[PATCH 1/3] wildmatch: fix exponential behavior
From: Phillip Wood <phillip.wood@dunelm.org.uk>
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 5230f605e1 (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 6f1a31f0aa (wildmatch: advance faster in <asterisk> + <literal> patterns, 2013-01-01) and 46983441ae (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 <derrickstolee@github.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
t/t3070-wildmatch.sh | 9 +++++++++
wildmatch.c | 12 ++++++++----
2 files changed, 17 insertions(+), 4 deletions(-)
Show changes to 2 files +17 −4
t/t3070-wildmatch.sh, wildmatch.c
diff --git a/t/t3070-wildmatch.sh b/t/t3070-wildmatch.sh
index 5d871fde96..b91a7cb712 100755
--- a/t/t3070-wildmatch.sh
+++ b/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
diff --git a/wildmatch.c b/wildmatch.c
index 7e5a7ea1ea..06861bd8bc 100644
--- a/wildmatch.c
+++ b/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)
--
2.39.2
