Commit 133cca19d75b for kernel

commit 133cca19d75b9264bc2bbcdf2c3b80e3da207649
Author: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Date:   Wed Jul 22 00:14:42 2026 +0200

    mptcp: fix BUILD_BUG_ON on legacy ARM config

    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@vger.kernel.org
    Suggested-by: Frank Ranner <frank.ranner@intel.com>
    Reported-by: kernel test robot <lkp@intel.com>
    Closes: https://lore.kernel.org/oe-kbuild-all/202605312026.Srgsz7Tp-lkp@intel.com/
    Closes: https://lore.kernel.org/oe-kbuild-all/202607031100.upQfRZTM-lkp@intel.com/
    Reviewed-by: Mat Martineau <martineau@kernel.org>
    Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
    Link: https://patch.msgid.link/20260722-net-mptcp-misc-fixes-7-2-rc5-v1-5-6fb595bc86ef@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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;