[PATCH net 5/5] mptcp: fix BUILD_BUG_ON on legacy ARM config

[Matthieu Baerts (NGI0)]

The 0-day bot managed to find kernel configs that cause build failures,
e.g. when using the StrongARM SA1100 target (ARMv4).

On such legacy ARM architecture, all structures are apparently aligned
to 32 bits, causing build issue here. Indeed, on such architecture,
'flags' size is not equivalent to sizeof(u16) as expected, but to
sizeof(u32).

Instead, use memset(). It was not used before to ensure a simple clear
operation was used by the compiler. But at the end, it shouldn't matter,
and the compiler should optimise this to the same operation with or
without memset() when -O above 0 is used. So let's switch to memset() to
fix this issue, and reduce this complexity.

Fixes: 5e939544f9d2 ("mptcp: fix uninit-value in mptcp_established_options")
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: Frank Ranner <frank.ranner@xxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202605312026.Srgsz7Tp-lkp@xxxxxxxxx/
Closes: https://lore.kernel.org/oe-kbuild-all/202607031100.upQfRZTM-lkp@xxxxxxxxx/
Reviewed-by: Mat Martineau <martineau@xxxxxxxxxx>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
---
 net/mptcp/options.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 1b74ca5b6a59..c664023d37ba 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -571,10 +571,7 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 	bool ret = false;
 
 	/* Zero `use_ack` and `use_map` flags with one shot. */
-	BUILD_BUG_ON(sizeof_field(struct mptcp_ext, flags) != sizeof(u16));
-	BUILD_BUG_ON(!IS_ALIGNED(offsetof(struct mptcp_ext, flags),
-				 sizeof(u16)));
-	*(u16 *)&opts->ext_copy.flags = 0;
+	memset(&opts->ext_copy.flags, 0, sizeof(opts->ext_copy.flags));
 	opts->csum_reqd = READ_ONCE(msk->csum_enabled);
 	mpext = skb ? mptcp_get_ext(skb) : NULL;
 

-- 
2.53.0