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

[Bart Van Assche]

On 7/27/21 12:27 AM, Bodo Stroesser wrote:
> What I meant is, that changing the source code in such a way, that
> writing a buffer in multiple writes works in general, could cause
> trouble in case userspace uses this.
>
> But for special syscall sequences your changes still change the result
> on existing configfs files. Example:
>
> 1) userspace program opens qfull_time_out
> 2) userspace program writes "90", count=2 to set timeout to 90 sec
> 3) userspace again wants to change timeout, so it writes "55", count=2
>
> Before the changes we end up with timeout being 55 seconds. After the
> change - due to data gathering - we finally have timeout 9055 seconds.

Hi Bodo,

How about replacing patches 1 and 2 from this series with the patch below?
Do you agree that this patch is sufficient to restore the behavior from
kernel v5.13 and before?

Thanks,

Bart.

Subject: [PATCH 1/3] configfs: Restore the kernel v5.13 text attribute write behavior

Instead of writing at the offset specified by the write() system call,
always write at offset zero.

Cc: Bodo Stroesser <bostroesser@xxxxxxxxx>
Cc: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
Cc: Yanko Kaneti <yaneti@xxxxxxxxxxx>
Cc: Brendan Higgins <brendanhiggins@xxxxxxxxxx>
Reported-by: Bodo Stroesser <bostroesser@xxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 fs/configfs/file.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/fs/configfs/file.c b/fs/configfs/file.c
index 5a0be9985bae..8adf6250b207 100644
--- a/fs/configfs/file.c
+++ b/fs/configfs/file.c
@@ -177,12 +177,11 @@ static ssize_t configfs_bin_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	return retval;
 }

-/* Fill [buffer, buffer + pos) with data coming from @from. */
-static int fill_write_buffer(struct configfs_buffer *buffer, loff_t pos,
+/* Fill @buffer with data coming from @from. */
+static int fill_write_buffer(struct configfs_buffer *buffer,
 			     struct iov_iter *from)
 {
-	loff_t to_copy;
-	int copied;
+	int to_copy, copied;
 	u8 *to;

 	if (!buffer->page)
@@ -190,11 +189,8 @@ static int fill_write_buffer(struct configfs_buffer *buffer, loff_t pos,
 	if (!buffer->page)
 		return -ENOMEM;

-	to_copy = SIMPLE_ATTR_SIZE - 1 - pos;
-	if (to_copy <= 0)
-		return 0;
-	to = buffer->page + pos;
-	copied = copy_from_iter(to, to_copy, from);
+	to = buffer->page;
+	copied = copy_from_iter(to, SIMPLE_ATTR_SIZE - 1, from);
 	buffer->needs_read_fill = 1;
 	/* if buf is assumed to contain a string, terminate it by \0,
 	 * so e.g. sscanf() can scan the string easily */
@@ -227,14 +223,14 @@ 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;

 	mutex_lock(&buffer->mutex);
-	len = fill_write_buffer(buffer, iocb->ki_pos, from);
+	len = fill_write_buffer(buffer, from);
 	if (len > 0)
 		len = flush_write_buffer(file, buffer, len);
 	if (len > 0)
-		iocb->ki_pos += len;
+		iocb->ki_pos = len;
 	mutex_unlock(&buffer->mutex);
 	return len;
 }