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Re: [Xen-devel] [PATCH net-next v4] xen-netback: Adding debugfs "io_ring_qX" files

On 11/08/14 11:38, Wei Liu wrote:
On Sun, Aug 10, 2014 at 10:57:51PM +0800, SeeChen Ng wrote:
Hi, I'm a noob to linux kernel, I tried to figure out the usage of the function
"simple_write_to_buffer", and I got confused about the code here.

+static ssize_t
+xenvif_write_io_ring(struct file *filp, const char __user *buf, size_t count,
+                    loff_t *ppos)
+       struct xenvif_queue *queue =
+               ((struct seq_file *)filp->private_data)->private;
+       int len;
+       char write[sizeof(XENVIF_KICK_STR)];
+       /* don't allow partial writes and check the length */
+       if (*ppos != 0)
+               return 0;
+       if (count < sizeof(XENVIF_KICK_STR) - 1)
+               return -ENOSPC;
The statement here is trying to verify the value of "count" is smaller
than the size of array "write", make sure that the array got
enough space for the write operation, right?

Yes I think so.

So, I think the statement should be:

          if (count >= sizeof(XENVIF_KICK_STR))
                  return -ENOSPC;

  * sizeof(XENVIF_KICK_STR) = 5
  * count is the number of bytes needs to be written, in the case of "kick"
"count" is the number of bytes in the incoming buffer. We don't necessarily need all of them.

    it's 5 because the tailing '\0' is also counted.

so the correct fix should be

   if (count > sizeof(XENVIF_KICK_STR))
           return -ENOSPC;
That would fail if e.g. there is a newline character after the 4 letters of "kick". I've crafted this check in this way so we don't have to worry what is after that 4 character. This check makes sure there is at least 4 character, and strncmp check exactly those ones. If there is anything after that, we don't care about it.

Do you want to submit a proper patch to fix it?

You can have a look at Documentation/SubmittingPatches for general
instructions on how to submit patch. And have a look at
Documentation/networking/netdev-FAQ.txt for network related patches.

If you have no interest in fixing it I can fix it myself.


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