From: Neil Horman <nhorman@tuxdriver.com>
Date: Wed, 27 Feb 2013 15:09:31 -0500

On Wed, Feb 27, 2013 at 11:43:51AM -0800, Guenter Roeck wrote:

quoted

Building sctp may fail with:

In function ‘copy_from_user’,
    inlined from ‘sctp_getsockopt_assoc_stats’ at
    net/sctp/socket.c:5656:20:
arch/x86/include/asm/uaccess_32.h:211:26: error: call to
    ‘copy_from_user_overflow’ declared with attribute error: copy_from_user()
    buffer size is not provably correct

if built with W=1 due to a missing parameter size validation.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 net/sctp/socket.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index cedd9bf..0a5f2bf 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c

@@ -5652,6 +5652,8 @@ static int sctp_getsockopt_assoc_stats(struct sock *sk, int len,
 	/* User must provide at least the assoc id */
 	if (len < sizeof(sctp_assoc_t))
 		return -EINVAL;
+	if (len > sizeof(struct sctp_assoc_stats))
+		len = sizeof(struct sctp_assoc_stats);
 
 	if (copy_from_user(&sas, optval, len))
 		return -EFAULT;

-- 
1.7.9.7

Theres more than that going on here.  This will fix the warning, but the
function is written such that, if you pass in a size that is greater than the
size of a struct sctp_association, but less than a struct sctp_assoc_stats.  I'm
not sure that a partial stat struct is really that useful to people.  What if
you were to check for max(struct sctp_association, struct sctp_assoc_stats) as
your minimum length check, then just did a copy_from_user of that length.  It
would save you having to compute two lengths separately, since you could then
just do a copy_to_user(...,sizeof(struct sctp_assoc_stats), at the bottom of
that function.

Genreally, getsockopt() implementations happily give partial return values
when the user gives a too small length.