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. 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. If, however, we get a premature EOF from ->splice_read(), have sent at least one byte and SPLICE_F_MORE was not set by the caller, ->splice_eof() will be invoked. Signed-off-by: David Howells <dhowells@xxxxxxxxxx> cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> cc: Jakub Kicinski <kuba@xxxxxxxxxx> cc: Jens Axboe <axboe@xxxxxxxxx> cc: Christoph Hellwig <hch@xxxxxx> cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> cc: Jan Kara <jack@xxxxxxx> cc: Jeff Layton <jlayton@xxxxxxxxxx> cc: David Hildenbrand <david@xxxxxxxxxx> cc: Christian Brauner <brauner@xxxxxxxxxx> cc: Chuck Lever <chuck.lever@xxxxxxxxxx> cc: Boris Pismenny <borisp@xxxxxxxxxx> cc: John Fastabend <john.fastabend@xxxxxxxxx> cc: Eric Dumazet <edumazet@xxxxxxxxxx> cc: "David S. Miller" <davem@xxxxxxxxxxxxx> cc: Paolo Abeni <pabeni@xxxxxxxxxx> cc: linux-fsdevel@xxxxxxxxxxxxxxx cc: linux-block@xxxxxxxxxxxxxxx cc: linux-mm@xxxxxxxxx cc: netdev@xxxxxxxxxxxxxxx Link: https://lore.kernel.org/r/499791.1685485603@xxxxxxxxxxxxxxxxxxxxxx/ [1] Link: https://lore.kernel.org/r/1591392508-14592-1-git-send-email-pooja.trivedi@xxxxxxxxxxxxx/ [2] Link: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=d452d48b9f8b1a7f8152d33ef52cfd7fe1735b0a [3] --- Notes: ver #4) - Use ->splice_eof() to signal a premature EOF to the splice output. fs/splice.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-) diff --git a/fs/splice.c b/fs/splice.c index 67dbd85db207..67ddaac1f5c5 100644 --- a/fs/splice.c +++ b/fs/splice.c @@ -1063,13 +1063,17 @@ 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)); @@ -1085,14 +1089,12 @@ 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
