rfkill: prep for rfkill API changes

We've designed the /dev/rfkill API in a way that we
can increase the event struct by adding members at
the end, should it become necessary. To validate the
events, userspace and the kernel need to have the
proper event size to check for -- when reading from
the other end they need to verify that it's at least
version 1 of the event API, with the current struct
size, so define a constant for that and make the
code a little more 'future proof'.

Not that I expect that we'll have to change the event
size any time soon, but it's better to write the code
in a way that lends itself to extending.

Due to the current size of the event struct, the code
is currently equivalent, but should the event struct
ever need to be increased the new code might not need
changing.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/net/rfkill/core.c b/net/rfkill/core.c
index 79693fe2001e..47497c97c8d9 100644
--- a/net/rfkill/core.c
+++ b/net/rfkill/core.c
@@ -1076,10 +1076,16 @@ static ssize_t rfkill_fop_write(struct file *file, const char __user *buf,
         struct rfkill_event ev;
 
         /* we don't need the 'hard' variable but accept it */
-        if (count < sizeof(ev) - 1)
+        if (count < RFKILL_EVENT_SIZE_V1 - 1)
                 return -EINVAL;
 
-        if (copy_from_user(&ev, buf, sizeof(ev) - 1))
+        /*
+         * Copy as much data as we can accept into our 'ev' buffer,
+         * but tell userspace how much we've copied so it can determine
+         * our API version even in a write() call, if it cares.
+         */
+        count = min(count, sizeof(ev));
+        if (copy_from_user(&ev, buf, count))
                 return -EFAULT;
 
         if (ev.op != RFKILL_OP_CHANGE && ev.op != RFKILL_OP_CHANGE_ALL)