Make ElfCache follow .gnu_debuglink

Summary:
If you split out debug info into a separate file, folly::Symbolizer
didn't find it, and failed to include file and line number info in
backtraces.

This adds a new open mode which follows the .gnu_debuginfo link, and
uses it from ElfCache and SignalSafeElfCache.

Reviewed By: meyering, luciang

Differential Revision: D3852311

fbshipit-source-id: fec9e57378ae59fa1b82d41a163bb9cfcf9ca23c

folly/experimental/symbolizer/Elf.cpp

@@ -58,8 +58,9 @@ void ElfFile::open(const char* name, bool readOnly) {
   }
 }
 
-int ElfFile::openNoThrow(const char* name, bool readOnly, const char** msg)
-  noexcept {
+int ElfFile::openNoThrow(const char* name,
+                         bool readOnly,
+                         const char** msg) noexcept {
   FOLLY_SAFE_CHECK(fd_ == -1, "File already open");
   fd_ = ::open(name, readOnly ? O_RDONLY : O_RDWR);
   if (fd_ == -1) {
@@ -95,6 +96,43 @@ int ElfFile::openNoThrow(const char* name, bool readOnly, const char** msg)
   return kSuccess;
 }
 
+int ElfFile::openAndFollow(const char* name,
+                           bool readOnly,
+                           const char** msg) noexcept {
+  auto result = openNoThrow(name, readOnly, msg);
+  if (!readOnly || result != kSuccess) return result;
+
+  /* NOTE .gnu_debuglink specifies only the name of the debugging info file
+   * (with no directory components). GDB checks 3 different directories, but
+   * ElfFile only supports the first version:
+   *     - dirname(name)
+   *     - dirname(name) + /.debug/
+   *     - X/dirname(name)/ - where X is set in gdb's `debug-file-directory`.
+   */
+  auto dirend = strrchr(name, '/');
+  // include ending '/' if any.
+  auto dirlen = dirend != nullptr ? dirend + 1 - name : 0;
+
+  auto debuginfo = getSectionByName(".gnu_debuglink");
+  if (!debuginfo) return result;
+
+  // The section starts with the filename, with any leading directory
+  // components removed, followed by a zero byte.
+  auto debugFileName = getSectionBody(*debuginfo);
+  auto debugFileLen = strlen(debugFileName.begin());
+  if (dirlen + debugFileLen >= PATH_MAX) {
+    return result;
+  }
+
+  char linkname[PATH_MAX];
+  memcpy(linkname, name, dirlen);
+  memcpy(linkname + dirlen, debugFileName.begin(), debugFileLen + 1);
+  reset();
+  result = openNoThrow(linkname, readOnly, msg);
+  if (result == kSuccess) return result;
+  return openNoThrow(name, readOnly, msg);
+}
+
 ElfFile::~ElfFile() {
   reset();
 }

folly/experimental/symbolizer/Elf.h

@@ -58,9 +58,14 @@ class ElfFile {
     kSystemError = -1,
     kInvalidElfFile = -2,
   };
+  // Open the ELF file. Does not throw on error.
   int openNoThrow(const char* name, bool readOnly=true,
                   const char** msg=nullptr) noexcept;
 
+  // Like openNoThrow, but follow .gnu_debuglink if present
+  int openAndFollow(const char* name, bool readOnly=true,
+                    const char** msg=nullptr) noexcept;
+
   // Open the ELF file. Throws on error.
   void open(const char* name, bool readOnly=true);
 

folly/experimental/symbolizer/ElfCache.cpp

@@ -48,7 +48,7 @@ std::shared_ptr<ElfFile> SignalSafeElfCache::getFile(StringPiece p) {
   auto& f = slots_[n];
 
   const char* msg = "";
-  int r = f->openNoThrow(path.data(), true, &msg);
+  int r = f->openAndFollow(path.data(), true, &msg);
   if (r != ElfFile::kSuccess) {
     return nullptr;
   }
@@ -77,7 +77,7 @@ std::shared_ptr<ElfFile> ElfCache::getFile(StringPiece p) {
 
   // No negative caching
   const char* msg = "";
-  int r = entry->file.openNoThrow(path.c_str(), true, &msg);
+  int r = entry->file.openAndFollow(path.c_str(), true, &msg);
   if (r != ElfFile::kSuccess) {
     return nullptr;
   }

folly/experimental/symbolizer/test/gnu_debuglink_test.sh

+#!/bin/bash
+
+crash="$1"
+rm -f "$crash."{debuginfo,strip}
+objcopy --only-keep-debug "$crash" "$crash.debuginfo"
+objcopy --strip-debug --add-gnu-debuglink="$crash.debuginfo" "$crash" "$crash.strip"
+
+echo '{"op":"start","test":"gnu_debuglink_test"}';
+start=$(date +%s)
+if "$crash.strip" 2>&1 | grep 'Crash.cpp:[0-9]*$' > /dev/null; then
+    result='"status":"passed"';
+else
+    result='"status":"failed"';
+fi
+end=$(date +%s)
+echo '{"op":"test_done","test":"gnu_debuglink_test",'"$result"'}'
+echo '{"op":"all_done","results":[{"name":"gnu_debuglink_test",'"$result"',"start_time":'"$start"',"end_time":'"$end"',"details":"nothing"}]}'