[PATCH] afs: Fix afs_getattr() to refetch file status if callback break occurred

[David Howells]

If a callback break occurs (change notification), afs_getattr() needs to
issue an FS.FetchStatus RPC operation to update the status of the file
being examined by the stat-family of system calls.

Fix afs_getattr() to do this if AFS_VNODE_CB_PROMISED has been cleared on a
vnode by a callback break.  Skip this if AT_STATX_DONT_SYNC is set.

This can be tested by appending to a file on one AFS client and then using
"stat -L" to examine its length on a machine running kafs.  This can also
be watched through tracing on the kafs machine.  The callback break is
seen:

     kworker/1:1-46      [001] .....   978.910812: afs_cb_call: c=0000005f YFSCB.CallBack
     kworker/1:1-46      [001] ...1.   978.910829: afs_cb_break: 100058:23b4c:242d2c2 b=2 s=1 break-cb
     kworker/1:1-46      [001] .....   978.911062: afs_call_done:    c=0000005f ret=0 ab=0 [0000000082994ead]

And then the stat command generated no traffic if unpatched, but with this
change a call to fetch the status can be observed:

            stat-4471    [000] .....   986.744122: afs_make_fs_call: c=000000ab 100058:023b4c:242d2c2 YFS.FetchStatus
            stat-4471    [000] .....   986.745578: afs_call_done:    c=000000ab ret=0 ab=0 [0000000087fc8c84]

Fixes: 08e0e7c82eea ("[AF_RXRPC]: Make the in-kernel AFS filesystem use AF_RXRPC.")
Reported-by: Markus Suvanto <markus.suvanto at gmail.com>
Signed-off-by: David Howells <dhowells at redhat.com>
cc: Marc Dionne <marc.dionne at auristor.com>
cc: linux-afs at lists.infradead.org
Tested-by: Markus Suvanto <markus.suvanto at gmail.com>
Tested-by: kafs-testing+fedora34_64checkkafs-build-496 at auristor.com
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216010
Link: https://lore.kernel.org/r/165308359800.162686.14122417881564420962.stgit@warthog.procyon.org.uk/ # v1
---

 fs/afs/inode.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/afs/inode.c b/fs/afs/inode.c
index 2fe402483ad5..30b066299d39 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -740,10 +740,22 @@ int afs_getattr(struct user_namespace *mnt_userns, const struct path *path,
 {
 	struct inode *inode = d_inode(path->dentry);
 	struct afs_vnode *vnode = AFS_FS_I(inode);
-	int seq = 0;
+	struct key *key;
+	int ret, seq = 0;
 
 	_enter("{ ino=%lu v=%u }", inode->i_ino, inode->i_generation);
 
+	if (!(query_flags & AT_STATX_DONT_SYNC) &&
+	    !test_bit(AFS_VNODE_CB_PROMISED, &vnode->flags)) {
+		key = afs_request_key(vnode->volume->cell);
+		if (IS_ERR(key))
+			return PTR_ERR(key);
+		ret = afs_validate(vnode, key);
+		key_put(key);
+		if (ret < 0)
+			return ret;
+	}
+
 	do {
 		read_seqbegin_or_lock(&vnode->cb_lock, &seq);
 		generic_fillattr(&init_user_ns, inode, stat);