Re: [PATCH v3 1/2] fanotify: report thread pidfds for FAN_REPORT_TID

[Amir Goldstein]

On Sat, May 30, 2026 at 3:37 AM AnonymeMeow <anonymemeow@xxxxxxxxx> wrote:
>
> The FAN_REPORT_PIDFD and FAN_REPORT_TID flags used to be mutually
> exclusive because by the time the pidfd support was introduced to
> fanotify, pidfds could only be created for thread group leaders. Now
> that the pidfd API supports thread-specific pidfds via PIDFD_THREAD,
> this restriction can be lifted.
>
> Signed-off-by: AnonymeMeow <anonymemeow@xxxxxxxxx>

Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>

Thanks,
Amir.

> ---
>  fs/notify/fanotify/fanotify_user.c | 27 ++++++++-------------------
>  1 file changed, 8 insertions(+), 19 deletions(-)
>
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index ae904451dfc0..ebdd48942029 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -19,6 +19,7 @@
>  #include <linux/memcontrol.h>
>  #include <linux/statfs.h>
>  #include <linux/exportfs.h>
> +#include <linux/pidfd.h>
>
>  #include <asm/ioctls.h>
>
> @@ -903,25 +904,21 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>                 metadata.fd = fd >= 0 ? fd : FAN_NOFD;
>
>         if (pidfd_mode) {
> -               /*
> -                * Complain if the FAN_REPORT_PIDFD and FAN_REPORT_TID mutual
> -                * exclusion is ever lifted. At the time of incoporating pidfd
> -                * support within fanotify, the pidfd API only supported the
> -                * creation of pidfds for thread-group leaders.
> -                */
> -               WARN_ON_ONCE(FAN_GROUP_FLAG(group, FAN_REPORT_TID));
> +               unsigned int tid_mode = FAN_GROUP_FLAG(group, FAN_REPORT_TID);
> +               enum pid_type pidtype = tid_mode ? PIDTYPE_PID : PIDTYPE_TGID;
> +               unsigned int pidfd_flags = tid_mode ? PIDFD_THREAD : 0;
>
>                 /*
> -                * The PIDTYPE_TGID check for an event->pid is performed
> +                * The pid_has_task() check for an event->pid is performed
>                  * preemptively in an attempt to catch out cases where the event
> -                * listener reads events after the event generating process has
> +                * listener reads events after the event generating task has
>                  * already terminated.  Depending on flag FAN_REPORT_FD_ERROR,
>                  * report either -ESRCH or FAN_NOPIDFD to the event listener in
>                  * those cases with all other pidfd creation errors reported as
>                  * the error code itself or as FAN_EPIDFD.
>                  */
> -               if (metadata.pid && pid_has_task(event->pid, PIDTYPE_TGID))
> -                       pidfd = pidfd_prepare(event->pid, 0, &pidfd_file);
> +               if (metadata.pid && pid_has_task(event->pid, pidtype))
> +                       pidfd = pidfd_prepare(event->pid, pidfd_flags, &pidfd_file);
>
>                 if (!FAN_GROUP_FLAG(group, FAN_REPORT_FD_ERROR) && pidfd < 0)
>                         pidfd = pidfd == -ESRCH ? FAN_NOPIDFD : FAN_EPIDFD;
> @@ -1628,14 +1625,6 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
>  #endif
>                 return -EINVAL;
>
> -       /*
> -        * A pidfd can only be returned for a thread-group leader; thus
> -        * FAN_REPORT_PIDFD and FAN_REPORT_TID need to remain mutually
> -        * exclusive.
> -        */
> -       if ((flags & FAN_REPORT_PIDFD) && (flags & FAN_REPORT_TID))
> -               return -EINVAL;
> -
>         /* Don't allow mixing mnt events with inode events for now */
>         if (flags & FAN_REPORT_MNT) {
>                 if (class != FAN_CLASS_NOTIF)
> --
> 2.54.0
>