Re: Splicing to/from a tty

[Al Viro <viro@zeniv.linux.org.uk>]

On Sun, Jan 24, 2021 at 11:11:42AM -0800, Linus Torvalds wrote:
> Al,
>  coming back to this because rc5 is imminent..
> 
> On Mon, Jan 18, 2021 at 11:45 AM Al Viro <viro@zeniv-ca> wrote:
> >
> > do_splice_direct() does something that do_splice() won't - it
> > handles non-pipe to non-pipe case.  Which is how sendfile(2) is
> > normally used, of course.
> >
> > I'll look into that in more details, but IMO bothering with
> > internal pipe is just plain wrong for those cases.
> 
> You clearly thought about this, with the emails about odd error cases,
> but I get the feeling that for fixing the current "you can't
> sendfile() to a pipe" regression (including stable) we should do the
> one-liner. No?
> 
> I agree that it would be better fixed by just having sendfile()
> basically turn into splice() for the pipe target case, but I haven't
> seen any patches from you, so I assume it wasn't 100% trivial.
> 
> Hmm?

Just to make clear - sendfile() regular-to-pipe is *not* the same
issue as splice to/from tty.  The latter needs ->splice_read()
and ->splice_write() in tty_fops; the former can be dealt with
by teaching do_sendifile() to use the guts of do_splice() in
case when in or out happens to be a pipe (with some rearrangement
of checks) instead of bothering with do_splice_direct().

The only commonality between these two is that both got broken
by the same patch series.  Johannes' patch is an attempt to
deal with regular-to-pipe sendfile(2), and it's not a good
way to handle that.  Neither it, nor the variant I proposed
would do a damn thing for tty (and sendfile(2) never worked
for source other than regular or block anyway).  FWIW, the real
check in do_splice_direct() should be "has FMODE_PREAD", regardless
of the position we are asking to read from - do_splice_direct()
is basically
	while left to copy
		splice_read from in to internal pipe
		splice_write from internal pipe to out
and if splice_write ends up with short copy, we advance the position
by the amount written and discard everything left in the internal pipe.
Using it for something non-seekable would end up with data silently
lost on short copy.

Note that decision against splice(2) between non-pipes appears
to have been deliberate and unless Jens (and mingo?) decide
they are OK with that change, I'm *not* adding that functionality
to do_splice().

Anyway, the series is in vfs.git #work.sendfile, 5.11-rc1-based

Shortlog:
Al Viro (3):
      do_splice_to(): move the logics for limiting the read length in
      take the guts of file-to-pipe splice into a helper function
      teach sendfile(2) to handle send-to-pipe directly

Diffstat:
 fs/internal.h   |  9 +++++++++
 fs/read_write.c | 19 +++++++++++++------
 fs/splice.c     | 42 +++++++++++++++++++++++-------------------
 3 files changed, 45 insertions(+), 25 deletions(-)

Individual patches in followups; first two are equivalent transformations
(fairly obvious cleanup and taking a part of do_splice() into a helper),
while the third one makes do_sendfile() use that new helper for file-to-pipe
case.