Re: [PATCH 2/2] fanotify: Add pidfd support to the fanotify API

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Apr 16, 2021 at 2:22 AM Matthew Bobrowski <repnop@google.com> wrote:
>
> Introduce a new flag FAN_REPORT_PIDFD for fanotify_init(2) which
> allows userspace applications to control whether a pidfd is to be
> returned instead of a pid for `struct fanotify_event_metadata.pid`.
>
> FAN_REPORT_PIDFD is mutually exclusive with FAN_REPORT_TID as the
> pidfd API is currently restricted to only support pidfd generation for
> thread-group leaders. Attempting to set them both when calling
> fanotify_init(2) will result in -EINVAL being returned to the
> caller. As the pidfd API evolves and support is added for tids, this
> is something that could be relaxed in the future.
>
> If pidfd creation fails, the pid in struct fanotify_event_metadata is
> set to FAN_NOPIDFD(-1).

Hi Matthew,

All in all looks good, just a few small nits.

> Falling back and providing a pid instead of a
> pidfd on pidfd creation failures was considered, although this could
> possibly lead to confusion and unpredictability within userspace
> applications as distinguishing between whether an actual pidfd or pid
> was returned could be difficult, so it's best to be explicit.

I don't think this should have been even "considered" so I see little
value in this paragraph in commit message.

>
> Signed-off-by: Matthew Bobrowski <repnop@google.com>
> ---
>  fs/notify/fanotify/fanotify_user.c | 33 +++++++++++++++++++++++++++---
>  include/linux/fanotify.h           |  2 +-
>  include/uapi/linux/fanotify.h      |  2 ++
>  3 files changed, 33 insertions(+), 4 deletions(-)
>
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 9e0c1afac8bd..fd8ae88796a8 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -329,7 +329,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>         struct fanotify_info *info = fanotify_event_info(event);
>         unsigned int fid_mode = FAN_GROUP_FLAG(group, FANOTIFY_FID_BITS);
>         struct file *f = NULL;
> -       int ret, fd = FAN_NOFD;
> +       int ret, pidfd, fd = FAN_NOFD;
>         int info_type = 0;
>
>         pr_debug("%s: group=%p event=%p\n", __func__, group, event);
> @@ -340,7 +340,25 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>         metadata.vers = FANOTIFY_METADATA_VERSION;
>         metadata.reserved = 0;
>         metadata.mask = event->mask & FANOTIFY_OUTGOING_EVENTS;
> -       metadata.pid = pid_vnr(event->pid);
> +
> +       if (FAN_GROUP_FLAG(group, FAN_REPORT_PIDFD) &&
> +               pid_has_task(event->pid, PIDTYPE_TGID)) {
> +               /*
> +                * Given FAN_REPORT_PIDFD is to be mutually exclusive with
> +                * FAN_REPORT_TID, panic here if the mutual exclusion is ever
> +                * blindly lifted without pidfds for threads actually being
> +                * supported.
> +                */
> +               WARN_ON(FAN_GROUP_FLAG(group, FAN_REPORT_TID));

Better WARN_ON_ONCE() the outcome of this error is not terrible.
Also in the comment above I would not refer to this warning as "panic".

> +
> +               pidfd = pidfd_create(event->pid, 0);
> +               if (unlikely(pidfd < 0))
> +                       metadata.pid = FAN_NOPIDFD;
> +               else
> +                       metadata.pid = pidfd;
> +       } else {
> +               metadata.pid = pid_vnr(event->pid);
> +       }

You should rebase your work on:
git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git fsnotify
and resolve conflicts with "unprivileged listener" code.

Need to make sure that pidfd is not reported to an unprivileged
listener even if group was initialized by a privileged process.
This is a conscious conservative choice that we made for reporting
pid info to unprivileged listener that can be revisited in the future.

>
>         if (path && path->mnt && path->dentry) {
>                 fd = create_fd(group, path, &f);
> @@ -941,6 +959,15 @@ 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_TID and FAN_REPORT_PIDFD need to be mutually
> +        * exclusive. Once the pidfd API supports the creation of pidfds on
> +        * individual threads, then we can look at removing this conditional.
> +        */
> +       if ((flags & FAN_REPORT_PIDFD) && (flags & FAN_REPORT_TID))
> +               return -EINVAL;
> +
>         if (event_f_flags & ~FANOTIFY_INIT_ALL_EVENT_F_BITS)
>                 return -EINVAL;
>
> @@ -1312,7 +1339,7 @@ SYSCALL32_DEFINE6(fanotify_mark,
>   */
>  static int __init fanotify_user_setup(void)
>  {
> -       BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 10);
> +       BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 11);
>         BUILD_BUG_ON(HWEIGHT32(FANOTIFY_MARK_FLAGS) != 9);
>
>         fanotify_mark_cache = KMEM_CACHE(fsnotify_mark,
> diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
> index 3e9c56ee651f..894740a6f4e0 100644
> --- a/include/linux/fanotify.h
> +++ b/include/linux/fanotify.h
> @@ -21,7 +21,7 @@
>  #define FANOTIFY_FID_BITS      (FAN_REPORT_FID | FAN_REPORT_DFID_NAME)
>
>  #define FANOTIFY_INIT_FLAGS    (FANOTIFY_CLASS_BITS | FANOTIFY_FID_BITS | \
> -                                FAN_REPORT_TID | \
> +                                FAN_REPORT_TID | FAN_REPORT_PIDFD | \
>                                  FAN_CLOEXEC | FAN_NONBLOCK | \
>                                  FAN_UNLIMITED_QUEUE | FAN_UNLIMITED_MARKS)
>

FAN_REPORT_PIDFD should be added to FANOTIFY_ADMIN_INIT_FLAGS
from fsnotify branch.

Thanks,
Amir.