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Date: Sat, 18 Jul 2020 09:56:48 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Hydro Flask <hydroflask@...mail.com>
Cc: musl@...ts.openwall.com, Carlos O'Donell <carlos@...hat.com>,
Florian Weimer <fweimer@...hat.com>
Subject: Re: Idea: futex() system call entry point
* Hydro Flask <hydroflask@...mail.com> [2020-07-17 18:21:27 -0700]:
> On 2020-07-17 16:30, Rich Felker wrote:
> > On Fri, Jul 17, 2020 at 02:37:27PM -0700, Hydro Flask wrote:
> > > Maybe a less complex suggestion is to expose a syscall_cp()
> > > function, so you can get cancellation point functionality for any
> > > system call. I actually quite like that option. How does that sound?
> >
> > In the specific case of futex waits, it's not clear to me that there's
> > any side effect for which you need to know in the cancellation handler
> > whether it occurred, so why can't you just enable async cancel around
> > syscall() and disable it again after?
>
> Oh I hadn't thought of that. That's actually a pretty good short-term
> solution. So you're saying:
>
> int fuxex_wait(int *uaddr, int val, const struct timespec *timeout)
> {
> int old, ret;
>
> /* pthread_setcanceltype() automatically calls
> pthread_testcancel() if async is enabled */
> ret = pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, &old);
> if (ret) {
> errno = ret;
> ret = -1;
> }
> ret = syscall(SYS_futex, uaddr, FUTEX_WAIT, val, timeout);
note that passing libc types (timespec)
to raw syscall is broken (at least on
32bit targets, but in general a libc
type may not match kernel types if this
has to be portable to other libcs).
you have to locally create a type that
is known to match the kernel abi on the
targets you care about and translate
between the libc type and that.
> old = pthread_setcanceltype(old, &old);
> if (old) abort();
> return ret;
> }
>
> I think you're right that even if the futex call succeeds, it's fine to
> cancel since it does not mutate any meaningful observable state. I think
> that should satisfy all my requirements when doing this on musl.
> pthread_testcancel/pthread_setcanceltype should be AS-safe in musl if
> cancellation is disabled or the interrupted code is AC-safe.
>
> That should likely also work in other libcs assuming a sane implementation
> of all the required functions involved. Thank you
>
> Hydro
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