[libunwind] Bare-metal DWARF: set dso_base to 0

[rprichard]

Summary

Previously, DwarfFDECache::findFDE used 0 as a special value meaning
"search the entire cache, including dynamically-registered FDEs".
Switch this special value to -1, which doesn't make sense as a DSO
base.

Fixes PR47335.

[whitequark]

Looks overall reasonable to me, but it's been years since I touched that code or used an environment involving it, so someone else would have to sign off on it.

[compnerd]

Why not use -1 as the indicator instead of 0?

[rprichard]

In D86748#2262437, @compnerd wrote:

Why not use -1 as the indicator instead of 0?

Just to be sure, the proposal is to use 0 for dso_base and -1 for DwarfFDECache::findFDE to mean "search everything"?

I think that will work, because -1 seems unlikely to appear as a genuine dso_base.

[rprichard]

Use -1 to indicate "search all cache entries".

[compnerd]

Yes, sorry I wasn't clear about that, but you understood correctly. I was suggesting using -1 as the search all, since it doesn't make sense as a DSO base. Minor comment about a possible constexpr improvement, but doesn't require a re-review.

libunwind/src/UnwindCursor.hpp

84

I wonder if we can get away with constexpr in addition to const.

[rprichard]

libunwind/src/UnwindCursor.hpp

84

constexpr should work -- libunwind doesn't use it yet, but it uses other C++11 features (e.g. https://reviews.llvm.org/D86411#2233300). I'll switch it from const to constexpr, which also implies const for a variable.

libunwind/src/AddressSpace.hpp

#if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
     info.dwarf_section                 = (uintptr_t)dyldInfo.dwarf_section;
     info.dwarf_section_length          = dyldInfo.dwarf_section_length;
  #endif
     info.compact_unwind_section        = (uintptr_t)dyldInfo.compact_unwind_section;
     info.compact_unwind_section_length = dyldInfo.compact_unwind_section_length;
     return true;
   }
 #elif defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND) && defined(_LIBUNWIND_IS_BAREMETAL)
+  info.dso_base = 0;
   // Bare metal is statically linked, so no need to ask the dynamic loader
   info.dwarf_section_length = (uintptr_t)(&__eh_frame_end - &__eh_frame_start);
   info.dwarf_section =        (uintptr_t)(&__eh_frame_start);
   _LIBUNWIND_TRACE_UNWINDING("findUnwindSections: section %p length %p",
                              (void *)info.dwarf_section, (void *)info.dwarf_section_length);
 #if defined(_LIBUNWIND_SUPPORT_DWARF_INDEX)
   info.dwarf_index_section =        (uintptr_t)(&__eh_frame_hdr_start);
   info.dwarf_index_section_length = (uintptr_t)(&__eh_frame_hdr_end - &__eh_frame_hdr_start);

libunwind/src/UnwindCursor.hpp

 namespace libunwind {
 
 #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
 /// Cache of recently found FDEs.
 template <typename A>
 class _LIBUNWIND_HIDDEN DwarfFDECache {
   typedef typename A::pint_t pint_t;
 public:
+  static constexpr pint_t kSearchAll = static_cast<pint_t>(-1);
   static pint_t findFDE(pint_t mh, pint_t pc);
   static void add(pint_t mh, pint_t ip_start, pint_t ip_end, pint_t fde);
   static void removeAllIn(pint_t mh);
   static void iterateCacheEntries(void (*func)(unw_word_t ip_start,
                                                unw_word_t ip_end,
                                                unw_word_t fde, unw_word_t mh));
 
 private:

 bool DwarfFDECache<A>::_registeredForDyldUnloads = false;
 #endif
 
 template <typename A>
 typename A::pint_t DwarfFDECache<A>::findFDE(pint_t mh, pint_t pc) {
   pint_t result = 0;
   _LIBUNWIND_LOG_IF_FALSE(_lock.lock_shared());
   for (entry *p = _buffer; p < _bufferUsed; ++p) {
-    if ((mh == p->mh) || (mh == 0)) {
+    if ((mh == p->mh) || (mh == kSearchAll)) {
       if ((p->ip_start <= pc) && (pc < p->ip_end)) {
         result = p->fde;
         break;
       }
     }
   }
   _LIBUNWIND_LOG_IF_FALSE(_lock.unlock_shared());
   return result;
#if defined(_LIBUNWIND_ARM_EHABI)
     if (sects.arm_section != 0 && this->getInfoFromEHABISection(pc, sects))
       return;
 #endif
   }
 
 #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
   // There is no static unwind info for this pc. Look to see if an FDE was
   // dynamically registered for it.
-  pint_t cachedFDE = DwarfFDECache<A>::findFDE(0, pc);
+  pint_t cachedFDE = DwarfFDECache<A>::findFDE(DwarfFDECache<A>::kSearchAll,
+                                               pc);
   if (cachedFDE != 0) {
     typename CFI_Parser<A>::FDE_Info fdeInfo;
     typename CFI_Parser<A>::CIE_Info cieInfo;
     if (!CFI_Parser<A>::decodeFDE(_addressSpace, cachedFDE, &fdeInfo, &cieInfo))
       if (getInfoFromFdeCie(fdeInfo, cieInfo, pc, 0))
         return;
   }