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authoraszlig <aszlig@redmoonstudios.org>2015-01-02T02·27+0100
committerEelco Dolstra <eelco.dolstra@logicblox.com>2015-01-02T11·53+0100
commitbbd45ac80fa8ab80acd317ed8d5cc8b3448f7387 (patch)
tree802f5a1f05916dcd31bcdd4421a9027f6972071b
parent411b237ee55012b0f7b790ed509f4f4102a811ae (diff)
libutil: Improve errmsg on readLink size mismatch.
A message like "error: reading symbolic link `...' : Success" really is
quite confusing, so let's not indicate "success" but rather point out
the real issue.

We could also limit the check of this to just check for non-negative
values, but this would introduce a race condition between stat() and
readlink() if the link target changes between those two calls, thus
leading to a buffer overflow vulnerability.

Reported by @Ericson2314 on IRC. Happened due to a possible ntfs-3g bug
where a relative symlink returned the absolute path (st_)size in stat()
while readlink() returned the relative size.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Tested-by: John Ericson <Ericson2314@Yahoo.com>
-rw-r--r--src/libutil/util.cc6
1 files changed, 5 insertions, 1 deletions
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index dcdb438e03b2..879f9fb5fddd 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -193,8 +193,12 @@ Path readLink(const Path & path)
     if (!S_ISLNK(st.st_mode))
         throw Error(format("‘%1%’ is not a symlink") % path);
     char buf[st.st_size];
-    if (readlink(path.c_str(), buf, st.st_size) != st.st_size)
+    ssize_t rlsize = readlink(path.c_str(), buf, st.st_size);
+    if (rlsize == -1)
         throw SysError(format("reading symbolic link ‘%1%’") % path);
+    else if (rlsize != st.st_size)
+        throw Error(format("symbolic link ‘%1%’ size mismatch %2% != %3%")
+            % path % rlsize % st.st_size);
     return string(buf, st.st_size);
 }