Reorder debug symbol search path on linux

[labath]

Summary

lldb was failing to locate the split debug symbols for libc. The previous search order was:

• the directory containing the original library
• the current directory
• /usr/lib/debug

This was failing to locate the debug symbols for libc (at least on my machine), because the
.gnu_debuglink for libc (libc.so.6) was "libc-2.19.so". However, libc.so.6 itself is a symlink to
libc-2.19.so, located in the same directory. This meant that under the default search path lldb
would locate the same file, which obviously provides no new information. Moving the module
directory to the last place in the search path allows lldb to correctly locate the debug version
of libc under /usr/lib/debug and indeed makes additional information visible in lldb output.

If the user has a newer version of debug symbols in the library directory than those in
/usr/lib/debug (perhaps because he is working on the library), then those should get picked up
anyway, because the newer library should have a different UUID.

[emaste]

because the .gnu_debuglink for libc (libc.so.6) was "libc-2.19.so".

That seems rather odd, the debuglink is typically to <filename>.debug. What distro uses the same name without an extension?

If the user has a newer version of debug symbols in the library directory than those in /usr/lib/debug (perhaps because he is working on the library), then those should get picked up anyway, because the newer library should have a different UUID.

Note that on FreeBSD we use .gnu_debuglink but do not (yet) have build-ID in binaries/libraries.

Also this change doesn't match GDB's ordering, which is:

• /usr/lib/debug/.build-id/ab/cdef1234.debug
• /usr/bin/ls.debug
• /usr/bin/.debug/ls.debug
• /usr/lib/debug/usr/bin/ls.debug.

(from https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html)

Moving up the files.push_back (dirname + "/.build-id/" + uuid_str); at line 81 may be what you want instead.

test/functionalities/unwind/noreturn/TestNoreturnUnwind.py

47–49

This is needed because you now have debug data for libc?

[labath]

This is google's custom version of ubuntu, but I don't think they've changed anything in this respect.
$ dpkg -l libc6:amd64
ii libc6:amd64 2.19-0ubuntu6.5 amd64 Embedded GNU C Library: Shared libraries

$ objdump -j .gnu_debuglink -s /lib/x86_64-linux-gnu/libc.so.6

/lib/x86_64-linux-gnu/libc.so.6: file format elf64-x86-64

Contents of section .gnu_debuglink:
0000 6c696263 2d322e31 392e736f 00000000 libc-2.19.so....
0010 ae65f08d .e..

Funnily enough, GDB seems to manage to find the debug information just fine. Maybe it just detects it's the same file underneath and skips it. I'll hold off from committing and try to investigate more...

thanks.

test/functionalities/unwind/noreturn/TestNoreturnUnwind.py

47–49

Yes.

I have a note that it would be nice to keep resolving the symbol to "abort" and not some internal libc mess. However, this is a separate issue (ps, gdb also resolves this do __GI_abort).

source/Host/common/Symbols.cpp

 Symbols::LocateExecutableSymbolFile (const ModuleSpec &module_spec)
 {
     const char *symbol_filename = module_spec.GetSymbolFileSpec().GetFilename().AsCString();
     if (!symbol_filename || !symbol_filename[0])
         return FileSpec();
 
     FileSpecList debug_file_search_paths (Target::GetDefaultDebugFileSearchPaths());
 
-    // Add module directory.
-    const ConstString &file_dir = module_spec.GetFileSpec().GetDirectory();
-    debug_file_search_paths.AppendIfUnique (FileSpec(file_dir.AsCString("."), true));
-
     // Add current working directory.
     debug_file_search_paths.AppendIfUnique (FileSpec(".", true));
 
     // Add /usr/lib/debug directory.
     debug_file_search_paths.AppendIfUnique (FileSpec("/usr/lib/debug", true));
 
+    // Add module directory.
+    const ConstString &file_dir = module_spec.GetFileSpec().GetDirectory();
+    debug_file_search_paths.AppendIfUnique (FileSpec(file_dir.AsCString("."), true));
+
     std::string uuid_str;
     const UUID &module_uuid = module_spec.GetUUID();
     if (module_uuid.IsValid())
     {
         // Some debug files are stored in the .build-id directory like this:
         //   /usr/lib/debug/.build-id/ff/e7fe727889ad82bb153de2ad065b2189693315.debug
         uuid_str = module_uuid.GetAsString("");
         uuid_str.insert (2, 1, '/');

test/functionalities/unwind/noreturn/TestNoreturnUnwind.py

def noreturn_unwind_tests (self):
         if process.GetState() != lldb.eStateStopped:
             self.fail("Process should be in the 'stopped' state, "
                       "instead the actual state is: '%s'" %
                       lldbutil.state_type_to_str(process.GetState()))
 
         thread = process.GetThreadAtIndex(0)
         abort_frame_number = 0
         for f in thread.frames:
-            if f.GetFunctionName() == "abort":
+            # We use endswith() to look for abort() since some C libraries mangle the symbol into
+            # __GI_abort or similar.
+            if f.GetFunctionName().endswith("abort"):
                 break
             abort_frame_number = abort_frame_number + 1
 
         if self.TraceOn():
             print "Backtrace once we're stopped:"
             for f in thread.frames:
                 print "  %d %s" % (f.GetFrameID(), f.GetFunctionName())