Allow folly::symbolizer::ElfFile to work with cores

Summary:
In cores we're only interested in the ElfPhdrs (and there generally
aren't any sections), so apart from some asserts, the code already
does everything we need.

Reviewed By: paulbiss

Differential Revision: D19161810

fbshipit-source-id: fe6e2f34413d61067156858fc652ffffd910433c

folly/experimental/symbolizer/Elf.cpp

@@ -237,7 +237,8 @@ ElfFile::OpenResult ElfFile::init() noexcept {
   }
 
   // We only support executable and shared object files
-  if (elfHeader.e_type != ET_EXEC && elfHeader.e_type != ET_DYN) {
+  if (elfHeader.e_type != ET_EXEC && elfHeader.e_type != ET_DYN &&
+      elfHeader.e_type != ET_CORE) {
     return {kInvalidElfFile, "invalid ELF file type"};
   }
 
@@ -250,7 +251,9 @@ ElfFile::OpenResult ElfFile::init() noexcept {
   }
 
   if (elfHeader.e_shentsize != sizeof(ElfShdr)) {
-    return {kInvalidElfFile, "invalid section header entry size"};
+    if (elfHeader.e_shentsize != 0 || elfHeader.e_type != ET_CORE) {
+      return {kInvalidElfFile, "invalid section header entry size"};
+    }
   }
 
   // Program headers are sorted by load address, so the first PT_LOAD

folly/experimental/symbolizer/Elf.h

@@ -46,8 +46,8 @@ using ElfSym = ElfW(Sym);
  * ELF file parser.
  *
  * We handle native files only (32-bit files on a 32-bit platform, 64-bit files
- * on a 64-bit platform), and only executables (ET_EXEC) and shared objects
- * (ET_DYN).
+ * on a 64-bit platform), and only executables (ET_EXEC) shared objects
+ * (ET_DYN) and core files (ET_CORE).
  */
 class ElfFile {
  public:
@@ -300,8 +300,9 @@ class ElfFile {
     //       I don't have a use-case for that right now, just assert that
     //       nobody wants this. We can always add it later.
     FOLLY_SAFE_CHECK(
-        elfHeader().e_type == ET_EXEC || elfHeader().e_type == ET_DYN,
-        "Only exectuables and shared objects are supported");
+        elfHeader().e_type == ET_EXEC || elfHeader().e_type == ET_DYN ||
+            elfHeader().e_type == ET_CORE,
+        "Only exectuables, shared objects and cores are supported");
     FOLLY_SAFE_CHECK(
         addr >= section.sh_addr &&
             (addr + sizeof(T)) <= (section.sh_addr + section.sh_size),