On Fri, Mar 04, 2022 at 02:11:50PM +0100, Marco Elver wrote: > On Fri, 4 Mar 2022 at 13:02, Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote: > > > > On Fri, Mar 04, 2022 at 12:50:21PM +0100, Marco Elver wrote: > > > On Fri, 4 Mar 2022 at 07:34, Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote: > > > > > > > > Changes from v1: > > > > Now SLAB passes requests larger than order-1 page > > > > to page allocator. > > > > > > > > Adjusted comments from Matthew, Vlastimil, Rientjes. > > > > Thank you for feedback! > > > > > > > > BTW, I have no idea what __ksize() should return when an object that > > > > is not allocated from slab is passed. both 0 and folio_size() > > > > seems wrong to me. > > > > > > Didn't we say 0 would be the safer of the two options? > > > https://lkml.kernel.org/r/0e02416f-ef43-dc8a-9e8e-50ff63dd3c61@xxxxxxx > > > > > > > Oh sorry, I didn't understand why 0 was safer when I was reading it. > > > > Reading again, 0 is safer because kasan does not unpoison for > > wrongly passed object, right? > > Not quite. KASAN can tell if something is wrong, i.e. invalid object. > Similarly, if you are able to tell if the passed pointer is not a > valid object some other way, you can do something better - namely, > return 0. > > The intuition here is that the caller has a pointer to an > invalid object, and wants to use ksize() to determine its size, and > most likely access all those bytes. Arguably, at that point the kernel > is already in a degrading state. But we can try to not let things get > worse by having ksize() return 0, in the hopes that it will stop > corrupting more memory. It won't work in all cases, but should avoid > things like "s = ksize(obj); touch_all_bytes(obj, s)" where the size > bounds the memory accessed corrupting random memory. Oh, it's to prevent to corrupt memory further in failure case, like memset(obj, 0, s); > The other reason is that a caller could actually check the size, and > if 0, do something else. Few callers will do so, because nobody > expects that their code has a bug. :-) and making it able to check errors by caller. Thank you so much for kind explanation. I'll add what Vlastimil suggested in next series. Thanks! -- Thank you, You are awesome! Hyeonggon :-)
