Re: [PATCH v2 2/2] tests: add pidfd poll tests

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jul 25, 2019 at 1:57 PM Yann Droneaud <ydroneaud@opteya.com> wrote:
>
> Hi,
>
> Le jeudi 25 juillet 2019 à 08:48 -0700, Suren Baghdasaryan a écrit :
> > On Thu, Jul 25, 2019 at 4:58 AM Yann Droneaud <ydroneaud@opteya.com> wrote:
> > > Le mercredi 24 juillet 2019 à 17:22 -0700, Suren Baghdasaryan a écrit :
> > > > This adds testing for polling on pidfd of a process being killed. Test runs
> > > > 10000 iterations by default to stress test pidfd polling functionality.
> > > > It accepts an optional command-line parameter to override the number or
> > > > iterations to run.
> > > > Specifically, it tests for:
> > > > - pidfd_open on a child process succeeds
> > > > - pidfd_send_signal on a child process succeeds
> > > > - polling on pidfd succeeds and returns exactly one event
> > > > - returned event is POLLIN
> > > > - event is received within 3 secs of the process being killed
> > > >
> > > > 10000 iterations was chosen because of the race condition being tested
> > > > which is not consistently reproducible but usually is revealed after less
> > > > than 2000 iterations.
> > > > Reveals race fixed by commit b191d6491be6 ("pidfd: fix a poll race when setting exit_state")
> > > >
> > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > > ---
> > > >  tools/testing/selftests/pidfd/.gitignore      |   1 +
> > > >  tools/testing/selftests/pidfd/Makefile        |   2 +-
> > > >  .../testing/selftests/pidfd/pidfd_poll_test.c | 112 ++++++++++++++++++
> > > >  3 files changed, 114 insertions(+), 1 deletion(-)
> > > >  create mode 100644 tools/testing/selftests/pidfd/pidfd_poll_test.c
> > > >
> > > > diff --git a/tools/testing/selftests/pidfd/.gitignore b/tools/testing/selftests/pidfd/.gitignore
> > > > index 16d84d117bc0..a67896347d34 100644
> > > > --- a/tools/testing/selftests/pidfd/.gitignore
> > > > +++ b/tools/testing/selftests/pidfd/.gitignore
> > > > @@ -1,2 +1,3 @@
> > > >  pidfd_open_test
> > > > +pidfd_poll_test
> > > >  pidfd_test
> > > > diff --git a/tools/testing/selftests/pidfd/Makefile b/tools/testing/selftests/pidfd/Makefile
> > > > index 720b2d884b3c..ed58b7108d18 100644
> > > > --- a/tools/testing/selftests/pidfd/Makefile
> > > > +++ b/tools/testing/selftests/pidfd/Makefile
> > > > @@ -1,7 +1,7 @@
> > > >  # SPDX-License-Identifier: GPL-2.0-only
> > > >  CFLAGS += -g -I../../../../usr/include/ -lpthread
> > > >
> > > > -TEST_GEN_PROGS := pidfd_test pidfd_open_test
> > > > +TEST_GEN_PROGS := pidfd_test pidfd_open_test pidfd_poll_test
> > > >
> > > >  include ../lib.mk
> > > >
> > > > diff --git a/tools/testing/selftests/pidfd/pidfd_poll_test.c b/tools/testing/selftests/pidfd/pidfd_poll_test.c
> > > > new file mode 100644
> > > > index 000000000000..d45c612a0fe5
> > > > --- /dev/null
> > > > +++ b/tools/testing/selftests/pidfd/pidfd_poll_test.c
> > > > @@ -0,0 +1,112 @@
> > > > +// SPDX-License-Identifier: GPL-2.0
> > > > +
> > > > +#define _GNU_SOURCE
> > > > +#include <errno.h>
> > > > +#include <linux/types.h>
> > > > +#include <linux/wait.h>
> > > > +#include <poll.h>
> > > > +#include <signal.h>
> > > > +#include <stdbool.h>
> > > > +#include <stdio.h>
> > > > +#include <stdlib.h>
> > > > +#include <string.h>
> > > > +#include <syscall.h>
> > > > +#include <sys/wait.h>
> > > > +#include <unistd.h>
> > > > +
> > > > +#include "pidfd.h"
> > > > +#include "../kselftest.h"
> > > > +
> > > > +static bool timeout;
> > > > +
> > > > +static void handle_alarm(int sig)
> > > > +{
> > > > +     timeout = true;
> > > > +}
> > > > +
> > > > +int main(int argc, char **argv)
> > > > +{
> > > > +     struct pollfd fds;
> > > > +     int iter, nevents;
> > > > +     int nr_iterations = 10000;
> > > > +
> > > > +     fds.events = POLLIN;
> > > > +
> > > > +     if (argc > 2)
> > > > +             ksft_exit_fail_msg("Unexpected command line argument\n");
> > > > +
> > > > +     if (argc == 2) {
> > > > +             nr_iterations = atoi(argv[1]);
> > > > +             if (nr_iterations <= 0)
