linux-kernel - [PATCH] afs: Fix afs_write_end() when called with copied == 0

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Date:   Sat, 14 Nov 2020 17:15:51 +0000
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org
Cc:     dhowells@...hat.com, linux-afs@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] afs: Fix afs_write_end() when called with copied == 0

When afs_write_end() is called with copied == 0, it tries to set the dirty
region, but there's no way to actually encode a 0-length region in the
encoding in page->private.  "0,0", for example, indicates a 1-byte region
at offset 0.  The maths miscalculates this and sets it incorrectly.

Fix it to just do nothing but unlock and put the page in this case.  We
don't actually need to mark the page dirty as nothing presumably changed.

Fixes: 65dd2d6072d3 ("afs: Alter dirty range encoding in page->private")
Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/afs/write.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/afs/write.c b/fs/afs/write.c
index 50371207f327..f34d13d294fa 100644
--- a/fs/afs/write.c
+++ b/fs/afs/write.c
@@ -174,6 +174,9 @@ int afs_write_end(struct file *file, struct address_space *mapping,
 	_enter("{%llx:%llu},{%lx}",
 	       vnode->fid.vid, vnode->fid.vnode, page->index);

+	if (copied == 0)
+		goto out;
+
 	maybe_i_size = pos + copied;

 	i_size = i_size_read(&vnode->vfs_inode);