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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Mon, 3 Jul 2023 18:02:07 +0300
Message-ID: <CAOQ4uxi=M08f3zRFf8xzHo=0VjP+O7Qkih_GNQs50jv4S8VCaA@mail.gmail.com>
Subject: Re: [PATCH v5 1/3] splice: always fsnotify_access(in),
 fsnotify_modify(out) on success
To:     =?UTF-8?Q?Ahelenia_Ziemia=C5=84ska?= 
        <nabijaczleweli@nabijaczleweli.xyz>
Cc:     Christian Brauner <brauner@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jan Kara <jack@suse.cz>,
        Chung-Chiang Cheng <cccheng@synology.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
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On Mon, Jul 3, 2023 at 5:42=E2=80=AFPM Ahelenia Ziemia=C5=84ska
<nabijaczleweli@nabijaczleweli.xyz> wrote:
>
> The current behaviour caused an asymmetry where some write APIs
> (write, sendfile) would notify the written-to/read-from objects,
> but splice wouldn't.
>
> This affected userspace which uses inotify, most notably coreutils
> tail -f, to monitor pipes.
> If the pipe buffer had been filled by a splice-family function:
>   * tail wouldn't know and thus wouldn't service the pipe, and
>   * all writes to the pipe would block because it's full,
> thus service was denied.
> (For the particular case of tail -f this could be worked around
>  with ---disable-inotify.)
>
> Fixes: 983652c69199 ("splice: report related fsnotify events")
> Link: https://lore.kernel.org/linux-fsdevel/jbyihkyk5dtaohdwjyivambb2gffy=
js3dodpofafnkkunxq7bu@jngkdxx65pux/t/#u
> Link: https://bugs.debian.org/1039488
> Signed-off-by: Ahelenia Ziemia=C5=84ska <nabijaczleweli@nabijaczleweli.xy=
z>
> Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> Acked-by: Jan Kara <jack@suse.cz>
> ---
>  fs/splice.c | 31 ++++++++++++++-----------------
>  1 file changed, 14 insertions(+), 17 deletions(-)
>
> diff --git a/fs/splice.c b/fs/splice.c
> index 3e06611d19ae..6ae6da52eba9 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -1154,10 +1154,8 @@ long do_splice(struct file *in, loff_t *off_in, st=
ruct file *out,
>                 if ((in->f_flags | out->f_flags) & O_NONBLOCK)
>                         flags |=3D SPLICE_F_NONBLOCK;
>
> -               return splice_pipe_to_pipe(ipipe, opipe, len, flags);
> -       }
> -
> -       if (ipipe) {
> +               ret =3D splice_pipe_to_pipe(ipipe, opipe, len, flags);
> +       } else if (ipipe) {
>                 if (off_in)
>                         return -ESPIPE;
>                 if (off_out) {
> @@ -1182,18 +1180,11 @@ long do_splice(struct file *in, loff_t *off_in, s=
truct file *out,
>                 ret =3D do_splice_from(ipipe, out, &offset, len, flags);
>                 file_end_write(out);
>
> -               if (ret > 0)
> -                       fsnotify_modify(out);
> -
>                 if (!off_out)
>                         out->f_pos =3D offset;
>                 else
>                         *off_out =3D offset;
> -
> -               return ret;
> -       }
> -
> -       if (opipe) {
> +       } else if (opipe) {
>                 if (off_out)
>                         return -ESPIPE;
>                 if (off_in) {
> @@ -1209,18 +1200,24 @@ long do_splice(struct file *in, loff_t *off_in, s=
truct file *out,
>
>                 ret =3D splice_file_to_pipe(in, opipe, &offset, len, flag=
s);
>
> -               if (ret > 0)
> -                       fsnotify_access(in);
> -
>                 if (!off_in)
>                         in->f_pos =3D offset;
>                 else
>                         *off_in =3D offset;
> +       } else
> +               return -EINVAL;

I must have missed this in previous reviews -
mixing if {} else (without {}) is an anti pattern that causes bugs.

No need to send v6 just for this - it can be fixed up on commit.

>
> -               return ret;
> +       if (ret > 0) {
> +               /*
> +                * Generate modify out before access in:
> +                * do_splice_from() may've already sent modify out,
> +                * and this ensures the events get merged.
> +                */
> +               fsnotify_modify(out);
> +               fsnotify_access(in);
>         }
>
> -       return -EINVAL;
> +       return ret;
>  }
>
>  static long __do_splice(struct file *in, loff_t __user *off_in,
> --
> 2.39.2
>