On 06/25, Andy Lutomirski wrote: > > On Wed, Jun 25, 2014 at 10:32 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > On 06/25, Andy Lutomirski wrote: > >> > >> Write the filter, then smp_mb (or maybe a weaker barrier is okay), > >> then set the bit. > > > > Yes, exactly, this is what I meant. Plas rmb() in __secure_computing(). > > > > But I still can't understand the rest of your discussion about the > > ordering we need ;) > > Let me try again from scratch. > > Currently there are three relevant variables: TIF_SECCOMP, > seccomp.mode, and seccomp.filter. __secure_computing needs > seccomp.mode and seccomp.filter to be in sync, and it wants (but > doesn't really need) TIF_SECCOMP to be in sync as well. > > My suggestion is to rearrange it a bit. Move mode into seccomp.filter > (so that filter == NULL implies no seccomp) and don't check > TIF_SECCOMP in secure_computing. Then turning on seccomp is entirely > atomic except for the fact that the seccomp hooks won't be called if > filter != NULL but !TIF_SECCOMP. This removes all ordering > requirements. Ah, got it, thanks. Perhaps I missed somehing, but to me this looks like unnecessary complication at first glance. We alredy have TIF_SECCOMP, we need it anyway, and we should only care about the case when this bit is actually set, so that we can race with the 1st call of __secure_computing(). Otherwise we are fine: we can miss the new filter anyway, ->mode can't be changed it is already nonzero. > Alternatively, __secure_computing could still BUG_ON(!seccomp.filter). > In that case, filter needs to be set before TIF_SECCOMP is set, but > that's straightforward. Yep. And this is how seccomp_assign_mode() already works? It is called after we change ->filter chain, it changes ->mode before set(TIF_SECCOMP) just it lacks a barrier. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
