# [PATCH] gc: fix cast in compare_tasks_by_selection() 2 messages from 2020-11-17 to 2020-11-17. Participants: René Scharfe, Jeff King. Thread: https://gitlist.dev/t/54660 ## René Scharfe, 2020-11-17 21:59 Subject: [PATCH] gc: fix cast in compare_tasks_by_selection() Message-ID: URL: https://gitlist.dev/e/d80423b7-c89d-99a2-ce49-8308822f7769%40web.de ``` compare_tasks_by_selection() is used with QSORT and gets passed pointers to the elements of "static struct maintenance_task tasks[]". It casts the *addresses* of these passed pointers to element pointers, though, and thus effectively compares some unrelated values from the stack. Fix the casts to actually compare array elements. Detected by USan (make SANITIZE=undefined test). Signed-off-by: René Scharfe --- builtin/gc.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/builtin/gc.c b/builtin/gc.c index 5cd2a43f9f..986b760b30 100644 --- a/builtin/gc.c +++ b/builtin/gc.c @@ -1213,10 +1213,8 @@ static struct maintenance_task tasks[] = { static int compare_tasks_by_selection(const void *a_, const void *b_) { - const struct maintenance_task *a, *b; - - a = (const struct maintenance_task *)&a_; - b = (const struct maintenance_task *)&b_; + const struct maintenance_task *a = a_; + const struct maintenance_task *b = b_; return b->selected_order - a->selected_order; } -- 2.29.2 ``` ## Jeff King, 2020-11-17 22:30 Subject: Re: [PATCH] gc: fix cast in compare_tasks_by_selection() Message-ID: <20201117223011.GA642234@coredump.intra.peff.net> URL: https://gitlist.dev/e/20201117223011.GA642234%40coredump.intra.peff.net In-Reply-To: ``` On Tue, Nov 17, 2020 at 10:59:49PM +0100, René Scharfe wrote: > compare_tasks_by_selection() is used with QSORT and gets passed pointers > to the elements of "static struct maintenance_task tasks[]". It casts > the *addresses* of these passed pointers to element pointers, though, > and thus effectively compares some unrelated values from the stack. Fix > the casts to actually compare array elements. > > Detected by USan (make SANITIZE=undefined test). I checked the caller here, and indeed, it's passing an array-of-struct so your patch is doing the right thing (not that I doubted it, but that's what review is for). This qsort void-pointer convention seems to create a lot of confusion (not just the lack of type-safety, but the fact that it's getting a pointer to the element). I felt like we had a discussion about this a while ago, and indeed, I found: https://lore.kernel.org/git/7b95417a-c8fb-4f1e-cb09-c36804a3a4d0@web.de/ The whole sub-thread is worth reading, but the macro you arrived at in: https://lore.kernel.org/git/c141fb44-904f-e8b6-119f-7d2d6bcfd81a@web.de/ seems pretty reasonable. -Peff ```