[PATCH v2 14/15] smb: client: distinguish real EOF from a stale remote_i_size on read

[Paulo Alcantara <pc@manguebit.org>]

smb2_readv_callback() sets NETFS_SREQ_HIT_EOF whenever a short read
lines up with netfs_read_remote_i_size(inode), the server's EOF as the
client currently believes it. That belief can be stale: after a lease
downgrade and handle reopen, the tracked remote_i_size can sit below
the client's own i_size while an extending write hasn't reached the
server yet. A read in that gap comes back short for a reason that has
nothing to do with the file's real size, but was still marked HIT_EOF,
and netfs reports a short read for it as-is.

Only treat it as real EOF when the position is also at or past the
client's own i_size; otherwise mark it NETFS_SREQ_CLEAR_TAIL instead,
which tells netfs the shortfall is safe to zero-fill rather than
report as a short read.

This is what fsx (generic/363) sees as "short read: 0x0 bytes instead
of 0x<n>" against a Windows server.

Fixes: 1da29f2c39b6 ("netfs, cifs: Fix handling of short DIO read")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org
---
 fs/smb/client/smb2pdu.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
index 3d7ead36d1a0..cfe3c4b4abe7 100644
--- a/fs/smb/client/smb2pdu.c
+++ b/fs/smb/client/smb2pdu.c
@@ -4796,14 +4796,20 @@ smb2_readv_callback(struct TCP_Server_Info *server, struct mid_q_entry *mid)
 				     rdata->got_bytes);
 
 	if (rdata->result == -ENODATA) {
-		__set_bit(NETFS_SREQ_HIT_EOF, &rdata->subreq.flags);
 		rdata->result = 0;
+		if (rdata->subreq.start + rdata->subreq.transferred >= i_size_read(inode))
+			__set_bit(NETFS_SREQ_HIT_EOF, &rdata->subreq.flags);
+		else
+			__set_bit(NETFS_SREQ_CLEAR_TAIL, &rdata->subreq.flags);
 	} else {
 		size_t trans = rdata->subreq.transferred + rdata->got_bytes;
 		if (trans < rdata->subreq.len &&
 		    rdata->subreq.start + trans >= netfs_read_remote_i_size(inode)) {
-			__set_bit(NETFS_SREQ_HIT_EOF, &rdata->subreq.flags);
 			rdata->result = 0;
+			if (rdata->subreq.start + trans >= i_size_read(inode))
+				__set_bit(NETFS_SREQ_HIT_EOF, &rdata->subreq.flags);
+			else
+				__set_bit(NETFS_SREQ_CLEAR_TAIL, &rdata->subreq.flags);
 		}
 		if (rdata->got_bytes)
 			__set_bit(NETFS_SREQ_MADE_PROGRESS, &rdata->subreq.flags);
-- 
2.55.0