wildmatch: fix exponential behavior

When dowild() cannot match a '*' or '/**/' wildcard then it must return
WM_ABORT_TO_STARSTAR or WM_ABORT_ALL respectively. Failure to observe
this results in unnecessary backtracking and the time taken for a failed
match increases exponentially with the number of wildcards in the
pattern [1]. Unfortunately in some instances dowild() returns WM_NOMATCH
for a failed match resulting in long match times for patterns containing
multiple wildcards as can be seen in the following benchmark.
(Note that the timings in the Benchmark 1 are really measuring the time
to execute test-tool rather than the time to match the pattern)

Benchmark 1: t/helper/test-tool wildmatch wildmatch aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab "*a"
  Time (mean ± σ):      22.8 ms ±   1.7 ms    [User: 12.1 ms, System: 10.6 ms]
  Range (min … max):    19.4 ms …  26.9 ms    113 runs

  Warning: Ignoring non-zero exit code.

Benchmark 2: t/helper/test-tool wildmatch wildmatch aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab "*a*a*a*a*a*a*a*a*a"
  Time (mean ± σ):      5.244 s ±  0.228 s    [User: 5.229 s, System: 0.010 s]
  Range (min … max):    4.969 s …  5.707 s    10 runs

  Warning: Ignoring non-zero exit code.

Summary
  't/helper/test-tool wildmatch wildmatch aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab "*a"' ran
  230.37 ± 20.04 times faster than 't/helper/test-tool wildmatch wildmatch aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab "*a*a*a*a*a*a*a*a*a"'

The security implications are limited as it only affects operations that
are potentially DoS vectors. For example by creating a blob containing
such a pattern a malicious user can exploit this behavior to use large
amounts of CPU time on a remote server by pushing the blob and then
creating a new clone with --filter=sparse:oid. However this filter type
is usually disabled as it is known to consume large amounts of CPU time
even without this bug.

The WM_MATCH changed in the first hunk of this patch comes from the
original implementation imported from rsync in 5230f60 (Import
wildmatch from rsync, 2012-10-15). Compared to the others converted here
it is fairly harmless as it only triggers at the end of the pattern and
so will only cause a single unnecessary backtrack. The others introduced
by 6f1a31f (wildmatch: advance faster in <asterisk> + <literal>
patterns, 2013-01-01) and 4698344 (wildmatch: make a special case for
"*/" with FNM_PATHNAME, 2013-01-01) are more pernicious and will cause
exponential behavior.

A new test is added to protect against future regressions.

[1] https://research.swtch.com/glob

Helped-by: Derrick Stolee <[email protected]>
Signed-off-by: Phillip Wood <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phillipwood

t/t3070-wildmatch.sh

@@ -431,4 +431,13 @@ match 1 1 1 1 'a' '[B-a]'
 match 0 1 0 1 'z' '[Z-y]'
 match 1 1 1 1 'Z' '[Z-y]'
 
+test_expect_success 'matching does not exhibit exponential behavior' '
+	test-tool wildmatch wildmatch \
+		aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab \
+		"*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a" &
+	pid=$! &&
+	sleep 2 &&
+	! kill $!
+'
+
 test_done

wildmatch.c

@@ -114,7 +114,7 @@ static int dowild(const uchar *p, const uchar *text, unsigned int flags)
 				 * only if there are no more slash characters. */
 				if (!match_slash) {
 					if (strchr((char *)text, '/'))
-						return WM_NOMATCH;
+						return WM_ABORT_TO_STARSTAR;
 				}
 				return WM_MATCH;
 			} else if (!match_slash && *p == '/') {
@@ -125,7 +125,7 @@ static int dowild(const uchar *p, const uchar *text, unsigned int flags)
 				 */
 				const char *slash = strchr((char*)text, '/');
 				if (!slash)
-					return WM_NOMATCH;
+					return WM_ABORT_ALL;
 				text = (const uchar*)slash;
 				/* the slash is consumed by the top-level for loop */
 				break;
@@ -153,8 +153,12 @@ static int dowild(const uchar *p, const uchar *text, unsigned int flags)
 							break;
 						text++;
 					}
-					if (t_ch != p_ch)
-						return WM_NOMATCH;
+					if (t_ch != p_ch) {
+						if (match_slash)
+							return WM_ABORT_ALL;
+						else
+							return WM_ABORT_TO_STARSTAR;
+					}
 				}
 				if ((matched = dowild(p, text, flags)) != WM_NOMATCH) {
 					if (!match_slash || matched != WM_ABORT_TO_STARSTAR)