[PATCH] pickaxe: fix segfault with '-S<...> --pickaxe-regex'

["SZEDER Gábor" <szeder.dev@gmail.com>]

'git {log,diff,...} -S<...> --pickaxe-regex' can segfault as a result
of out-of-bounds memory reads.

diffcore-pickaxe.c:contains() looks for all matches of the given regex
in a buffer in a loop, advancing the buffer pointer to the end of the
last match in each iteration.  When we switched to REG_STARTEND in
b7d36ffca (regex: use regexec_buf(), 2016-09-21), we started passing
the size of that buffer to the regexp engine, too.  Unfortunately,
this buffer size is never updated on subsequent iterations, and as the
buffer pointer advances on each iteration, this "bufptr+bufsize"
points past the end of the buffer.  This results in segmentation
fault, if that memory can't be accessed.  In case of 'git log' it can
also result in erroneously listed commits, if the memory past the end
of buffer is accessible and happens to contain data matching the
regex.

Make sure that the buffer size is reduced on each iteration as the
buffer pointer is advanced, thus maintaining the correct end of buffer
location.

The new test is flaky, I've never seen it fail on my Linux box, but
this is expected according to db5dfa331 (regex: -G<pattern> feeds a
non NUL-terminated string to regexec() and fails, 2016-09-21).  And
based on that commit message I would expect the new test without the
fix to fail reliably on Windows.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---

 diffcore-pickaxe.c      | 5 ++++-
 t/t4062-diff-pickaxe.sh | 5 +++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/diffcore-pickaxe.c b/diffcore-pickaxe.c
index 9795ca1c1..03f84b714 100644
--- a/diffcore-pickaxe.c
+++ b/diffcore-pickaxe.c
@@ -85,8 +85,11 @@ static unsigned int contains(mmfile_t *mf, regex_t *regexp, kwset_t kws)
 		       !regexec_buf(regexp, data, sz, 1, &regmatch, flags)) {
 			flags |= REG_NOTBOL;
 			data += regmatch.rm_eo;
-			if (*data && regmatch.rm_so == regmatch.rm_eo)
+			sz -= regmatch.rm_eo;
+			if (*data && regmatch.rm_so == regmatch.rm_eo) {
 				data++;
+				sz--;
+			}
 			cnt++;
 		}
 
diff --git a/t/t4062-diff-pickaxe.sh b/t/t4062-diff-pickaxe.sh
index f0bf50bda..7c4903f49 100755
--- a/t/t4062-diff-pickaxe.sh
+++ b/t/t4062-diff-pickaxe.sh
@@ -19,4 +19,9 @@ test_expect_success '-G matches' '
 	test 4096-zeroes.txt = "$(cat out)"
 '
 
+test_expect_success '-S --pickaxe-regex' '
+	git diff --name-only -S0 --pickaxe-regex HEAD^ >out &&
+	verbose test 4096-zeroes.txt = "$(cat out)"
+'
+
 test_done
-- 
2.12.0.377.g15f6ffe90