nfs: use credential guards in nfs_local_call_write()

Use credential guards for scoped credential override with automatic
restoration on scope exit.

Link: https://patch.msgid.link/20251103-work-creds-guards-simple-v1-11-a3e156839e7f@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/nfs/localio.c

@@ -781,18 +781,11 @@ static void nfs_local_write_aio_complete(struct kiocb *kiocb, long ret)
 	nfs_local_pgio_aio_complete(iocb); /* Calls nfs_local_write_aio_complete_work */
 }
 
-static void nfs_local_call_write(struct work_struct *work)
+static ssize_t do_nfs_local_call_write(struct nfs_local_kiocb *iocb,
+				       struct file *filp)
 {
-	struct nfs_local_kiocb *iocb =
-		container_of(work, struct nfs_local_kiocb, work);
-	struct file *filp = iocb->kiocb.ki_filp;
-	unsigned long old_flags = current->flags;
-	const struct cred *save_cred;
 	ssize_t status;
 
-	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
-	save_cred = override_creds(filp->f_cred);
-
 	file_start_write(filp);
 	for (int i = 0; i < iocb->n_iters ; i++) {
 		if (iocb->iter_is_dio_aligned[i]) {
@@ -837,7 +830,22 @@ static void nfs_local_call_write(struct work_struct *work)
 	}
 	file_end_write(filp);
 
-	revert_creds(save_cred);
+	return status;
+}
+
+static void nfs_local_call_write(struct work_struct *work)
+{
+	struct nfs_local_kiocb *iocb =
+		container_of(work, struct nfs_local_kiocb, work);
+	struct file *filp = iocb->kiocb.ki_filp;
+	unsigned long old_flags = current->flags;
+	ssize_t status;
+
+	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
+
+	scoped_with_creds(filp->f_cred)
+		status = do_nfs_local_call_write(iocb, filp);
+
 	current->flags = old_flags;
 
 	if (status != -EIOCBQUEUED) {