Re: Corrupt name-rev output

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

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;