From 133cca19d75b9264bc2bbcdf2c3b80e3da207649 Mon Sep 17 00:00:00 2001 From: "Matthieu Baerts (NGI0)" Date: Wed, 22 Jul 2026 00:14:42 +0200 Subject: [PATCH] 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 Reported-by: kernel test robot 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 Signed-off-by: Matthieu Baerts (NGI0) Link: https://patch.msgid.link/20260722-net-mptcp-misc-fixes-7-2-rc5-v1-5-6fb595bc86ef@kernel.org Signed-off-by: Jakub Kicinski --- 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;