Re: [PATCH v2 4/4] bisect--helper: double-check run command on exit code 126 and 127

From: René Scharfe <l.s.r@web.de>
Date: Feb 4, 2022, 17:16 UTC
Message-ID: <0d70e31e-f6d1-dca5-c0e8-e2288e3e1c4e@web.de>
In-Reply-To: <xmqqa6f7pime.fsf@gitster.g>

Am 04.02.22 um 01:42 schrieb Junio C Hamano:

Show 59 quoted lines

> René Scharfe <l.s.r@web.de> writes:
>
>> When a run command cannot be executed or found, shells return exit code
>> 126 or 127, respectively.  Valid run commands are allowed to return
>> these codes as well to indicate bad revisions, though, for historical
>> reasons.  This means typos can cause bogus bisect runs that go over the
>> full distance and end up reporting invalid results.
>>
>> The best solution would be to reserve exit codes 126 and 127, like
>> 71b0251cdd (Bisect run: "skip" current commit if script exit code is
>> 125., 2007-10-26) did for 125, and abort bisect run when we get them.
>> That might be inconvenient for those who relied on the documentation
>> stating that 126 and 127 can be used for bad revisions, though.
>
> I think the basic idea is sound and useful.  How happy are we who
> was involved in the discussion with this result?
>
>> +static int get_first_good(const char *refname, const struct object_id *oid,
>> +			  int flag, void *cb_data)
>> +{
>> +	oidcpy(cb_data, oid);
>> +	return 1;
>> +}
>
> OK, this iterates and stops at the first one.
>
>> +static int verify_good(const struct bisect_terms *terms,
>> +		       const char **quoted_argv)
>> +{
>> +	int rc;
>> +	enum bisect_error res;
>> +	struct object_id good_rev;
>> +	struct object_id current_rev;
>> +	char *good_glob = xstrfmt("%s-*", terms->term_good);
>> +	int no_checkout = ref_exists("BISECT_HEAD");
>> +
>> +	for_each_glob_ref_in(get_first_good, good_glob, "refs/bisect/",
>> +			     &good_rev);
>> +	free(good_glob);
>> +
>> +	if (read_ref(no_checkout ? "BISECT_HEAD" : "HEAD", &current_rev))
>> +		return -1;
>
>  * Could the current_rev already be marked as "good", in which case
>    we can avoid cost of rewriting working tree files to a
>    potentially distant revision?  I often do manual tests to mark
>    "bisect good" or "bisect bad" before using "bisect run".
>
>  * Can we have *no* rev that is marked as "good"?  I think we made
>    it possible to say "my time is more valuable than machine cycles,
>    so I'll only tell you that this revision is broken and give you
>    no limit on the bottom side of the history.  still assume that
>    there was only one good-to-bad transition in the history and find
>    it" by supplying only one "bad" and no "good" when starting to
>    bisect.  And in such a case, ...
>
>> +	res = bisect_checkout(&good_rev, no_checkout);
>
> ... this would feed an uninitialized object_id to bisect_checkout.

bisect_run() starts by calling bisect_next_check() with a current_term parameter value of NULL. It checks if the good rev is missing and calls decide_next(), which returns -1 if current_term is NULL unless both good and bad revs are present. bisect_next_check() passes this value along. bisect_run() exits if it's non-zero.

So AFAICS the uninitialized access would only happen if the good rev ref was deleted between the bisect_next_check() call and the verify_good() call. I considered this scenario to be practically impossible with the current code. We can handle it more gracefully by doing something like in the patch below.

Supporting a bad-only git bisect run would take more work -- perhaps by making verify_good() pick a root commit to check as an assumed good rev (plus fix whatever else caused the current code to pass NULL as current_term).

René

---
 builtin/bisect--helper.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/builtin/bisect--helper.c b/builtin/bisect--helper.c
index 50783a586c..e1e58de3b2 100644
--- a/builtin/bisect--helper.c
+++ b/builtin/bisect--helper.c
@@ -1106,9 +1106,12 @@ static int verify_good(const struct bisect_terms *terms,
 	char *good_glob = xstrfmt("%s-*", terms->term_good);
 	int no_checkout = ref_exists("BISECT_HEAD");

+	oidcpy(&good_rev, null_oid());
 	for_each_glob_ref_in(get_first_good, good_glob, "refs/bisect/",
 			     &good_rev);
 	free(good_glob);
+	if (is_null_oid(&good_rev))
+		return -1;

 	if (read_ref(no_checkout ? "BISECT_HEAD" : "HEAD", &current_rev))
 		return -1;
--
2.35.0