[PATCH v2] nfsd: recall deleg if a requested dir attr change can't be encoded

[Jeff Layton]

When the client requested NOTIFY4_CHANGE_DIR_ATTRS,
nfsd4_cb_notify_prepare() tried to append the dir attribute change to
the CB_NOTIFY, but silently dropped it if the attrmask space
reservation failed or nfsd4_encode_dir_attr_change() failed to marshal
the change into the buffer. It then returned true and transmitted a
CB_NOTIFY lacking the requested notification, leaving the client's
cached directory attributes stale with no indication.

RFC 8881 s10.9.4 requires the server to recall the delegation when the
directory changes in a way that is not covered by the notification.
Treat a failure to encode the dir attribute change like the per-event
encode failures already handled in the loop above: set error and fall
through to out_recall.

The granted notification mask and the requested dir attribute mask come
from independent fields of the GET_DIR_DELEGATION request, so a client
can be granted NOTIFY4_CHANGE_DIR_ATTRS while requesting no specific dir
attributes. nfsd4_encode_dir_attr_change() now distinguishes a genuine
encode failure (which forces the recall) from that benign case (which
simply omits the event).

While we're in there, also remove an unneeded check for
NOTIFY4_CHANGE_DIR_ATTRS from nfsd4_encode_dir_attr_change().

Fixes: 757a16cd93d5 ("nfsd: add support to CB_NOTIFY for dir attribute changes")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
This version should address Chuck's comments about the NULL pointer
handling.
---
Changes in v2:
- Allow nfsd4_encode_dir_attr_change() to return ERR_PTR when there are errors
- Drop unneeded NOTIFY4_CHANGE_DIR_ATTRS check
- Link to v1: https://lore.kernel.org/r/20260617-dir-deleg-fixes-v1-0-32b85b366c29@xxxxxxxxxx
---
 fs/nfsd/nfs4state.c | 43 ++++++++++++++++++++++++++++---------------
 fs/nfsd/nfs4xdr.c   | 30 +++++++++++++++++-------------
 2 files changed, 45 insertions(+), 28 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index b830aed7ae39..4ae5d65c056a 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -3598,23 +3598,36 @@ nfsd4_cb_notify_prepare(struct nfsd4_callback *cb)
 put_event:
 		nfsd_notify_event_put(nne);
 	}
-	if (!error) {
-		if (dp->dl_notify_mask & BIT(NOTIFY4_CHANGE_DIR_ATTRS)) {
-			u32 *maskp = (u32 *)xdr_reserve_space(&stream, sizeof(*maskp));
+	if (!error && (dp->dl_notify_mask & BIT(NOTIFY4_CHANGE_DIR_ATTRS))) {
+		u32 *maskp = (u32 *)xdr_reserve_space(&stream, sizeof(*maskp));
+		u8 *p;
 
-			if (maskp) {
-				u8 *p = nfsd4_encode_dir_attr_change(&stream, dp, nf);
-
-				if (p) {
-					*maskp = BIT(NOTIFY4_CHANGE_DIR_ATTRS);
-					ncn->ncn_nf[count].notify_mask.count = 1;
-					ncn->ncn_nf[count].notify_mask.element = maskp;
-					ncn->ncn_nf[count].notify_vals.data = p;
-					ncn->ncn_nf[count].notify_vals.len = (u8 *)stream.p - p;
-					++count;
-				}
-			}
+		if (maskp)
+			p = nfsd4_encode_dir_attr_change(&stream, dp, nf);
+		else
+			p = ERR_PTR(-ENOBUFS);
+
+		if (IS_ERR(p)) {
+			/*
+			 * The client asked to be told about dir attr changes
+			 * but the change could not be encoded. RFC 8881
+			 * s10.9.4 requires the server to recall the delegation
+			 * rather than drop a requested notification, so fall
+			 * through to recall. A NULL return instead means there
+			 * were no attributes to report, so omit the event in
+			 * that case.
+			 */
+			error = true;
+		} else if (p) {
+			*maskp = BIT(NOTIFY4_CHANGE_DIR_ATTRS);
+			ncn->ncn_nf[count].notify_mask.count = 1;
+			ncn->ncn_nf[count].notify_mask.element = maskp;
+			ncn->ncn_nf[count].notify_vals.data = p;
+			ncn->ncn_nf[count].notify_vals.len = (u8 *)stream.p - p;
+			++count;
 		}
+	}
+	if (!error) {
 		ncn->ncn_nf_cnt = count;
 		nfsd_file_put(nf);
 		return true;
diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index 5282e00805af..f46115a38af7 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -4439,28 +4439,32 @@ u8 *nfsd4_encode_notify_event(struct xdr_stream *xdr, struct nfsd_notify_event *
  * @nf: nfsd_file opened on the directory
  *
  * Encode a dir attr change event.
+ *
+ * Return: a pointer to the start of the encoded event on success; NULL
+ * if there were no requested attributes to report, in which case the
+ * caller should omit the event; or an ERR_PTR if the event was requested
+ * but could not be marshalled into @xdr, in which case the caller should
+ * recall the delegation.
  */
 u8 *nfsd4_encode_dir_attr_change(struct xdr_stream *xdr, struct nfs4_delegation *dp,
 				 struct nfsd_file *nf)
 {
 	struct dentry *dentry = nf->nf_file->f_path.dentry;
 	struct notify_attr4 na = { };
-	bool ret;
-	u8 *p = NULL;
-
-	if (!(dp->dl_notify_mask & BIT(NOTIFY4_CHANGE_DIR_ATTRS)))
-		return NULL;
+	u8 *p;
 
 	/* RFC 8881 s10.4.3: ne_file must be a zero-length string for dir attrs */
-	ret = nfsd4_setup_notify_entry4(&na.na_changed_entry, xdr,
-					dentry, dp, nf, "", 0);
+	if (!nfsd4_setup_notify_entry4(&na.na_changed_entry, xdr,
+				       dentry, dp, nf, "", 0))
+		return ERR_PTR(-ENOBUFS);
 
-	/* Don't bother with the event if we're not encoding attrs */
-	if (ret && na.na_changed_entry.ne_attrs.attr_vals.len) {
-		p = (u8 *)xdr->p;
-		if (!xdrgen_encode_notify_attr4(xdr, &na))
-			p = NULL;
-	}
+	/* No requested attributes to report; omit the event */
+	if (!na.na_changed_entry.ne_attrs.attr_vals.len)
+		return NULL;
+
+	p = (u8 *)xdr->p;
+	if (!xdrgen_encode_notify_attr4(xdr, &na))
+		return ERR_PTR(-ENOBUFS);
 	return p;
 }
 

---
base-commit: 6f90c7618528b5ca5887f8c6057f26dcc7a27a99
change-id: 20260617-dir-deleg-fixes-ecac84095b84

Best regards,
-- 
Jeff Layton <jlayton@xxxxxxxxxx>