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SubjectRe: [PATCH RFC 1/2] Add polling support to pidfd
On Fri, Apr 19, 2019 at 09:18:59PM +0200, Christian Brauner wrote:
> On Fri, Apr 19, 2019 at 03:02:47PM -0400, Joel Fernandes wrote:
> > On Thu, Apr 18, 2019 at 07:26:44PM +0200, Christian Brauner wrote:
> > > On April 18, 2019 7:23:38 PM GMT+02:00, Jann Horn <jannh@google.com> wrote:
> > > >On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov <oleg@redhat.com> wrote:
> > > >> On 04/16, Joel Fernandes wrote:
> > > >> > On Tue, Apr 16, 2019 at 02:04:31PM +0200, Oleg Nesterov wrote:
> > > >> > >
> > > >> > > Could you explain when it should return POLLIN? When the whole
> > > >process exits?
> > > >> >
> > > >> > It returns POLLIN when the task is dead or doesn't exist anymore,
> > > >or when it
> > > >> > is in a zombie state and there's no other thread in the thread
> > > >group.
> > > >>
> > > >> IOW, when the whole thread group exits, so it can't be used to
> > > >monitor sub-threads.
> > > >>
> > > >> just in case... speaking of this patch it doesn't modify
> > > >proc_tid_base_operations,
> > > >> so you can't poll("/proc/sub-thread-tid") anyway, but iiuc you are
> > > >going to use
> > > >> the anonymous file returned by CLONE_PIDFD ?
> > > >
> > > >I don't think procfs works that way. /proc/sub-thread-tid has
> > > >proc_tgid_base_operations despite not being a thread group leader.
> > > >(Yes, that's kinda weird.) AFAICS the WARN_ON_ONCE() in this code can
> > > >be hit trivially, and then the code will misbehave.
> > > >
> > > >@Joel: I think you'll have to either rewrite this to explicitly bail
> > > >out if you're dealing with a thread group leader, or make the code
> > > >work for threads, too.
> > >
> > > The latter case probably being preferred if this API is supposed to be
> > > useable for thread management in userspace.
> >
> > At the moment, we are not planning to use this for sub-thread management. I
> > am reworking this patch to only work on clone(2) pidfds which makes the above
>
> Indeed and agreed.
>
> > discussion about /proc a bit unnecessary I think. Per the latest CLONE_PIDFD
> > patches, CLONE_THREAD with pidfd is not supported.
>
> Yes. We have no one asking for it right now and we can easily add this
> later.
>
> Admittedly I haven't gotten around to reviewing the patches here yet
> completely. But one thing about using POLLIN. FreeBSD is using POLLHUP
> on process exit which I think is nice as well. How about returning
> POLLIN | POLLHUP on process exit?
> We already do things like this. For example, when you proxy between
> ttys. If the process that you're reading data from has exited and closed
> it's end you still can't usually simply exit because it might have still
> buffered data that you want to read. The way one can deal with this
> from userspace is that you can observe a (POLLHUP | POLLIN) event and
> you keep on reading until you only observe a POLLHUP without a POLLIN
> event at which point you know you have read
> all data.
> I like the semantics for pidfds as well as it would indicate:
> - POLLHUP -> process has exited
> - POLLIN -> information can be read

Actually I think a bit different about this, in my opinion the pidfd should
always be readable (we would store the exit status somewhere in the future
which would be readable, even after task_struct is dead). So I was thinking
we always return EPOLLIN. If process has not exited, then it blocks.

However, we also are returning EPOLLERR in previous patch if the task_struct
has been reaped (task == NULL). I could change that to EPOLLHUP.

So the update patch looks like below. Thoughts?

---8<-----------------------

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 6a803a0b75df..eb279b5f4115 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3069,8 +3069,45 @@ static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx)
tgid_base_stuff, ARRAY_SIZE(tgid_base_stuff));
}

