[PATCH v4] nfsd: fix file change detection in CB_GETATTR

[Scott Mayhew <smayhew@redhat.com>]

RFC 8881, section 10.4.3 doesn't say anything about caching the file
size in the delegation record, nor does it say anything about comparing
a cached file size with the size reported by the client in the
CB_GETATTR reply for the purpose of determining if the client holds
modified data for the file.

What section 10.4.3 of RFC 8881 does say is that the server should
compare the *current* file size with the size reported by the client
holding the delegation in the CB_GETATTR reply, and if they differ to
treat it as a modification regardless of the change attribute retrieved
via the CB_GETATTR.

Doing otherwise would cause the server to believe the client holding the
delegation has a modified version of the file, even if the client
flushed the modifications to the server prior to the CB_GETATTR.  This
would have the added side effect of subsequent CB_GETATTRs causing
updates to the mtime, ctime, and change attribute even if the client
holding the delegation makes no further updates to the file.

Modify nfsd4_deleg_getattr_conflict() to obtain the current file size
via i_size_read().  Retain the ncf_cur_fsize field, since it's a
convenient way to return the file size back to nfsd4_encode_fattr4(),
but don't use it for the purpose of detecting file changes.  Remove the
unnecessary initialization of ncf_cur_fsize in nfs4_open_delegation().

Also, if we recall the delegation (because the client didn't respond to
the CB_GETATTR), then skip the logic that checks the nfs4_cb_fattr
fields.

Fixes: c5967721e106 ("NFSD: handle GETATTR conflict with write delegation")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
---

Chuck - if you want this to go to stable, it applies cleanly against
6.17.y, 6.18.y, and 6.19.y.  The first hunk fails to apply against
6.12.y due to a context difference because that branch doesn't have
delegated timestamp support.

 fs/nfsd/nfs4state.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index fa657badf5f8..6319f6c96f6f 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -6459,7 +6459,6 @@ nfs4_open_delegation(struct svc_rqst *rqstp, struct nfsd4_open *open,
 		}
 		open->op_delegate_type = deleg_ts ? OPEN_DELEGATE_WRITE_ATTRS_DELEG :
 						    OPEN_DELEGATE_WRITE;
-		dp->dl_cb_fattr.ncf_cur_fsize = stat.size;
 		dp->dl_cb_fattr.ncf_initial_cinfo = nfsd4_change_attribute(&stat);
 		dp->dl_atime = stat.atime;
 		dp->dl_ctime = stat.ctime;
@@ -9516,11 +9515,15 @@ nfsd4_deleg_getattr_conflict(struct svc_rqst *rqstp, struct dentry *dentry,
 		if (status != nfserr_jukebox ||
 		    !nfsd_wait_for_delegreturn(rqstp, inode))
 			goto out_status;
+		status = nfs_ok;
+		goto out_status;
+	}
+	if (!ncf->ncf_file_modified) {
+		if (ncf->ncf_initial_cinfo != ncf->ncf_cb_change)
+			ncf->ncf_file_modified = true;
+		else if (i_size_read(inode) != ncf->ncf_cb_fsize)
+			ncf->ncf_file_modified = true;
 	}
-	if (!ncf->ncf_file_modified &&
-	    (ncf->ncf_initial_cinfo != ncf->ncf_cb_change ||
-	     ncf->ncf_cur_fsize != ncf->ncf_cb_fsize))
-		ncf->ncf_file_modified = true;
 	if (ncf->ncf_file_modified) {
 		int err;
 
-- 
2.53.0