Re: [PATCH mptcp-net] mptcp: fix BUILD_BUG_ON on legacy ARM config

On Tue, 7 Jul 2026, Matthieu Baerts (NGI0) wrote:

> 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")
> Suggested-by: Frank Ranner <[email protected]>
> Reported-by: kernel test robot <[email protected]>
> Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
> Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
> Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
> ---
> 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 0ca60314d667..47d4a1b520d8 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;

This change LGTM, thanks Matthieu

Reviewed-by: Mat Martineau <[email protected]>