Re: [PATCH] commit-reach: fix sorting commits by generation
- From
René Scharfe <l.s.r@web.de>
- Date
- Oct 22, 2018, 21:53 UTC
- Message-ID
- <13bdda53-b751-182b-4aa8-d9a5c03f422d@web.de>
- In-Reply-To
- <20181022211037.22719-1-t.gummerer@gmail.com>
Am 22.10.2018 um 23:10 schrieb Thomas Gummerer:
Show 15 quoted lines
> compare_commit_by_gen is used to sort a list of pointers to 'struct > commit'. The comparison function for qsort is called with pointers to > the objects it needs to compare, so when sorting a list of 'struct > commit *', the arguments are of type 'struct commit **'. However, > currently the comparison function casts it's arguments to 'struct > commit *' and uses those, leading to out of bounds memory access and > potentially to wrong results. Fix that. > > Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com> > --- > > I noticed this by running the test suite through valgrind. I'm not > familiar with this code, so I'm not sure why this didn't cause any > issues or how they would manifest, but this seems like the right fix > for this function either way.
Right; I sent a similar patch a while ago, but it seems to have fallen through the cracks:
https://public-inbox.org/git/d1b58614-989f-5998-6c53-c19eee409a2f@web.de/
Anyway, your implied question was discussed back then. Derrick wrote:
The reason to sort is to hopefully minimize the amount we walk by exploring the "lower" commits first. This is a performance-only thing, not a correctness issue (which is why the bug exists). Even then, it is just a heuristic.
Does b6723e4671 in pu (commit-reach: fix first-parent heuristic) change that picture? Did a quick test and found no performance difference with and without the fix on top, i.e. proper sorting didn't seem to matter.
Show 19 quoted lines
> commit-reach.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/commit-reach.c b/commit-reach.c
> index bc522d6840..9efddfd7a0 100644
> --- a/commit-reach.c
> +++ b/commit-reach.c
> @@ -516,8 +516,8 @@ int commit_contains(struct ref_filter *filter, struct commit *commit,
>
> static int compare_commits_by_gen(const void *_a, const void *_b)
> {
> - const struct commit *a = (const struct commit *)_a;
> - const struct commit *b = (const struct commit *)_b;
> + const struct commit *a = *(const struct commit **)_a;
> + const struct commit *b = *(const struct commit **)_b;
>
> if (a->generation < b->generation)
> return -1;
> Looks good to me.
René