+static unsigned int proc_tgid_base_poll(struct file *file, struct poll_table_struct *pts)
+{
+ int poll_flags = 0;
+ struct task_struct *task;
+ struct pid *pid;
+
+ task = get_proc_task(file->f_path.dentry->d_inode);
+
+ WARN_ON_ONCE(task && !thread_group_leader(task));
+
+ /*
+ * tasklist_lock must be held because to avoid racing with
+ * changes in exit_state and wake up. Basically to avoid:
+ *
+ * P0: read exit_state = 0
+ * P1: write exit_state = EXIT_DEAD
+ * P1: Do a wake up - wq is empty, so do nothing
+ * P0: Queue for polling - wait forever.
+ */
+ read_lock(&tasklist_lock);
+ if (!task)
+ poll_flags = POLLIN | POLLRDNORM | POLLHUP;
+ else if (task->exit_state && thread_group_empty(task))
+ poll_flags = POLLIN | POLLRDNORM;
+
+ if (!poll_flags) {
+ pid = proc_pid(file->f_path.dentry->d_inode);
+ poll_wait(file, &pid->wait_pidfd, pts);
+ }
+ read_unlock(&tasklist_lock);
+
+ if (task)
+ put_task_struct(task);
+ return poll_flags;
+}
+
static const struct file_operations proc_tgid_base_operations = {
.read = generic_read_dir,
+ .poll = proc_tgid_base_poll,
.iterate_shared = proc_tgid_base_readdir,
.llseek = generic_file_llseek,
};
diff --git a/include/linux/pid.h b/include/linux/pid.h
index b6f4ba16065a..2e0dcbc6d14e 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -3,6 +3,7 @@
#define _LINUX_PID_H

#include <linux/rculist.h>
+#include <linux/wait.h>

enum pid_type
{
@@ -60,6 +61,8 @@ struct pid
unsigned int level;
/* lists of tasks that use this pid */
struct hlist_head tasks[PIDTYPE_MAX];
+ /* wait queue for pidfd pollers */
+ wait_queue_head_t wait_pidfd;
struct rcu_head rcu;
struct upid numbers[1];
};
diff --git a/kernel/pid.c b/kernel/pid.c
index 20881598bdfa..5c90c239242f 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -214,6 +214,8 @@ struct pid *alloc_pid(struct pid_namespace *ns)
for (type = 0; type < PIDTYPE_MAX; ++type)
INIT_HLIST_HEAD(&pid->tasks[type]);

+ init_waitqueue_head(&pid->wait_pidfd);
+
upid = pid->numbers + ns->level;
spin_lock_irq(&pidmap_lock);
if (!(ns->pid_allocated & PIDNS_ADDING))
diff --git a/kernel/signal.c b/kernel/signal.c
index f98448cf2def..e3781703ef7e 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1800,6 +1800,17 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
return ret;
}

+static void do_wakeup_pidfd_pollers(struct task_struct *task)
+{
+ struct pid *pid;
+
+ lockdep_assert_held(&tasklist_lock);
+
+ pid = get_task_pid(task, PIDTYPE_PID);
+ wake_up_all(&pid->wait_pidfd);
+ put_pid(pid);
+}
+
/*
* Let a parent know about the death of a child.
* For a stopped/continued status change, use do_notify_parent_cldstop instead.
@@ -1823,6 +1834,9 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
BUG_ON(!tsk->ptrace &&
(tsk->group_leader != tsk || !thread_group_empty(tsk)));

+ /* Wake up all pidfd waiters */
+ do_wakeup_pidfd_pollers(tsk);
+
if (sig != SIGCHLD) {
/*
* This is only possible if parent == real_parent.
diff --git a/tools/testing/selftests/pidfd/Makefile b/tools/testing/selftests/pidfd/Makefile
index deaf8073bc06..4b31c14f273c 100644
--- a/tools/testing/selftests/pidfd/Makefile
+++ b/tools/testing/selftests/pidfd/Makefile
@@ -1,4 +1,4 @@
-CFLAGS += -g -I../../../../usr/include/
+CFLAGS += -g -I../../../../usr/include/ -lpthread

TEST_GEN_PROGS := pidfd_test

diff --git a/tools/testing/selftests/pidfd/pidfd_test.c b/tools/testing/selftests/pidfd/pidfd_test.c
index d59378a93782..57ae217339e9 100644
--- a/tools/testing/selftests/pidfd/pidfd_test.c
+++ b/tools/testing/selftests/pidfd/pidfd_test.c
@@ -4,18 +4,26 @@
#include <errno.h>
#include <fcntl.h>
#include <linux/types.h>
+#include <pthread.h>
#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <syscall.h>
+#include <sys/epoll.h>
+#include <sys/mman.h>
#include <sys/mount.h>
#include <sys/wait.h>
+#include <time.h>
#include <unistd.h>

#include "../kselftest.h"

