common_prefix: simplify and fix scanning for prefixes

common_prefix() scans backwards from the far end of each 'next'
pathspec, starting from 'len', shortening the 'prefix' using 'path' as
a reference.

However, there is a small opportunity for an out-of-bounds access
because len is unconditionally set to prefix-1 after a "direct match"
test failed.  This means that if 'next' is shorter than prefix+2, we
read past it.

Instead of a minimal fix, simplify the loop: scan *forward* over the
'next' entry, remembering the last '/' where it matched the prefix
known so far.  This is far easier to read and also has the advantage
that we only scan over each entry once.

Acked-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/dir.c b/dir.c
index 5615f33af187f381f8c2dfe7ab53910fe165fd59..7f912c76f268e2f27857a67d565e77be0e43660d 100644 (file)
--- a/dir.c
+++ b/dir.c
@@ -31,22 +31,22 @@ static int common_prefix(const char **pathspec)
        if (!slash)
                return 0;
 
+       /*
+        * The first 'prefix' characters of 'path' are common leading
+        * path components among the pathspecs we have seen so far,
+        * including the trailing slash.
+        */
        prefix = slash - path + 1;
        while ((next = *++pathspec) != NULL) {
-               int len = strlen(next);
-               if (len >= prefix && !memcmp(path, next, prefix))
+               int len, last_matching_slash = -1;
+               for (len = 0; len < prefix && next[len] == path[len]; len++)
+                       if (next[len] == '/')
+                               last_matching_slash = len;
+               if (len == prefix)
                        continue;
-               len = prefix - 1;
-               for (;;) {
-                       if (!len)
-                               return 0;
-                       if (next[--len] != '/')
-                               continue;
-                       if (memcmp(path, next, len+1))
-                               continue;
-                       prefix = len + 1;
-                       break;
-               }
+               if (last_matching_slash < 0)
+                       return 0;
+               prefix = last_matching_slash + 1;
        }
        return prefix;
 }