On Mon, Jul 3, 2023 at 5:42 PM Ahelenia Ziemiańska
<nabijaczleweli@xxxxxxxxxxxxxxxxxx> 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/jbyihkyk5dtaohdwjyivambb2gffyjs3dodpofafnkkunxq7bu@jngkdxx65pux/t/#u
> Link: https://bugs.debian.org/1039488
> Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Acked-by: Jan Kara <jack@xxxxxxx>
> ---
> 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, struct file *out,
> if ((in->f_flags | out->f_flags) & O_NONBLOCK)
> flags |= SPLICE_F_NONBLOCK;
>
> - return splice_pipe_to_pipe(ipipe, opipe, len, flags);
> - }
> -
> - if (ipipe) {
> + ret = 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, struct file *out,
> ret = do_splice_from(ipipe, out, &offset, len, flags);
> file_end_write(out);
>
> - if (ret > 0)
> - fsnotify_modify(out);
> -
> if (!off_out)
> out->f_pos = offset;
> else
> *off_out = 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, struct file *out,
>
> ret = splice_file_to_pipe(in, opipe, &offset, len, flags);
>
> - if (ret > 0)
> - fsnotify_access(in);
> -
> if (!off_in)
> in->f_pos = offset;
> else
> *off_in = 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
>