+#define CHILD_THREAD_MIN_WAIT 3 /* seconds */
+#define MAX_EVENTS 5
+#define __NR_pidfd_send_signal 424
+
static inline int sys_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
unsigned int flags)
{
@@ -30,6 +38,22 @@ static void set_signal_received_on_sigusr1(int sig)
signal_received = 1;
}

+static int open_pidfd(const char *test_name, pid_t pid)
+{
+ char buf[256];
+ int pidfd;
+
+ snprintf(buf, sizeof(buf), "/proc/%d", pid);
+ pidfd = open(buf, O_DIRECTORY | O_CLOEXEC);
+
+ if (pidfd < 0)
+ ksft_exit_fail_msg(
+ "%s test: Failed to open process file descriptor\n",
+ test_name);
+
+ return pidfd;
+}
+
/*
* Straightforward test to see whether pidfd_send_signal() works is to send
* a signal to ourself.
@@ -87,7 +111,6 @@ static int wait_for_pid(pid_t pid)
static int test_pidfd_send_signal_exited_fail(void)
{
int pidfd, ret, saved_errno;
- char buf[256];
pid_t pid;
const char *test_name = "pidfd_send_signal signal exited process";

@@ -99,17 +122,10 @@ static int test_pidfd_send_signal_exited_fail(void)
if (pid == 0)
_exit(EXIT_SUCCESS);

- snprintf(buf, sizeof(buf), "/proc/%d", pid);
-
- pidfd = open(buf, O_DIRECTORY | O_CLOEXEC);
+ pidfd = open_pidfd(test_name, pid);

(void)wait_for_pid(pid);

- if (pidfd < 0)
- ksft_exit_fail_msg(
- "%s test: Failed to open process file descriptor\n",
- test_name);
-
ret = sys_pidfd_send_signal(pidfd, 0, NULL, 0);
saved_errno = errno;
close(pidfd);
@@ -368,10 +384,179 @@ static int test_pidfd_send_signal_syscall_support(void)
return 0;
}

