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>

1 files changed, 6 insertions, 2 deletions

diff --git a/nix/libutil/util.cc b/nix/libutil/util.cc
index 7998664ed0..410d0f2830 100644
--- a/nix/libutil/util.cc
+++ b/nix/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)
-        throw SysError(format("reading symbolic link `%1%'") % path);
+    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);
 }