nfsd: remove unsafe BUG_ON from set_change_info

At one time, nfsd would scrape inode information directly out of struct
inode in order to populate the change_info4. At that time, the BUG_ON in
set_change_info made some sense, since having it unset meant a coding
error.

More recently, it calls vfs_getattr to get this information, which can
fail. If that fails, fh_pre_saved can end up not being set. While this
situation is unfortunate, we don't need to crash the box.

Move set_change_info to nfs4proc.c since all of the callers are there.
Revise the condition for setting "atomic" to also check for
fh_pre_saved. Drop the BUG_ON and just have it zero out both
change_attr4s when this occurs.

Reported-by: Boyang Xue <bxue@redhat.com>
Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2223560
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4proc.c

@@ -382,6 +382,38 @@ nfsd4_create_file(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	return status;
 }
 
+/**
+ * set_change_info - set up the change_info4 for a reply
+ * @cinfo: pointer to nfsd4_change_info to be populated
+ * @fhp: pointer to svc_fh to use as source
+ *
+ * Many operations in NFSv4 require change_info4 in the reply. This function
+ * populates that from the info that we (should!) have already collected. In
+ * the event that we didn't get any pre-attrs, just zero out both.
+ */
+static void
+set_change_info(struct nfsd4_change_info *cinfo, struct svc_fh *fhp)
+{
+	cinfo->atomic = (u32)(fhp->fh_pre_saved && fhp->fh_post_saved && !fhp->fh_no_atomic_attr);
+	cinfo->before_change = fhp->fh_pre_change;
+	cinfo->after_change = fhp->fh_post_change;
+
+	/*
+	 * If fetching the pre-change attributes failed, then we should
+	 * have already failed the whole operation. We could have still
+	 * failed to fetch post-change attributes however.
+	 *
+	 * If we didn't get post-op attrs, just zero-out the after
+	 * field since we don't know what it should be. If the pre_saved
+	 * field isn't set for some reason, throw warning and just copy
+	 * whatever is in the after field.
+	 */
+	if (WARN_ON_ONCE(!fhp->fh_pre_saved))
+		cinfo->before_change = 0;
+	if (!fhp->fh_post_saved)
+		cinfo->after_change = 0;
+}
+
 static __be32
 do_open_lookup(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate, struct nfsd4_open *open, struct svc_fh **resfh)
 {

fs/nfsd/xdr4.h

@@ -774,17 +774,6 @@ void warn_on_nonidempotent_op(struct nfsd4_op *op);
 
 #define NFS4_SVC_XDRSIZE		sizeof(struct nfsd4_compoundargs)
 
-static inline void
-set_change_info(struct nfsd4_change_info *cinfo, struct svc_fh *fhp)
-{
-	BUG_ON(!fhp->fh_pre_saved);
-	cinfo->atomic = (u32)(fhp->fh_post_saved && !fhp->fh_no_atomic_attr);
-
-	cinfo->before_change = fhp->fh_pre_change;
-	cinfo->after_change = fhp->fh_post_change;
-}
-
-
 bool nfsd4_mach_creds_match(struct nfs4_client *cl, struct svc_rqst *rqstp);
 bool nfs4svc_decode_compoundargs(struct svc_rqst *rqstp, struct xdr_stream *xdr);
 bool nfs4svc_encode_compoundres(struct svc_rqst *rqstp, struct xdr_stream *xdr);