On Fri, Apr 16, 2021 at 2:22 AM Matthew Bobrowski <repnop@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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.
