On May 7, 2016 7:38 AM, "Stas Sergeev" <[email protected]> wrote: > > 03.05.2016 20:31, Andy Lutomirski пишет: > >> If a signal stack is set up with SS_AUTODISARM, then the kernel >> inherently avoids incorrectly resetting the signal stack if signals >> recurse: the signal stack will be reset on the first signal >> delivery. This means that we don't need check the stack pointer >> when delivering signals if SS_AUTODISARM is set. >> >> This will make segmented x86 programs more robust: currently there's >> a hole that could be triggered if ESP/RSP appears to point to the >> signal stack but actually doesn't due to a nonzero SS base. >> >> Signed-off-by: Stas Sergeev <[email protected]> >> Cc: Al Viro <[email protected]> >> Cc: Aleksa Sarai <[email protected]> >> Cc: Amanieu d'Antras <[email protected]> >> Cc: Andrea Arcangeli <[email protected]> >> Cc: Andrew Morton <[email protected]> >> Cc: Andy Lutomirski <[email protected]> >> Cc: Borislav Petkov <[email protected]> >> Cc: Brian Gerst <[email protected]> >> Cc: Denys Vlasenko <[email protected]> >> Cc: Eric W. Biederman <[email protected]> >> Cc: Frederic Weisbecker <[email protected]> >> Cc: H. Peter Anvin <[email protected]> >> Cc: Heinrich Schuchardt <[email protected]> >> Cc: Jason Low <[email protected]> >> Cc: Josh Triplett <[email protected]> >> Cc: Konstantin Khlebnikov <[email protected]> >> Cc: Linus Torvalds <[email protected]> >> Cc: Oleg Nesterov <[email protected]> >> Cc: Palmer Dabbelt <[email protected]> >> Cc: Paul Moore <[email protected]> >> Cc: Pavel Emelyanov <[email protected]> >> Cc: Peter Zijlstra <[email protected]> >> Cc: Richard Weinberger <[email protected]> >> Cc: Sasha Levin <[email protected]> >> Cc: Shuah Khan <[email protected]> >> Cc: Tejun Heo <[email protected]> >> Cc: Thomas Gleixner <[email protected]> >> Cc: Vladimir Davydov <[email protected]> >> Cc: [email protected] >> Cc: [email protected] >> Signed-off-by: Andy Lutomirski <[email protected]> >> --- >> include/linux/sched.h | 12 ++++++++++++ >> 1 file changed, 12 insertions(+) >> >> diff --git a/include/linux/sched.h b/include/linux/sched.h >> index 2950c5cd3005..8f03a93348b9 100644 >> --- a/include/linux/sched.h >> +++ b/include/linux/sched.h >> @@ -2576,6 +2576,18 @@ static inline int kill_cad_pid(int sig, int priv) >> */ >> static inline int on_sig_stack(unsigned long sp) >> { >> + /* >> + * If the signal stack is AUTODISARM then, by construction, we >> + * can't be on the signal stack unless user code deliberately set >> + * SS_AUTODISARM when we were already on the it. > > "on the it" -> "on it". > > Anyway, I am a bit puzzled with this patch. > You say "unless user code deliberately set > > SS_AUTODISARM when we were already on the it" > so what happens in case it actually does? >
Stack corruption. Don't do that. > Without your patch: if user sets up the same sas - no stack switch. > if user sets up different sas - stack switch on nested signal. > > With your patch: stack switch in any case, so if user > set up same sas - stack corruption by nested signal. > > Or am I missing the intention? The intention is to make everything completely explicit. With SS_AUTODISARM, the kernel knows directly whether you're on the signal stack, and there should be no need to look at sp. If you set SS_AUTODISARM and get a signal, the signal stack gets disarmed. If you take a nested signal, it's delivered normally. When you return all the way out, the signal stack is re-armed. For DOSEMU, this means that no 16-bit register state can possibly cause a signal to be delivered wrong, because the register state when a signal is raised won't affect delivery, which seems like a good thing to me. If this behavior would be problematic for you, can you explain why?
