Re: [PATCH v3] splice: only read in as much information as there is pipe buffer space

["Andreas Grünbacher" <andreas.gruenbacher@gmail.com>]

Hi Darrick,

Am Di., 15. Okt. 2019 um 07:33 Uhr schrieb Darrick J. Wong
<darrick.wong@oracle.com>:
> From: Darrick J. Wong <darrick.wong@oracle.com>
>
> Andreas Grünbacher reports that on the two filesystems that support
> iomap directio, it's possible for splice() to return -EAGAIN (instead of
> a short splice) if the pipe being written to has less space available in
> its pipe buffers than the length supplied by the calling process.
>
> Months ago we fixed splice_direct_to_actor to clamp the length of the
> read request to the size of the splice pipe.  Do the same to do_splice.
>
> Fixes: 17614445576b6 ("splice: don't read more than available pipe space")
> Reported-by: syzbot+3c01db6025f26530cf8d@syzkaller.appspotmail.com
> Reported-by: Andreas Grünbacher <andreas.gruenbacher@gmail.com>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

I've done some minimal testing on top of 5.4-rc3. This patch fixes the
splice issue and also passes the syzbot reproducer. I'll add it to the
set of patches I regularly run fstests on now, but we already know
fstests doesn't cover splice in the greatest depth possible, so that's
unlikely to reveal anything new.

Thanks,
Andreas

> ---
>  fs/splice.c |   14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/fs/splice.c b/fs/splice.c
> index 98412721f056..e509239d7e06 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -945,12 +945,13 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
>         WARN_ON_ONCE(pipe->nrbufs != 0);
>
>         while (len) {
> +               unsigned int pipe_pages;
>                 size_t read_len;
>                 loff_t pos = sd->pos, prev_pos = pos;
>
>                 /* Don't try to read more the pipe has space for. */
> -               read_len = min_t(size_t, len,
> -                                (pipe->buffers - pipe->nrbufs) << PAGE_SHIFT);
> +               pipe_pages = pipe->buffers - pipe->nrbufs;
> +               read_len = min(len, (size_t)pipe_pages << PAGE_SHIFT);
>                 ret = do_splice_to(in, &pos, pipe, read_len, flags);
>                 if (unlikely(ret <= 0))
>                         goto out_release;
> @@ -1180,8 +1181,15 @@ static long do_splice(struct file *in, loff_t __user *off_in,
>
>                 pipe_lock(opipe);
>                 ret = wait_for_space(opipe, flags);
> -               if (!ret)
> +               if (!ret) {
> +                       unsigned int pipe_pages;
> +
> +                       /* Don't try to read more the pipe has space for. */
> +                       pipe_pages = opipe->buffers - opipe->nrbufs;
> +                       len = min(len, (size_t)pipe_pages << PAGE_SHIFT);
> +
>                         ret = do_splice_to(in, &offset, opipe, len, flags);
> +               }
>                 pipe_unlock(opipe);
>                 if (ret > 0)
>                         wakeup_pipe_readers(opipe);