[PATCH 1/2] Fix finding split debug info files not located by the build-id mechanism

[Mark Wielaard <mjw at redhat dot com>]

From: Dodji Seketeli <dodji@seketeli.org>

[This bug has been found by using elfutils in the context of
 libabigail.  The initial bug report is
 https://sourceware.org/bugzilla/show_bug.cgi?id=18792.  The
 interesting comments start at is
 https://sourceware.org/bugzilla/show_bug.cgi?id=18792#c4]

suppose we have a debug info file that is located at a
/prefix1/usr/lib/debug/prefix2/libfoo.so.  Suppose also that the debug
info describes a binary that is located at /prefix1/prefix2/libfoo.so

Suppose the debug_link property inside the binary
/prefix1/prefix2/libfoo.so correctly contains the string "libfoo.so"
that designates the name of the debug info file.

The problem is, when find_debuginfo_in_path() is called with its
file_name parameter set to "/prefix1/prefix2/libfoo.so" and
mod->dwfl->callbacks->debuginfo_path set to
"/prefix1/lib/debug/", it fails to locate the debug
info file libfoo.so under "/prefix1/usr/lib/debug/prefix2/".

This patch fixes the issue by making find_debuginfo_in_path() try all
the sub-strings of "/prefix1/prefix2/libfoo.so "under"
"/prefix1/usr/lib/debug/", to find libfoo.so.  That is, it tries, in
order:
 - /prefix1/usr/lib/debug/prefix1/prefix2/libfoo.so
 - /prefix1/usr/lib/debug/prefix2/libfoo.so <-- and boom, it finds it!

Note that the patch tries the variations between the two candidates
above too.

The patch uses a goto.  I dislike gotos like anyone else, but then
here, not using this would imply a bigger change of the logic of that
function.  So I am proposing the scheme based on the goto instead.

The patch lacks a test case, but then it's a bit cumbersome to write
one right now.  I'll defer that to later when people agree with the
general approach of the patch.

	* libdwfl/find-debuginfo.c (find_debuginfo_in_path): Try to locate
	the debug info file named debuglink_file under
	mod->dwfl->callbacks->debuginfo_path, by looking at the set of
	sub-trees under mod->dwfl->callbacks->debuginfo_path which is
	common to the set of non-absolute parent trees of file_name.

Signed-off-by: Dodji Seketeli <dodji@seketeli.org>
Signed-off-by: Mark Wielaard <mjw@redhat.com>
---
 libdwfl/ChangeLog        |  9 +++++++++
 libdwfl/find-debuginfo.c | 13 ++++++++++++-
 libdwfl/libdwfl.h        | 14 ++++++++------
 3 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index 5059be6..3e36aa7 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,12 @@
+2015-08-14  Dodji Seketeli  <dodji@seketeli.org>
+
+	* find-debuginfo.c (find_debuginfo_in_path): Try to locate the
+	debug info file named debuglink_file under
+	mod->dwfl->callbacks->debuginfo_path, by looking at
+	the set of sub-trees under mod->dwfl->callbacks->debuginfo_path
+	which is common to the set of non-absolute parent trees of
+	file_name.
+
 2015-06-18  Mark Wielaard  <mjw@redhat.com>
 
 	* find-debuginfo.c (try_open): Free fname on all failure paths.
diff --git a/libdwfl/find-debuginfo.c b/libdwfl/find-debuginfo.c
index 1faa494..6b8d1ac 100644
--- a/libdwfl/find-debuginfo.c
+++ b/libdwfl/find-debuginfo.c
@@ -252,7 +252,15 @@ find_debuginfo_in_path (Dwfl_Module *mod, const char *file_name,
 	  dir = p;
 	  if (mod->dw == NULL)
 	    {
-	      subdir = file_dirname + 1;
+	      subdir = file_dirname;
+	      /* We want to explore all sub-subdirs.  Chop off one slash
+		 at a time.  */
+	    explore_dir:
+	      subdir = strchr (subdir, '/');
+	      if (subdir != NULL)
+		subdir = subdir + 1;
+	      if (subdir && *subdir == 0)
+		continue;
 	      file = debuglink_file;
 	    }
 	  else
@@ -292,6 +300,9 @@ find_debuginfo_in_path (Dwfl_Module *mod, const char *file_name,
 		  }
 		break;
 	      }
+	    /* If possible try again with a sub-subdir.  */
+	    if (mod->dw == NULL && subdir)
+	      goto explore_dir;
 	    continue;
 	  default:
 	    goto fail_free;
diff --git a/libdwfl/libdwfl.h b/libdwfl/libdwfl.h
index 2bb4f45..1098c83 100644
--- a/libdwfl/libdwfl.h
+++ b/libdwfl/libdwfl.h
@@ -275,12 +275,14 @@ extern int dwfl_module_build_id (Dwfl_Module *mod,
    In searches for debuginfo by name, if the remainder of the
    element is empty, the directory containing the main file is
    tried; if it's an absolute path name, the absolute directory path
-   containing the main file is taken as a subdirectory of this path;
-   a relative path name is taken as a subdirectory of the directory
-   containing the main file.  Hence for /bin/ls, the default string
-   ":.debug:/usr/lib/debug" says to look in /bin, then /bin/.debug,
-   then /usr/lib/debug/bin, for the file name in the .gnu_debuglink
-   section (or "ls.debug" if none was found).  */
+   (and any subdirectory of that path) containing the main file is
+   taken as a subdirectory of this path; a relative path name is taken
+   as a subdirectory of the directory containing the main file.
+   Hence for /usr/bin/ls, the default string ":.debug:/usr/lib/debug"
+   says to look in /usr/bin, then /usr/bin/.debug, then the path subdirs
+   under /usr/lib/debug, in the order /usr/lib/debug/usr/bin, then
+   /usr/lib/debug/bin, and finally /usr/lib/debug, for the file name in
+   the .gnu_debuglink section (or "ls.debug" if none was found).  */
 
 /* Standard find_elf callback function working solely on build ID.
    This can be tried first by any find_elf callback, to use the
-- 
1.8.3.1