linux-ext4 - Re: [PATCH v7 09/18] fanotify: introduce FAN_PRE

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Message-ID: <CAOQ4uxhkPJ=atVPeQ3PsOKps3w8qxJgpvMR1wwT=-onc4KLV5Q@mail.gmail.com>
Date: Fri, 15 Nov 2024 12:28:01 +0100
From: Amir Goldstein <amir73il@...il.com>
To: Josef Bacik <josef@...icpanda.com>
Cc: kernel-team@...com, linux-fsdevel@...r.kernel.org, jack@...e.cz, 
	brauner@...nel.org, torvalds@...ux-foundation.org, linux-xfs@...r.kernel.org, 
	linux-btrfs@...r.kernel.org, linux-mm@...ck.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v7 09/18] fanotify: introduce FAN_PRE_ACCESS permission event

On Tue, Nov 12, 2024 at 6:56 PM Josef Bacik <josef@...icpanda.com> wrote:
>
> From: Amir Goldstein <amir73il@...il.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@...il.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 9cc4a9ac1515..2ec0cc9c85cf 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -1633,11 +1633,23 @@ 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;

Should we make this return -EOPNOTSUPP?

This way the LTP test could report the accurate message
"FAN_PRE_ACCESS not supported in kernel" vs.
"FAN_PRE_ACCESS not supported on XXX filesystem"

Thanks,
Amir.