
to use the same strategy.

[mstorsjo]

LGTM

libunwind/trunk/src/AddressSpace.hpp

#endif
             EHHeaderParser<LocalAddressSpace>::EHHeaderInfo hdrInfo;
             uintptr_t eh_frame_hdr_start = pinfo->dlpi_addr + phdr->p_vaddr;
 #if defined(__ANDROID__)
             if (pinfo->dlpi_addr == 0 && phdr->p_vaddr < image_base)
               eh_frame_hdr_start = eh_frame_hdr_start + image_base;
 #endif
             cbdata->sects->dwarf_index_section = eh_frame_hdr_start;
             cbdata->sects->dwarf_index_section_length = phdr->p_memsz;
-            EHHeaderParser<LocalAddressSpace>::decodeEHHdr(
+            found_hdr = EHHeaderParser<LocalAddressSpace>::decodeEHHdr(
                 *cbdata->addressSpace, eh_frame_hdr_start, phdr->p_memsz,
                 hdrInfo);
+            if (found_hdr)
               cbdata->sects->dwarf_section = hdrInfo.eh_frame_ptr;
-            found_hdr = true;
           }
         }
 
         if (found_obj && found_hdr) {
           cbdata->sects->dwarf_section_length = object_length;
           return true;
         } else {
           return false;

libunwind/trunk/src/EHHeaderParser.hpp

public:
   /// Information encoded in the EH frame header.
   struct EHHeaderInfo {
     pint_t eh_frame_ptr;
     size_t fde_count;
     pint_t table;
     uint8_t table_enc;
   };
 
-  static void decodeEHHdr(A &addressSpace, pint_t ehHdrStart, pint_t ehHdrEnd,
+  static bool decodeEHHdr(A &addressSpace, pint_t ehHdrStart, pint_t ehHdrEnd,
                           EHHeaderInfo &ehHdrInfo);
   static bool findFDE(A &addressSpace, pint_t pc, pint_t ehHdrStart,
                       uint32_t sectionLength,
                       typename CFI_Parser<A>::FDE_Info *fdeInfo,
                       typename CFI_Parser<A>::CIE_Info *cieInfo);
 
 private:
   static bool decodeTableEntry(A &addressSpace, pint_t &tableEntry,
                                pint_t ehHdrStart, pint_t ehHdrEnd,
                                uint8_t tableEnc,
                                typename CFI_Parser<A>::FDE_Info *fdeInfo,
                                typename CFI_Parser<A>::CIE_Info *cieInfo);
   static size_t getTableEntrySize(uint8_t tableEnc);
 };
 
 template <typename A>
-void EHHeaderParser<A>::decodeEHHdr(A &addressSpace, pint_t ehHdrStart,
+bool EHHeaderParser<A>::decodeEHHdr(A &addressSpace, pint_t ehHdrStart,
                                     pint_t ehHdrEnd, EHHeaderInfo &ehHdrInfo) {
   pint_t p = ehHdrStart;
   uint8_t version = addressSpace.get8(p++);
-  if (version != 1)
-    _LIBUNWIND_ABORT("Unsupported .eh_frame_hdr version");
+  if (version != 1) {
+    _LIBUNWIND_LOG0("Unsupported .eh_frame_hdr version");
+    return false;
+  }
 
   uint8_t eh_frame_ptr_enc = addressSpace.get8(p++);
   uint8_t fde_count_enc = addressSpace.get8(p++);
   ehHdrInfo.table_enc = addressSpace.get8(p++);
 
   ehHdrInfo.eh_frame_ptr =
       addressSpace.getEncodedP(p, ehHdrEnd, eh_frame_ptr_enc, ehHdrStart);
   ehHdrInfo.fde_count =
       fde_count_enc == DW_EH_PE_omit
           ? 0
           : addressSpace.getEncodedP(p, ehHdrEnd, fde_count_enc, ehHdrStart);
   ehHdrInfo.table = p;
+
+  return true;
 }
 
 template <typename A>
 bool EHHeaderParser<A>::decodeTableEntry(
     A &addressSpace, pint_t &tableEntry, pint_t ehHdrStart, pint_t ehHdrEnd,
     uint8_t tableEnc, typename CFI_Parser<A>::FDE_Info *fdeInfo,
     typename CFI_Parser<A>::CIE_Info *cieInfo) {
   // Have to decode the whole FDE for the PC range anyway, so just throw away

 template <typename A>
 bool EHHeaderParser<A>::findFDE(A &addressSpace, pint_t pc, pint_t ehHdrStart,
                                 uint32_t sectionLength,
                                 typename CFI_Parser<A>::FDE_Info *fdeInfo,
                                 typename CFI_Parser<A>::CIE_Info *cieInfo) {
   pint_t ehHdrEnd = ehHdrStart + sectionLength;
 
   EHHeaderParser<A>::EHHeaderInfo hdrInfo;
-  EHHeaderParser<A>::decodeEHHdr(addressSpace, ehHdrStart, ehHdrEnd, hdrInfo);
+  if (!EHHeaderParser<A>::decodeEHHdr(addressSpace, ehHdrStart, ehHdrEnd,
+                                      hdrInfo))
+    return false;
 
   size_t tableEntrySize = getTableEntrySize(hdrInfo.table_enc);
   pint_t tableEntry;
 
   size_t low = 0;
   for (size_t len = hdrInfo.fde_count; len > 1;) {
     size_t mid = low + (len / 2);
     tableEntry = hdrInfo.table + mid * tableEntrySize;