Fix a bug in ElfFile::getSectionByName

Summary:
It first looks for the name, then looks for a section with that
offset. But strings can be overlaid; eg .rela.debug_info and .debug_info
can be overlapped, so we won't find .debug_info at all.

Instead, just iterate the sections checking to see if the name matches
the one we're looking for.

Reviewed By: ricklavoie, palmtenor

Differential Revision: D9809817

fbshipit-source-id: b165719c6f9004ecc964096677114bce357ece30

folly/experimental/symbolizer/Elf.cpp

@@ -353,22 +353,15 @@ const ElfShdr* ElfFile::getSectionByName(const char* name) const {
     return nullptr; // no section name string table
   }
 
-  // Find offset in the section name string table of the requested name
   const ElfShdr& sectionNames = *getSectionByIndex(elfHeader().e_shstrndx);
-  const char* foundName = iterateStrings(
-      sectionNames, [&](const char* s) { return !strcmp(name, s); });
-  if (foundName == nullptr) {
-    return nullptr;
-  }
-
-  size_t offset = foundName - (file_ + sectionNames.sh_offset);
+  const char* start = file_ + sectionNames.sh_offset;
 
   // Find section with the appropriate sh_name offset
   const ElfShdr* foundSection = iterateSections([&](const ElfShdr& sh) {
-    if (sh.sh_name == offset) {
-      return true;
+    if (sh.sh_name >= sectionNames.sh_size) {
+      return false;
     }
-    return false;
+    return !strcmp(start + sh.sh_name, name);
   });
   return foundSection;
 }