Re: Corrupt name-rev output

["René Scharfe" <l.s.r@web.de>]

Am 17.05.22 um 12:15 schrieb Ævar Arnfjörð Bjarmason:
>
> On Fri, Apr 22 2022, René Scharfe wrote:
>
>> Am 21.04.22 um 19:55 schrieb René Scharfe:
>>> Am 21.04.22 um 04:11 schrieb Elijah Newren:
>>>
>>>> Reverting 2d53975488 fixes the problem.
>>>
>>> That's a good band-aid.
>> Or perhaps it's all we need.  I can't replicate the original reduction
>> of peak memory usage for the Chromium repo anymore.  In fact, the very
>> next commit, 079f970971 (name-rev: sort tip names before applying,
>> 2020-02-05), reduced the number of times free(3) is called there from
>> 44245 to 5, and 3656f84278 (name-rev: prefer shorter names over
>> following merges, 2021-12-04) brought that number down to zero.
>>
>> I can't reproduce the issue with the hardenedBSD repo, by the way, but
>> e.g. with 'git name-rev 58b82150da' in the Linux repo.
>>
>> --- >8 ---
>> Subject: [PATCH] Revert "name-rev: release unused name strings"
>>
>> This reverts commit 2d53975488df195e1431c3f90bfb5b60018d5bf6.
>>
>> 3656f84278 (name-rev: prefer shorter names over following merges,
>> 2021-12-04) broke the assumption of 2d53975488 (name-rev: release unused
>> name strings, 2020-02-04) that a better name for a child is a better
>> name for all of its ancestors as well, because it added a penalty for
>> generation > 0.  This leads to strings being free(3)'d that are still
>> needed.
>>
>> 079f970971 (name-rev: sort tip names before applying, 2020-02-05)
>> already reduced the number of free(3) calls for the use case that
>> motivated the original patch (name-rev --all in the Chromium repository)
>> from ca. 44000 to 5, and 3656f84278 eliminated even those few.  So this
>> revert won't affect name-rev's performance on that particular repo.
>>
>> Reported-by: Thomas Hurst <tom@hur.st>
>> Helped-by: Elijah Newren <newren@gmail.com>
>> Signed-off-by: René Scharfe <l.s.r@web.de>
>> ---
>>  builtin/name-rev.c | 21 +++++----------------
>>  1 file changed, 5 insertions(+), 16 deletions(-)
>>
>> diff --git a/builtin/name-rev.c b/builtin/name-rev.c
>> index c59b5699fe..02ea9d1633 100644
>> --- a/builtin/name-rev.c
>> +++ b/builtin/name-rev.c
>> @@ -18,7 +18,7 @@
>>  #define CUTOFF_DATE_SLOP 86400
>>
>>  struct rev_name {
>> -	char *tip_name;
>> +	const char *tip_name;
>>  	timestamp_t taggerdate;
>>  	int generation;
>>  	int distance;
>> @@ -84,7 +84,7 @@ static int commit_is_before_cutoff(struct commit *commit)
>>
>>  static int is_valid_rev_name(const struct rev_name *name)
>>  {
>> -	return name && (name->generation || name->tip_name);
>> +	return name && name->tip_name;
>>  }
>>
>>  static struct rev_name *get_commit_rev_name(const struct commit *commit)
>> @@ -146,20 +146,9 @@ static struct rev_name *create_or_update_name(struct commit *commit,
>>  {
>>  	struct rev_name *name = commit_rev_name_at(&rev_names, commit);
>>
>> -	if (is_valid_rev_name(name)) {
>> -		if (!is_better_name(name, taggerdate, generation, distance, from_tag))
>> -			return NULL;
>> -
>> -		/*
>> -		 * This string might still be shared with ancestors
>> -		 * (generation > 0).  We can release it here regardless,
>> -		 * because the new name that has just won will be better
>> -		 * for them as well, so name_rev() will replace these
>> -		 * stale pointers when it processes the parents.
>> -		 */
>> -		if (!name->generation)
>> -			free(name->tip_name);
>> -	}
>> +	if (is_valid_rev_name(name) &&
>> +	    !is_better_name(name, taggerdate, generation, distance, from_tag))
>> +		return NULL;
>>
>>  	name->taggerdate = taggerdate;
>>  	name->generation = generation;
>
> I haven't dug into whether it's a false positive, but with this change
> GCC's -fanalyzer has started complaining about a potential NULL
> dereference:
>
>     builtin/name-rev.c:230:50: error: dereference of NULL ‘name’ [CWE-476] [-Werror=analyzer-null-dereference]
>       230 |                                 generation = name->generation + 1;
>
> This "fixes" it, and passes all tests, but presumably a better fix
> involves the callers of get_commit_rev_name() (or that function itself)
> deciding if they're OK with NULL here earlier?:
>
> 	diff --git a/builtin/name-rev.c b/builtin/name-rev.c
> 	index 02ea9d16330..1d3a620ac72 100644
> 	--- a/builtin/name-rev.c
> 	+++ b/builtin/name-rev.c
> 	@@ -209,6 +209,7 @@ static void name_rev(struct commit *start_commit,
> 	 		struct rev_name *name = get_commit_rev_name(commit);
> 	 		struct commit_list *parents;
> 	 		int parent_number = 1;
> 	+		assert(name);
>
> 	 		parents_to_queue_nr = 0;
>
>

It's a false positive AFAICS because we set the name of the first commit
to go into the queue with create_or_update_name(), then go through its
parents and set their name as well before putting them into the queue to
traverse ancestors recursively.  I.e. everything we pull from the queue
must have a non-NULL name, right?

This begs the question why a free(3) call would make a difference to the
analyzer -- it doesn't affect NULL pointers after all.

Another good question is whether we should remove the validity check and
use commit_rev_name_peek() directly instead of get_commit_rev_name() in
the loop since we know it must be valid.  This might save a few cycles
and perhaps calm down the analyzer.

René