[PATCH net] sfc: fix potential stack corruption from running past stat bitmask

[Edward Cree]

From: Andrew Rybchenko <andrew.rybche...@oktetlabs.ru>

On 32-bit systems, mask is only an array of 3 longs, not 4, so don't try
to write to mask[3].
Also include build-time checks in case the size of the bitmask changes.

Fixes: 3c36a2aded8c ("sfc: display vadaptor statistics for all interfaces")
Signed-off-by: Edward Cree <ec...@solarflare.com>
---
 drivers/net/ethernet/sfc/ef10.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/sfc/ef10.c b/drivers/net/ethernet/sfc/ef10.c
index 1f30912..6e0a4f2 100644
--- a/drivers/net/ethernet/sfc/ef10.c
+++ b/drivers/net/ethernet/sfc/ef10.c
@@ -1304,13 +1304,14 @@ static void efx_ef10_get_stat_mask(struct efx_nic *efx, 
unsigned long *mask)
        }
 
 #if BITS_PER_LONG == 64
+       BUILD_BUG_ON(BITS_TO_LONGS(EF10_STAT_COUNT) != 2);
        mask[0] = raw_mask[0];
        mask[1] = raw_mask[1];
 #else
+       BUILD_BUG_ON(BITS_TO_LONGS(EF10_STAT_COUNT) != 3);
        mask[0] = raw_mask[0] & 0xffffffff;
        mask[1] = raw_mask[0] >> 32;
        mask[2] = raw_mask[1] & 0xffffffff;
-       mask[3] = raw_mask[1] >> 32;
 #endif
 }