USB: usbmon: fix read(2)

There's a bug in the usbmon binary reader: When using read() to fetch
the packets and a packet's data is partially read, the next read call
will once again return up to len_cap bytes of data. The b_read counter
is not regarded when determining the remaining chunk size.

So, when dumping USB data with "cat /dev/usbmon0 > usbmon.trace" while
reading from a USB storage device and analyzing the dump file
afterwards it will get out of sync after a couple of packets.

Signed-off-by: Ingo van Lil <inguin@gmx.de>
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 4 insertions, 1 deletions

diff --git a/drivers/usb/mon/mon_bin.c b/drivers/usb/mon/mon_bin.c
index c9de3f027aab..e06810aef2df 100644
--- a/drivers/usb/mon/mon_bin.c
+++ b/drivers/usb/mon/mon_bin.c
@@ -687,7 +687,10 @@ static ssize_t mon_bin_read(struct file *file, char __user *buf,
         }
 
         if (rp->b_read >= sizeof(struct mon_bin_hdr)) {
-                step_len = min(nbytes, (size_t)ep->len_cap);
+                step_len = ep->len_cap;
+                step_len -= rp->b_read - sizeof(struct mon_bin_hdr);
+                if (step_len > nbytes)
+                        step_len = nbytes;
                 offset = rp->b_out + PKT_SIZE;
                 offset += rp->b_read - sizeof(struct mon_bin_hdr);
                 if (offset >= rp->b_size)