Re: [PATCH v8 10/19] fanotify: introduce FAN_PRE_ACCESS permission event

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Nov 15, 2024 at 4:31 PM Josef Bacik <josef@toxicpanda.com> wrote:
>
> From: Amir Goldstein <amir73il@gmail.com>
>
> Similar to FAN_ACCESS_PERM permission event, but it is only allowed with
> class FAN_CLASS_PRE_CONTENT and only allowed on regular files and dirs.
>
> Unlike FAN_ACCESS_PERM, it is safe to write to the file being accessed
> in the context of the event handler.
>
> This pre-content event is meant to be used by hierarchical storage
> managers that want to fill the content of files on first read access.
>
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
>  fs/notify/fanotify/fanotify.c      |  3 ++-
>  fs/notify/fanotify/fanotify_user.c | 22 +++++++++++++++++++---
>  include/linux/fanotify.h           | 14 ++++++++++----
>  include/uapi/linux/fanotify.h      |  2 ++
>  4 files changed, 33 insertions(+), 8 deletions(-)
>
> diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
> index 2e6ba94ec405..da6c3c1c7edf 100644
> --- a/fs/notify/fanotify/fanotify.c
> +++ b/fs/notify/fanotify/fanotify.c
> @@ -916,8 +916,9 @@ static int fanotify_handle_event(struct fsnotify_group *group, u32 mask,
>         BUILD_BUG_ON(FAN_OPEN_EXEC_PERM != FS_OPEN_EXEC_PERM);
>         BUILD_BUG_ON(FAN_FS_ERROR != FS_ERROR);
>         BUILD_BUG_ON(FAN_RENAME != FS_RENAME);
> +       BUILD_BUG_ON(FAN_PRE_ACCESS != FS_PRE_ACCESS);
>
> -       BUILD_BUG_ON(HWEIGHT32(ALL_FANOTIFY_EVENT_BITS) != 21);
> +       BUILD_BUG_ON(HWEIGHT32(ALL_FANOTIFY_EVENT_BITS) != 22);
>
>         mask = fanotify_group_event_mask(group, iter_info, &match_mask,
>                                          mask, data, data_type, dir);
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 456cc3e92c88..5ea447e9e5a8 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -1640,11 +1640,23 @@ static int fanotify_events_supported(struct fsnotify_group *group,
>                                      unsigned int flags)
>  {
>         unsigned int mark_type = flags & FANOTIFY_MARK_TYPE_BITS;
> +       bool is_dir = d_is_dir(path->dentry);
>         /* Strict validation of events in non-dir inode mask with v5.17+ APIs */
>         bool strict_dir_events = FAN_GROUP_FLAG(group, FAN_REPORT_TARGET_FID) ||
>                                  (mask & FAN_RENAME) ||
>                                  (flags & FAN_MARK_IGNORE);
>
> +       /*
> +        * Filesystems need to opt-into pre-content evnets (a.k.a HSM)
> +        * and they are only supported on regular files and directories.
> +        */
> +       if (mask & FANOTIFY_PRE_CONTENT_EVENTS) {
> +               if (!(path->mnt->mnt_sb->s_iflags & SB_I_ALLOW_HSM))
> +                       return -EINVAL;

You missed my latest push of this change.
no worries, for final version want:

                return -EOPNOTSUPP;

> +               if (!is_dir && !d_is_reg(path->dentry))
> +                       return -EINVAL;
> +       }
> +
>         /*
>          * Some filesystems such as 'proc' acquire unusual locks when opening
>          * files. For them fanotify permission events have high chances of
> @@ -1677,7 +1689,7 @@ static int fanotify_events_supported(struct fsnotify_group *group,
>          * but because we always allowed it, error only when using new APIs.
>          */
>         if (strict_dir_events && mark_type == FAN_MARK_INODE &&
> -           !d_is_dir(path->dentry) && (mask & FANOTIFY_DIRONLY_EVENT_BITS))
> +           !is_dir && (mask & FANOTIFY_DIRONLY_EVENT_BITS))
>                 return -ENOTDIR;
>
>         return 0;
> @@ -1778,10 +1790,14 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
>                 return -EPERM;
>
>         /*
> -        * Permission events require minimum priority FAN_CLASS_CONTENT.
> +        * Permission events are not allowed for FAN_CLASS_NOTIF.
> +        * Pre-content permission events are not allowed for FAN_CLASS_CONTENT.
>          */
>         if (mask & FANOTIFY_PERM_EVENTS &&
> -           group->priority < FSNOTIFY_PRIO_CONTENT)
> +           group->priority == FSNOTIFY_PRIO_NORMAL)
> +               return -EINVAL;
> +       else if (mask & FANOTIFY_PRE_CONTENT_EVENTS &&
> +                group->priority == FSNOTIFY_PRIO_CONTENT)
>                 return -EINVAL;
>
>         if (mask & FAN_FS_ERROR &&
> diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
> index 89ff45bd6f01..c747af064d2c 100644
> --- a/include/linux/fanotify.h
> +++ b/include/linux/fanotify.h
> @@ -89,6 +89,16 @@
>  #define FANOTIFY_DIRENT_EVENTS (FAN_MOVE | FAN_CREATE | FAN_DELETE | \
>                                  FAN_RENAME)
>
> +/* Content events can be used to inspect file content */
> +#define FANOTIFY_CONTENT_PERM_EVENTS (FAN_OPEN_PERM | FAN_OPEN_EXEC_PERM | \
> +                                     FAN_ACCESS_PERM)
> +/* Pre-content events can be used to fill file content */
> +#define FANOTIFY_PRE_CONTENT_EVENTS  (FAN_PRE_ACCESS)
> +
> +/* Events that require a permission response from user */
> +#define FANOTIFY_PERM_EVENTS   (FANOTIFY_CONTENT_PERM_EVENTS | \
> +                                FANOTIFY_PRE_CONTENT_EVENTS)
> +
>  /* Events that can be reported with event->fd */
>  #define FANOTIFY_FD_EVENTS (FANOTIFY_PATH_EVENTS | FANOTIFY_PERM_EVENTS)
>
> @@ -104,10 +114,6 @@
>                                  FANOTIFY_INODE_EVENTS | \
>                                  FANOTIFY_ERROR_EVENTS)
>
> -/* Events that require a permission response from user */
> -#define FANOTIFY_PERM_EVENTS   (FAN_OPEN_PERM | FAN_ACCESS_PERM | \
> -                                FAN_OPEN_EXEC_PERM)
> -
>  /* Extra flags that may be reported with event or control handling of events */
>  #define FANOTIFY_EVENT_FLAGS   (FAN_EVENT_ON_CHILD | FAN_ONDIR)
>
> diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
> index 79072b6894f2..7596168c80eb 100644
> --- a/include/uapi/linux/fanotify.h
> +++ b/include/uapi/linux/fanotify.h
> @@ -27,6 +27,8 @@
>  #define FAN_OPEN_EXEC_PERM     0x00040000      /* File open/exec in perm check */
>  /* #define FAN_DIR_MODIFY      0x00080000 */   /* Deprecated (reserved) */
>
> +#define FAN_PRE_ACCESS         0x00100000      /* Pre-content access hook */
> +
>  #define FAN_EVENT_ON_CHILD     0x08000000      /* Interested in child events */
>
>  #define FAN_RENAME             0x10000000      /* File was renamed */
> --
> 2.43.0
>