Re: pidfd and O_NONBLOCK

[Christian Brauner <christian.brauner@ubuntu.com>]

On Tue, Aug 11, 2020 at 11:12:36AM -0700, Josh Triplett wrote:
> As far as I can tell, O_NONBLOCK has no effect on a pidfd. When calling
> waitid on a pidfd for a running process, it always blocks unless you
> provide WNOHANG.
> 
> I don't think anything depends on that behavior. Would it be possible to
> make O_NONBLOCK on a pidfd cause waitid on a running process to return
> EWOULDBLOCK?
> 
> This would make it easier to use pidfd in some non-blocking event loops.

Hey Josh,

Just to see I did a _horrible_ draft (cf. [1]) and it seems doable to me
and if you can provide a good rationale and a use-case then I think that
would be ok.

[1]:
diff --git a/kernel/exit.c b/kernel/exit.c
index 727150f28103..b43a0e126cee 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -934,6 +934,7 @@ struct wait_opts {

        wait_queue_entry_t              child_wait;
        int                     notask_error;
+       bool                    wo_pidfd_nonblock;
 };

 static int eligible_pid(struct wait_opts *wo, struct task_struct *p)
@@ -1462,6 +1463,11 @@ static long do_wait(struct wait_opts *wo)
 notask:
        retval = wo->notask_error;
        if (!retval && !(wo->wo_flags & WNOHANG)) {
+               if (wo->wo_pidfd_nonblock) {
+                       retval = -EWOULDBLOCK;
+                       goto end;
+               }
+
                retval = -ERESTARTSYS;
                if (!signal_pending(current)) {
                        schedule();
@@ -1495,9 +1501,10 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
                          int options, struct rusage *ru)
 {
        struct wait_opts wo;
+       struct fd f;
        struct pid *pid = NULL;
        enum pid_type type;
-       long ret;
+       long ret = 0;

        if (options & ~(WNOHANG|WNOWAIT|WEXITED|WSTOPPED|WCONTINUED|
                        __WNOTHREAD|__WCLONE|__WALL))
@@ -1505,6 +1512,7 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
        if (!(options & (WEXITED|WSTOPPED|WCONTINUED)))
                return -EINVAL;

+       wo.wo_pidfd_nonblock = false;
        switch (which) {
        case P_ALL:
                type = PIDTYPE_MAX;
@@ -1531,9 +1539,22 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
                if (upid < 0)
                        return -EINVAL;

-               pid = pidfd_get_pid(upid);
+               f = fdget(upid);
+               if (!f.file)
+                       return ERR_PTR(-EBADF);
+
+               pid = pidfd_pid(f.file);
+
+               if (f.file->f_flags & O_NONBLOCK)
+                       wo.wo_pidfd_nonblock = true;
+
                if (IS_ERR(pid))
-                       return PTR_ERR(pid);
+                       ret = PTR_ERR(pid);
+               else
+                       get_pid(pid);
+               fdput(f);
+               if (ret)
+                       return ret;
                break;
        default:
                return -EINVAL;