> > > > +                     ksft_exit_fail_msg("invalid input parameter %s\n",
> > > > +                                     argv[1]);
> > > > +     }
> > > > +
> > > > +     ksft_print_msg("running pidfd poll test for %d iterations\n",
> > > > +             nr_iterations);
> > > > +
> > > > +     for (iter = 0; iter < nr_iterations; iter++) {
> > > > +             int pidfd;
> > > > +             int child_pid = fork();
> > > > +
> > > > +             if (child_pid < 0) {
> > > > +                     if (errno == EAGAIN) {
> > > > +                             iter--;
> > > > +                             continue;
> > > > +                     }
> > > > +                     ksft_exit_fail_msg(
> > > > +                             "%s - failed to fork a child process\n",
> > > > +                             strerror(errno));
> > > > +             }
> > > > +
> > > > +             if (!child_pid) {
> > > > +                     /* Child process just sleeps for a min and exits */
> > > > +                     sleep(60);
> > > > +                     exit(EXIT_SUCCESS);
> > > > +             }
> > > > +
> > > > +             /* Parent kills the child and waits for its death */
> > > > +             pidfd = sys_pidfd_open(child_pid, 0);
> > > > +             if (pidfd < 0)
> > > > +                     ksft_exit_fail_msg("%s - pidfd_open failed\n",
> > > > +                                     strerror(errno));
> > > > +
> > > > +             /* Setup 3 sec alarm - plenty of time */
> > > > +             if (signal(SIGALRM, handle_alarm) == SIG_ERR)
> > > > +                     ksft_exit_fail_msg("%s - signal failed\n",
> > > > +                                     strerror(errno));
> > > > +             alarm(3);
> > > > +
> > > > +             /* Send SIGKILL to the child */
> > > > +             if (sys_pidfd_send_signal(pidfd, SIGKILL, NULL, 0))
> > > > +                     ksft_exit_fail_msg("%s - pidfd_send_signal failed\n",
> > > > +                                     strerror(errno));
> > > > +
> > > > +             /* Wait for the death notification */
> > > > +             fds.fd = pidfd;
> > > > +             nevents = poll(&fds, 1, -1);
> > > > +
> > > > +             /* Check for error conditions */
> > > > +             if (nevents < 0)
> > > > +                     ksft_exit_fail_msg("%s - poll failed\n",
> > > > +                                     strerror(errno));
> > > > +
> > > > +             if (nevents != 1)
> > > > +                     ksft_exit_fail_msg("unexpected poll result: %d\n",
> > > > +                                     nevents);
> > > > +
> > > > +             if (!(fds.revents & POLLIN))
> > > > +                     ksft_exit_fail_msg(
> > > > +                             "unexpected event type received: 0x%x\n",
> > > > +                             fds.revents);
> > > > +
> > > > +             if (timeout)
> > > > +                     ksft_exit_fail_msg(
> > > > +                             "death notification wait timeout\n");
> > > > +
> > > > +             close(pidfd);
> > >
> > > There's no call to wait(), or alike function. Is it required for the
> > > test to left zombies ?
> >
> > The test checks that death notification gets sent from kernel, which
> > by design should happen for any process that has non-zero
> > task->exit_state and that includes zombies (see
> > https://elixir.bootlin.com/linux/v5.3-rc1/source/kernel/fork.c#L1731
> > for that condition). So IIUC this code, the poll() should succeed no
> > matter if the task is zombie or dead. Or do I misunderstand your
> > question?
>
> OK, so I think it would be better to call wait() (in the future
> pidfd_wait()) to reap processes as they die.
>
> I'm not afraid by one zombie, but herding an army of 10000 zombie
> processes is another matter :)

Ah, I see what you mean now. I can respin this patch with a
wait(child_pid, NULL, 0) after close(pidfd) call. Would that address
your concern?

> Regards.
>
> --
> Yann Droneaud
> OPTEYA
>
>
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