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

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Oct 16, 2019 at 02:12:59PM +1100, Dave Chinner wrote:
> On Mon, Oct 14, 2019 at 03:09:40PM -0700, Darrick J. Wong wrote:
> > 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>
> > ---
> >  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;
> 
> define this as a size_t...
> 
> >  		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);
> 
> 		read_len = min_t(size_t, len, pipe_pages << PAGER_SHIFT);

If we make pipe_pages have type size_t then we don't need min_t here,
right?  Since len and read_len are already size_t.

--D

> >  		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);
> 
> And same here...
> 
> Cheers,
> 
> Dave.
> 
> -- 
> Dave Chinner
> david@fromorbit.com