+void *test_pidfd_poll_exec_thread(void *priv)
+{
+ char waittime[256];
+
+ ksft_print_msg("Child Thread: starting. pid %d tid %d ; and sleeping\n",
+ getpid(), syscall(SYS_gettid));
+ ksft_print_msg("Child Thread: doing exec of sleep\n");
+
+ sprintf(waittime, "%d", CHILD_THREAD_MIN_WAIT);
+ execl("/bin/sleep", "sleep", waittime, (char *)NULL);
+
+ ksft_print_msg("Child Thread: DONE. pid %d tid %d\n",
+ getpid(), syscall(SYS_gettid));
+ return NULL;
+}
+
+static int poll_pidfd(const char *test_name, int pidfd)
+{
+ int c;
+ int epoll_fd = epoll_create1(0);
+ struct epoll_event event, events[MAX_EVENTS];
+
+ if (epoll_fd == -1)
+ ksft_exit_fail_msg("%s test: Failed to create epoll file descriptor\n",
+ test_name);
+
+ event.events = EPOLLIN;
+ event.data.fd = pidfd;
+
+ if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, pidfd, &event)) {
+ ksft_print_msg("%s test: Failed to add epoll file descriptor: Skipping\n",
+ test_name);
+ _exit(PIDFD_SKIP);
+ }
+
+ c = epoll_wait(epoll_fd, events, MAX_EVENTS, 5000);
+ if (c != 1 || !(events[0].events & EPOLLIN))
+ ksft_exit_fail_msg("%s test: Unexpected epoll_wait result (c=%d, events=%x)\n",
+ test_name, c, events[0].events);
+
+ close(epoll_fd);
+ return events[0].events;
+
+}
+
+int test_pidfd_poll_exec(int use_waitpid)
+{
+ int pid, pidfd;
+ int status, ret;
+ pthread_t t1;
+ time_t prog_start = time(NULL);
+ const char *test_name = "pidfd_poll check for premature notification on child thread exec";
+
+ ksft_print_msg("Parent: pid: %d\n", getpid());
+ pid = fork();
+ if (pid == 0) {
+ ksft_print_msg("Child: starting. pid %d tid %d\n", getpid(),
+ syscall(SYS_gettid));
+ pthread_create(&t1, NULL, test_pidfd_poll_exec_thread, NULL);
+ /*
+ * Exec in the non-leader thread will destroy the leader immediately.
+ * If the wait in the parent returns too soon, the test fails.
+ */
+ while (1)
+ ;
+ }
+
+ ksft_print_msg("Parent: Waiting for Child (%d) to complete.\n", pid);
+
+ if (use_waitpid) {
+ ret = waitpid(pid, &status, 0);
+ if (ret == -1)
+ ksft_print_msg("Parent: error\n");
+
+ if (ret == pid)
+ ksft_print_msg("Parent: Child process waited for.\n");
+ } else {
+ pidfd = open_pidfd(test_name, pid);
+ poll_pidfd(test_name, pidfd);
+ }
+
+ time_t prog_time = time(NULL) - prog_start;
+
+ ksft_print_msg("Time waited for child: %lu\n", prog_time);
+
+ close(pidfd);
+
+ if (prog_time < CHILD_THREAD_MIN_WAIT || prog_time > CHILD_THREAD_MIN_WAIT + 2)
+ ksft_exit_fail_msg("%s test: Failed\n", test_name);
+ else
+ ksft_test_result_pass("%s test: Passed\n", test_name);
+}
+
+void *test_pidfd_poll_leader_exit_thread(void *priv)
+{
+ char waittime[256];
+
+ ksft_print_msg("Child Thread: starting. pid %d tid %d ; and sleeping\n",
+ getpid(), syscall(SYS_gettid));
+ sleep(CHILD_THREAD_MIN_WAIT);
+ ksft_print_msg("Child Thread: DONE. pid %d tid %d\n", getpid(), syscall(SYS_gettid));
+ return NULL;
+}
+
+static time_t *child_exit_secs;
+int test_pidfd_poll_leader_exit(int use_waitpid)
+{
+ int pid, pidfd;
+ int status, ret;
+ pthread_t t1, t2;
+ time_t prog_start = time(NULL);
+ const char *test_name = "pidfd_poll check for premature notification on non-empty"
+ "group leader exit";
+
+ child_exit_secs = mmap(NULL, sizeof *child_exit_secs, PROT_READ | PROT_WRITE,
+ MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+
+ ksft_print_msg("Parent: pid: %d\n", getpid());
+ pid = fork();
+ if (pid == 0) {
+ ksft_print_msg("Child: starting. pid %d tid %d\n", getpid(), syscall(SYS_gettid));
+ pthread_create(&t1, NULL, test_pidfd_poll_leader_exit_thread, NULL);
+ pthread_create(&t2, NULL, test_pidfd_poll_leader_exit_thread, NULL);
+
+ /*
+ * glibc exit calls exit_group syscall, so explicity call exit only
+ * so that only the group leader exits, leaving the threads alone.
+ */
+ *child_exit_secs = time(NULL);
+ syscall(SYS_exit, 0);
+ }
+
+ ksft_print_msg("Parent: Waiting for Child (%d) to complete.\n", pid);
+
+ if (use_waitpid) {
+ ret = waitpid(pid, &status, 0);
+ if (ret == -1)
+ ksft_print_msg("Parent: error\n");
+ } else {
+ /*
+ * This sleep tests for the case where if the child exits, and is in
+ * EXIT_ZOMBIE, but the thread group leader is non-empty, then the poll
+ * doesn't prematurely return even though there are active threads
+ */
+ sleep(1);
+ pidfd = open_pidfd(test_name, pid);
+ poll_pidfd(test_name, pidfd);
+ }
+
+ if (ret == pid)
+ ksft_print_msg("Parent: Child process waited for.\n");
+
+ time_t since_child_exit = time(NULL) - *child_exit_secs;
+
+ ksft_print_msg("Time since child exit: %lu\n", since_child_exit);
+
+ close(pidfd);
+
+ if (since_child_exit < CHILD_THREAD_MIN_WAIT ||
+ since_child_exit > CHILD_THREAD_MIN_WAIT + 2)
+ ksft_exit_fail_msg("%s test: Failed\n", test_name);
+ else
+ ksft_test_result_pass("%s test: Passed\n", test_name);
+}
+
int main(int argc, char **argv)
{
ksft_print_header();

+ test_pidfd_poll_exec(0);
+ test_pidfd_poll_exec(1);
+ test_pidfd_poll_leader_exit(0);
+ test_pidfd_poll_leader_exit(1);
test_pidfd_send_signal_syscall_support();
test_pidfd_send_signal_simple_success();
test_pidfd_send_signal_exited_fail();
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