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Re: extending wait4(2) or waitid(2) linux syscall
- From: Florian Weimer <fweimer at redhat dot com>
- To: Arnd Bergmann <arnd at arndb dot de>
- Cc: "Dmitry V. Levin" <ldv at altlinux dot org>, Albert ARIBAUD <albert dot aribaud at 3adev dot fr>, "H. Peter Anvin" <hpa at zytor dot com>, GNU C Library <libc-alpha at sourceware dot org>, Linux API <linux-api at vger dot kernel dot org>
- Date: Wed, 28 Nov 2018 10:31:23 +0100
- Subject: Re: extending wait4(2) or waitid(2) linux syscall
- References: <20170420152051.568f2050.albert.aribaud@3adev.fr> <20181115140441.GA2171@altlinux.org> <CAK8P3a0Gsqa8WTbALOUchRyEA7E2f3P1f=XQ8nD2xQaemfPpcQ@mail.gmail.com> <20181115153008.GC2171@altlinux.org> <CAK8P3a38zcMikOiaHXrKbc3URL6-6kETk=1F-Bx7LywPyRh=yg@mail.gmail.com> <87d0qrooj3.fsf@oldenburg.str.redhat.com> <CAK8P3a3pVS3_70PQ+SeiSZtqXFcb5HON0QGRypWfu=cfLPgN=w@mail.gmail.com>
* Arnd Bergmann:
> On Mon, Nov 26, 2018 at 4:18 PM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * Arnd Bergmann:
>>
>> > On Thu, Nov 15, 2018 at 7:30 AM Dmitry V. Levin <ldv@altlinux.org> wrote:
>> >> On Thu, Nov 15, 2018 at 06:39:03AM -0800, Arnd Bergmann wrote:
>> >> > On Thu, Nov 15, 2018 at 6:05 AM Dmitry V. Levin wrote:
>> >> > > On Thu, Apr 20, 2017 at 03:20:51PM +0200, Albert ARIBAUD wrote:
>> >>
>> >> 1. strace needs a race-free invocation of wait4(2) or waitid(2)
>> >> with a different signal mask, this cannot be achieved without
>> >> an extended version of syscall, similar to pselect6(2) extension
>> >> over select(2) and ppoll(2) extension over poll(2).
>> >>
>> >> Signal mask specification in linux requires two parameters:
>> >> "const sigset_t *sigmask" and "size_t sigsetsize".
>> >> Creating pwait6(2) as an extension of wait4(2) with two arguments
>> >> is straightforward.
>> >> Creating pwaitid(2) as an extension of waitid(2) that already has 5
>> >> arguments would require an indirection similar to pselect6(2).
>> >
>> > Getting back to this point: you could also do the same thing with
>> > the CLONE_FD approach from Josh Triplett[1] or Casey Dahlin's
>> > older waitfd() syscall, correct?
>>
>> A descriptor-based solution would not be useful to glibc because
>> applications assume that glibc does not (persistently) open any file
>> descriptors behind t heir back.
>
> Right, makes sense. What about a temporary file descriptor as discussed
> in the recent procfd() mail thread then? Would that work?
>
> /* for illustration, needs error handling and more features */
> int pwait(pid_t id, siginfo_t *infop)
> {
> char waitfd_file[MAX_PROCFD_LEN];
> struct pollfd pfd[1] = { {.events = POLLIN }};
>
> snprintf(waitfd_file, MAX_PROCFD_LEN, "/proc/%d/wait", pid);
> pfd.fd = open(waitfd_file, O_RDONLY);
> ppoll(&pfd, 1, NULL, sigmask);
> read(fd, infop, sizeof(*infop));
> close(fd);
>
> return 0;
> }
Together with an officiall supported way that allows us that the process
denoted by the PID is still the same that it was, it could work, I
think.
It would also nice to have the ability to launch processes without
making them visible to wait, and trigger SIGCHLD signals, but noticing
process exit seems like a tricky matter under these circumstances.
Thanks,
Florian