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Re: [PATCH] t4062: stop using repetition in regex




Am 09.08.2017 um 08:15 schrieb René Scharfe:
> Am 09.08.2017 um 07:29 schrieb Junio C Hamano:
>> René Scharfe <l.s.r@xxxxxx> writes:
>>
>>> Am 09.08.2017 um 00:26 schrieb Junio C Hamano:
>>>> ... but in the meantime, I think replacing the test with "0$" to
>>>> force the scanner to find either the end of line or the end of the
>>>> buffer may be a good workaround.  We do not have to care how many of
>>>> random bytes are in front of the last "0" in order to ensure that
>>>> the regexec_buf() does not overstep to 4097th byte, while seeing
>>>> that regexec() that does not know how long the haystack is has to do
>>>> so, no?
>>>
>>> Our regexec() calls strlen() (see my other reply).
>>>
>>> Using "0$" looks like the best option to me.
>>
>> Yeah, it seems that way.  If we want to be close/faithful to the
>> original, we could do "^0*$", but the part that is essential to
>> trigger the old bug is not the "we have many zeroes" (or "we have
>> 4096 zeroes") part, but "zero is at the end of the string" part, so
>> "0$" would be the minimal pattern that also would work for OBSD.
> 
> Thought about it a bit more.
> 
> "^0{4096}$" checks if the byte after the buffer is \n or \0 in the
> hope of triggering a segfault.  On Linux I can access that byte just
> fine; perhaps there is no guard page.  Also there is a 2 in 256
> chance of the byte being \n or \0 (provided its value is random),
> which would cause the test to falsely report success.
> 
> "0$" effectively looks for "0\n" or "0\0", which can only occur
> after the buffer.  If that string is found close enough then we
> may not trigger a segfault and report a false positive.
> 
> In the face of unreliable segfaults we need to reverse our strategy,
> I think.  Searching for something not in the buffer (e.g. "1") and
> considering matches and segfaults as confirmation that the bug is
> still present should avoid any false positives.  Right?

And that's not it either. *sigh*

If the 4097th byte is NUL or LF then we can only hope its access
triggers a segfault -- there is no other way to distinguish the
result from a legitimate match when limiting with REG_STARTEND.  So
we have to accept this flakiness.

We can check the value of that byte with [^0] and interpret a
match as failure, but that adds negation and makes the test more
complex.

^0*$ would falsely match if the 4097th byte (and possibly later
ones) is 0.  We need to make sure we check for end-of-line after
the 4096th byte, not later.

Sorry, Dscho, I thought we could take a shortcut here, but -- as
you wrote all along -- we can't.

So how about this?

-- >8 --
Subject: [PATCH] t4062: use less than 256 repetitions in regex

OpenBSD's regex library has a repetition limit (RE_DUP_MAX) of 255.
That's the minimum acceptable value according to POSIX.  In t4062 we use
4096 repetitions in the test "-G matches", though, causing it to fail.
Combine two repetition operators, both less than 256, to arrive at 4096
zeros instead of using a single one, to fix the test on OpenBSD.

Original-patch-by: David Coppa <dcoppa@xxxxxxxxxxx>
Signed-off-by: Rene Scharfe <l.s.r@xxxxxx>
---
 t/t4062-diff-pickaxe.sh | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/t/t4062-diff-pickaxe.sh b/t/t4062-diff-pickaxe.sh
index 7c4903f497..1130c8019b 100755
--- a/t/t4062-diff-pickaxe.sh
+++ b/t/t4062-diff-pickaxe.sh
@@ -14,8 +14,10 @@ test_expect_success setup '
 	test_tick &&
 	git commit -m "A 4k file"
 '
+
+# OpenBSD only supports up to 255 repetitions, so repeat twice for 64*64=4096.
 test_expect_success '-G matches' '
-	git diff --name-only -G "^0{4096}$" HEAD^ >out &&
+	git diff --name-only -G "^(0{64}){64}$" HEAD^ >out &&
 	test 4096-zeroes.txt = "$(cat out)"
 '
 
-- 
2.14.0