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From: Magne Hov <mhov@undo.io>
To: gdb-patches@sourceware.org
Subject: [PATCH v2] gdb/source.c: Fix undefined behaviour dereferencing empty string
Date: Wed, 21 Sep 2022 15:58:34 +0100	[thread overview]
Message-ID: <20220921145834.837969-1-mhov@undo.io> (raw)
In-Reply-To: <20220915183141.3484234-1-mhov@undo.io>

When a source file's dirname is solely made up of directory separators
we end up trying to dereference the last character of an empty string
with std::string::back, which results in undefined behaviour. A typical
use case where this can happen is when the root directory "/" is used as
a compilation directory.

With libstdc++.so.6.0.28 we get no out-of-bounds checks and the byte
preceding the storage of the empty string is returned. The character
value of this byte depends on heap implementation and usage, but when
this byte happens to hold the value of the directory separator character
we go on to call std::string::pop_back on the empty string which results
in an out_of_range exception which terminates GDB.

Fix this by using path_join. prepare_path_for_appending ensures that the
filename component is relative.

The testsuite has been run before and after the change and no
regressions were found.
---
 gdb/source.c | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/gdb/source.c b/gdb/source.c
index 3f498d552c4..25ad1ecb3da 100644
--- a/gdb/source.c
+++ b/gdb/source.c
@@ -1146,15 +1146,7 @@ find_and_open_source (const char *filename,
 	 helpful if part of the compilation directory was removed,
 	 e.g. using gcc's -fdebug-prefix-map, and we have added the missing
 	 prefix to source_path.  */
-      std::string cdir_filename (dirname);
-
-      /* Remove any trailing directory separators.  */
-      while (IS_DIR_SEPARATOR (cdir_filename.back ()))
-	cdir_filename.pop_back ();
-
-      /* Add our own directory separator.  */
-      cdir_filename.append (SLASH_STRING);
-      cdir_filename.append (filename_start);
+      std::string cdir_filename = path_join (dirname, filename_start);
 
       result = openp (path, OPF_SEARCH_IN_PATH | OPF_RETURN_REALPATH,
 		      cdir_filename.c_str (), OPEN_MODE, fullname);
-- 
2.25.1


  parent reply	other threads:[~2022-09-21 14:58 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-15 18:31 [PATCH] " Magne Hov
2022-09-20 19:07 ` Tom Tromey
2022-09-21 14:51   ` Magne Hov
2022-09-21 14:58 ` Magne Hov [this message]
2022-09-21 15:52   ` [PATCH v2] " Simon Marchi
2022-09-22 13:11     ` Magne Hov
2022-09-24  8:58   ` Magne Hov

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