[PATCH net-next v3 05/11] splice, net: Fix SPLICE_F_MORE signalling in splice_direct_to_actor()

[PATCH net-next v3 05/11] splice, net: Fix SPLICE_F_MORE signalling in splice_direct_to_actor()

[David Howells]

splice_direct_to_actor() doesn't manage SPLICE_F_MORE correctly[1] - and,
as a result, it incorrectly signals/fails to signal MSG_MORE when splicing
to a socket.  The problem I'm seeing happens when a short splice occurs
because we got a short read due to hitting the EOF on a file: as the length
read (read_len) is less than the remaining size to be spliced (len),
SPLICE_F_MORE (and thus MSG_MORE) is set.

The issue is that, for the moment, we have no way to know *why* the short
read occurred and so can't make a good decision on whether we *should* keep
MSG_MORE set.  Further, the argument can be made that it should be left to
userspace to decide how to handle it - userspace could perform some sort of
cancellation for example.

MSG_SENDPAGE_NOTLAST was added to work around this, but that is also set
incorrectly under some circumstances - for example if a short read fills a
single pipe_buffer, but the next read would return more (seqfile can do
this).

This was observed with the multi_chunk_sendfile tests in the tls kselftest
program.  Some of those tests would hang and time out when the last chunk
of file was less than the sendfile request size:

	build/kselftest/net/tls -r tls.12_aes_gcm.multi_chunk_sendfile

This has been observed before[2] and worked around in AF_TLS[3].

Fix this by making splice_direct_to_actor() always signal SPLICE_F_MORE if
we haven't yet hit the requested operation size.  SPLICE_F_MORE remains
signalled if the user passed it in to splice() but otherwise gets cleared
when we've read sufficient data to fulfill the request.  The cleanup of a
short splice to userspace is left to userspace.

[!] Note that this changes user-visible behaviour.  It will cause the
    multi_chunk_sendfile tests in the TLS kselftest to fail.  This failure
    in the testsuite will be addressed in a subsequent patch by making
    userspace do a zero-length send().

It appears that SPLICE_F_MORE is only used by splice-to-socket.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Christoph Hellwig <hch@lst.de>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Matthew Wilcox <willy@infradead.org>
cc: Jan Kara <jack@suse.cz>
cc: Jeff Layton <jlayton@kernel.org>
cc: David Hildenbrand <david@redhat.com>
cc: Christian Brauner <brauner@kernel.org>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Boris Pismenny <borisp@nvidia.com>
cc: John Fastabend <john.fastabend@gmail.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-fsdevel@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-mm@kvack.org
cc: netdev@vger.kernel.org

Link: https://lore.kernel.org/r/499791.1685485603@warthog.procyon.org.uk/ [1]
Link: https://lore.kernel.org/r/1591392508-14592-1-git-send-email-pooja.trivedi@stackpath.com/ [2]
Link: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=d452d48b9f8b1a7f8152d33ef52cfd7fe1735b0a [3]
---
 fs/splice.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index 9b1d43c0c562..c71bd8e03469 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1052,13 +1052,17 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
 	 */
 	bytes = 0;
 	len = sd->total_len;
+
+	/* Don't block on output, we have to drain the direct pipe. */
 	flags = sd->flags;
+	sd->flags &= ~SPLICE_F_NONBLOCK;
 
 	/*
-	 * Don't block on output, we have to drain the direct pipe.
+	 * We signal MORE until we've read sufficient data to fulfill the
+	 * request and we keep signalling it if the caller set it.
 	 */
-	sd->flags &= ~SPLICE_F_NONBLOCK;
 	more = sd->flags & SPLICE_F_MORE;
+	sd->flags |= SPLICE_F_MORE;
 
 	WARN_ON_ONCE(!pipe_empty(pipe->head, pipe->tail));
 
@@ -1074,14 +1078,12 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
 		sd->total_len = read_len;
 
 		/*
-		 * If more data is pending, set SPLICE_F_MORE
-		 * If this is the last data and SPLICE_F_MORE was not set
-		 * initially, clears it.
+		 * If we now have sufficient data to fulfill the request then
+		 * we clear SPLICE_F_MORE if it was not set initially.
 		 */
-		if (read_len < len)
-			sd->flags |= SPLICE_F_MORE;
-		else if (!more)
+		if (read_len >= len && !more)
 			sd->flags &= ~SPLICE_F_MORE;
+
 		/*
 		 * NOTE: nonblocking mode only applies to the input. We
 		 * must not do the output in nonblocking mode as then we

Re: [PATCH net-next v3 05/11] splice, net: Fix SPLICE_F_MORE signalling in splice_direct_to_actor()

[Linus Torvalds]

On Fri, Jun 2, 2023 at 11:08 AM David Howells <dhowells@redhat.com> wrote:
>
> Fix this by making splice_direct_to_actor() always signal SPLICE_F_MORE if
> we haven't yet hit the requested operation size.

Well, I certainly like this patch better than the previous versions,
just because it doesn't add random fd-specific code.

That said, I think it might be worth really documenting the behavior,
particularly for files where the kernel *could* know "the file is at
EOF, no more data".

I hope that if user space wants to splice() a file to a socket, said
user space would have done an 'fstat()' and actually pass in the file
size as the length to splice(). Because if they do, I think this
simplified patch does the right thing automatically.

But if user space instead passes in a "maximally big len", and just
depends on the kernel then doing tha

                ret = do_splice_to(in, &pos, pipe, len, flags);
                if (unlikely(ret <= 0))
                        goto out_release;

to stop splicing at EOF, then the last splice_write() will have had
SPLICE_F_MORE set, even though no more data is coming from the file,
of course.

And I think that's fine. But wasn't that effectively what the old code
was already doing because 'read_len' was smaller than 'len'? I thought
that was what you wanted to fix?

IOW, I thought you wanted to clear SPLICE_F_MORE when we hit EOF. This
still doesn't do that.

So now I'm confused about what your "fix" is. Your patch doesn't
actually seem to change existing behavior in splice_direct_to_actor().

I was expecting you to actually pass the 'sd' down to do_splice_to()
and then to ->splice_read(), so that the splice_read() function could
say "I have no more", and clear it.

But you didn't do that.

Am I misreading something, or did I miss another patch?

               Linus