[PATCH] fix symbol search loop at debuglink mode

Some distribs like ubuntu'18 creatively name files with debug symbols.
(same name without .debug suffix). And give no `build-id` way.

$dpkg -L libc6-dbg | grep -v gconv
/usr/lib/debug/.build-id/68/f36706eb2e6eee4046c4fdca2a19540b2f6113.debug
/usr/lib/debug/lib/x86_64-linux-gnu/libc-2.27.so

$ dpkg -L libc6
/lib/x86_64-linux-gnu/libc-2.27.so
/lib/x86_64-linux-gnu/libc.so.6

$ objdump -s -j .gnu_debuglink -j .note.gnu.build-id \
  /lib/x86_64-linux-gnu/libc-2.27.so

Contents of section .gnu_debuglink:
 0000 6c696263 2d322e32 372e736f 00000000  libc-2.27.so....
 0010 5ac51387                             Z...
Contents of section .note.gnu.build-id:
 0270 04000000 14000000 03000000 474e5500  ............GNU.
 0280 ce450eb0 1a5e5acc 7ce7b8c2 633b02cc  .E...^Z.|...c;..
 0290 1093339e                             ..3.

So elf_find_debugfile_by_debuglink("libc") loops back to
/lib/x86_64-linux-gnu/libc-2.27.so and can't lookup symbols for libc and
another standard libs. This patch breaks this loop and allows to lookup
for symbols at other places.

elf.c

@@ -917,6 +917,7 @@ static int
 elf_try_debugfile (struct backtrace_state *state, const char *prefix,
 		   size_t prefix_len, const char *prefix2, size_t prefix2_len,
 		   const char *debuglink_name,
+		   const char *ref_filename,
 		   backtrace_error_callback error_callback, void *data)
 {
   size_t debuglink_len;
@@ -936,6 +937,9 @@ elf_try_debugfile (struct backtrace_state *state, const char *prefix,
   memcpy (try + prefix_len + prefix2_len, debuglink_name, debuglink_len);
   try[prefix_len + prefix2_len + debuglink_len] = '\0';
 
+  if(ref_filename && *ref_filename && !strcmp(try, ref_filename))
+      return -1; // loop detected
+
   ret = backtrace_open (try, error_callback, data, &does_not_exist);
 
   backtrace_free (state, try, try_len, error_callback, data);
@@ -1026,7 +1030,8 @@ elf_find_debugfile_by_debuglink (struct backtrace_state *state,
     }
 
   ddescriptor = elf_try_debugfile (state, prefix, prefix_len, "", 0,
-				   debuglink_name, error_callback, data);
+				   debuglink_name, filename,
+                                   error_callback, data);
   if (ddescriptor >= 0)
     {
       ret = ddescriptor;
@@ -1037,7 +1042,7 @@ elf_find_debugfile_by_debuglink (struct backtrace_state *state,
 
   ddescriptor = elf_try_debugfile (state, prefix, prefix_len, ".debug/",
 				   strlen (".debug/"), debuglink_name,
-				   error_callback, data);
+                                   filename, error_callback, data);
   if (ddescriptor >= 0)
     {
       ret = ddescriptor;
@@ -1049,7 +1054,7 @@ elf_find_debugfile_by_debuglink (struct backtrace_state *state,
   ddescriptor = elf_try_debugfile (state, "/usr/lib/debug/",
 				   strlen ("/usr/lib/debug/"), prefix,
 				   prefix_len, debuglink_name,
-				   error_callback, data);
+				   filename, error_callback, data);
   if (ddescriptor >= 0)
     ret = ddescriptor;