linux-kernel - [PATCH 2/4] configfs: Fix writing at a non-zero offset

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Date:   Fri, 23 Jul 2021 14:23:51 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Christoph Hellwig <hch@....de>
Cc:     Joel Becker <jlbec@...lplan.org>, linux-kernel@...r.kernel.org,
        Bodo Stroesser <bostroesser@...il.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Yanko Kaneti <yaneti@...lera.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Bart Van Assche <bvanassche@....org>
Subject: [PATCH 2/4] configfs: Fix writing at a non-zero offset

Store data at the right offset when writing with a non-zero offset. I think
this patch fixes behavior introduced in the initial configfs commit. From a
source code comment in the initial configfs commit: "There is no easy way
for us to know if userspace is only doing a partial write, so we don't
support them."

Cc: Bodo Stroesser <bostroesser@...il.com>
Cc: Martin K. Petersen <martin.petersen@...cle.com>
Cc: Yanko Kaneti <yaneti@...lera.com>
Cc: Brendan Higgins <brendanhiggins@...gle.com>
Signed-off-by: Bart Van Assche <bvanassche@....org>
---
 fs/configfs/file.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/configfs/file.c b/fs/configfs/file.c
index 8121bb1b2121..3b2ddc2e5d42 100644
--- a/fs/configfs/file.c
+++ b/fs/configfs/file.c
@@ -226,14 +226,16 @@ static ssize_t configfs_write_iter(struct kiocb *iocb, struct iov_iter *from)
 {
 	struct file *file = iocb->ki_filp;
 	struct configfs_buffer *buffer = file->private_data;
-	ssize_t len;
+	int len, written = 0;
 
 	mutex_lock(&buffer->mutex);
 	len = fill_write_buffer(buffer, iocb->ki_pos, from);
 	if (len > 0)
-		len = flush_write_buffer(file, buffer, len);
-	if (len > 0)
-		iocb->ki_pos += len;
+		written = flush_write_buffer(file, buffer, iocb->ki_pos + len);
+	if (written > 0) {
+		len = max_t(int, 0, written - iocb->ki_pos);
+		iocb->ki_pos = written;
+	}
 	mutex_unlock(&buffer->mutex);
 	return len;
 }