netdev - [PATCH 03/16] backing-file: use credential guards for reads

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Message-Id: <20251103-work-creds-guards-simple-v1-3-a3e156839e7f@kernel.org>
Date: Mon, 03 Nov 2025 12:26:51 +0100
From: Christian Brauner <brauner@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-aio@...ck.org, linux-unionfs@...r.kernel.org, 
 linux-erofs@...ts.ozlabs.org, linux-nfs@...r.kernel.org, 
 linux-cifs@...r.kernel.org, samba-technical@...ts.samba.org, 
 cgroups@...r.kernel.org, netdev@...r.kernel.org, 
 Christian Brauner <brauner@...nel.org>
Subject: [PATCH 03/16] backing-file: use credential guards for reads

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

Signed-off-by: Christian Brauner <brauner@...nel.org>
---
 fs/backing-file.c | 52 ++++++++++++++++++++++++++++------------------------
 1 file changed, 28 insertions(+), 24 deletions(-)

diff --git a/fs/backing-file.c b/fs/backing-file.c
index 15a7f8031084..4cb7276e7ead 100644
--- a/fs/backing-file.c
+++ b/fs/backing-file.c
@@ -157,13 +157,37 @@ static int backing_aio_init_wq(struct kiocb *iocb)
 	return sb_init_dio_done_wq(sb);
 }
 
+static int do_backing_file_read_iter(struct file *file, struct iov_iter *iter,
+				     struct kiocb *iocb, int flags)
+{
+	struct backing_aio *aio = NULL;
+	int ret;
+
+	if (is_sync_kiocb(iocb)) {
+		rwf_t rwf = iocb_to_rw_flags(flags);
+
+		return vfs_iter_read(file, iter, &iocb->ki_pos, rwf);
+	}
+
+	aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
+	if (!aio)
+		return -ENOMEM;
+
+	aio->orig_iocb = iocb;
+	kiocb_clone(&aio->iocb, iocb, get_file(file));
+	aio->iocb.ki_complete = backing_aio_rw_complete;
+	refcount_set(&aio->ref, 2);
+	ret = vfs_iocb_iter_read(file, &aio->iocb, iter);
+	backing_aio_put(aio);
+	if (ret != -EIOCBQUEUED)
+		backing_aio_cleanup(aio, ret);
+	return ret;
+}
 
 ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
 			       struct kiocb *iocb, int flags,
 			       struct backing_file_ctx *ctx)
 {
-	struct backing_aio *aio = NULL;
-	const struct cred *old_cred;
 	ssize_t ret;
 
 	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
@@ -176,28 +200,8 @@ ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
 	    !(file->f_mode & FMODE_CAN_ODIRECT))
 		return -EINVAL;
 
-	old_cred = override_creds(ctx->cred);
-	if (is_sync_kiocb(iocb)) {
-		rwf_t rwf = iocb_to_rw_flags(flags);
-
-		ret = vfs_iter_read(file, iter, &iocb->ki_pos, rwf);
-	} else {
-		ret = -ENOMEM;
-		aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
-		if (!aio)
-			goto out;
-
-		aio->orig_iocb = iocb;
-		kiocb_clone(&aio->iocb, iocb, get_file(file));
-		aio->iocb.ki_complete = backing_aio_rw_complete;
-		refcount_set(&aio->ref, 2);
-		ret = vfs_iocb_iter_read(file, &aio->iocb, iter);
-		backing_aio_put(aio);
-		if (ret != -EIOCBQUEUED)
-			backing_aio_cleanup(aio, ret);
-	}
-out:
-	revert_creds(old_cred);
+	scoped_with_creds(ctx->cred)
+		do_backing_file_read_iter(file, iter, iocb, flags);
 
 	if (ctx->accessed)
 		ctx->accessed(iocb->ki_filp);

-- 
2.47.3