linux: Add /usr/lib/debug symbol path checking

Debian likes to hive off symbol information to separate -dbg packages,
installing the debug information in a parallel tree rooted at /usr/lib/debug.
libunwind doesn't know how to find these files but it's easy, per the tested,
attached patch, to make it so. I don't see /usr/lib/debug at
http://www.pathname.com/fhs/pub/fhs-2.3.html#USRLIBLIBRARIESFORPROGRAMMINGANDPA
but
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Debugging_with_gdb/separate-debug-files.html
says it's not just Debian and hints it might be driven by gdb. I do see mention
of /usr/lib/debug in libunwind already, but only in a DWARF-specific part of
the code.  A minor concern might be checking errno to see if the problem is
ENOENT before trying the alternate path. That might be better than flogging a
mmap problem or being out of file handles or whatever.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/os-linux.c b/src/os-linux.c
index 1cc9ba52..652d6f2f 100644
--- a/src/os-linux.c
+++ b/src/os-linux.c
@@ -37,6 +37,7 @@ tdep_get_elf_image (struct elf_image *ei, pid_t pid, unw_word_t ip,
   struct map_iterator mi;
   int found = 0, rc;
   unsigned long hi;
+  char debug_path[PATH_MAX];
 
   if (maps_init (&mi, pid) < 0)
     return -1;
@@ -57,7 +58,12 @@ tdep_get_elf_image (struct elf_image *ei, pid_t pid, unw_word_t ip,
     {
       strncpy(path, mi.path, pathlen);
     }
-  rc = elf_map_image (ei, mi.path);
+  snprintf (debug_path, sizeof (debug_path), "/usr/lib/debug%s", mi.path);
+  rc = elf_map_image (ei, debug_path);
+  if (rc != 0)
+    {
+      rc = elf_map_image (ei, mi.path);
+    }
   maps_close (&mi);
   return rc;
 }