On Mon, Jan 27, 2014 at 2:43 PM, Linus Torvalds <torva...@linux-foundation.org> wrote: > On Mon, Jan 27, 2014 at 2:32 PM, Al Viro <v...@zeniv.linux.org.uk> wrote: >> >> do_signal() is also a place where arbitrary changes to regs might've >> been done by tracer, so regs->cs might need to be checked in the same >> place where we validate regs->rip ;-/ > > Fair enough. But it would still be really easy, and make the common > case signal delivery a bit faster. > > Now, sadly, most signal delivery is then followed by sigreturn (the > exceptions being dying or doing a longjmp), so we'd still get the > iretq then. But it would cut the iretq's related to signals in half. > > We *could* try to do sigreturn with sysret and a small trampoline too, > of course. But I'm not sure how far I'd want to take it.
I once spent a while thinking about how to do this. The best I could come up with was to use something like 'ret 128' for the trampoline. (The issue here is that there's no good place to shove a global variable with the missing register values, fs and gs aren't really available for these games, and the red zone is in the way.) I think that sysret for sigreturn is probably not very interesting. On the other hand, sysret for #PF might be a huge win, despite being even scarier. (Or someone could politely ask Intel for a couple of non-serializing msrs that set the values of rcx and whatever other registers get clobbered by sysret.) --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
