[LTP] [PATCH] fanotify: report thread pidfds for FAN_REPORT_TID

[Amir Goldstein]

On Sun, May 24, 2026 at 12:25 PM AnonymeMeow <anonymemeow@gmail.com> 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.
>
> This patch allows listeners using FAN_REPORT_PIDFD | FAN_REPORT_TID
> to receive the pidfd referring to the thread that triggered the
> event.
>
> Signed-off-by: AnonymeMeow <anonymemeow@gmail.com>

Regarding the legal standpoint, as far as I can tell, commit d4563201f33a0
("Documentation: simplify and clarify DCO contribution example language")
explicitly allowed pseudonyms, so although unorthodox, this seems to be legit.

In any case, the contributing itself seems legit to me, so

Reviewed-by: Amir Goldstein <amir73il@gmail.com>

But before merging this, AnonymeMeow, since it is your first contribution
to the Linux kernel (right?) as well as a "I am not a robot" challenge
I would like to first see that the tests have been dealt with.

The LTP test fanotify20 explicitly checks that this flag combination is not
allowed. Need to discuss an acceptable solution with LTP developers -
remove this check depending on kernel version or whatever they choose.

The LTP test fanotify21 tests the FAN_REPORT_PIDFD functionality.
Please add two new variants for REPORT_TID:
        .test_variants = 4,

#define TST_VARIANT_FD_ERROR (tst_variant & 1)
#define TST_VARIANT_PIDFD_THREAD (tst_variant & 2)

The existing checks if (tst_variant) become if (TST_VARIANT_FD_ERROR)
so bit 0 represents the FAN_REPORT_FD_ERROR variation.

in do_setup() need to use if (TST_VARIANT_PIDFD_THREAD)
to add FAN_REPORT_TID and to use PIDFD_THREAD for pidfd.

Follow the footsteps of fd_error_unsupported to skip the
TST_VARIANT_PIDFD_THREAD variants on kernels where the flag
combination is not supported.

Good luck,
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
>