From: Jeff King Date: Tue, 17 Nov 2020 22:30:11 GMT Subject: Re: [PATCH] gc: fix cast in compare_tasks_by_selection() Message-ID: <20201117223011.GA642234@coredump.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