linux-kernel - Re: [PATCH 1/2] hyper-v: Mark inner union in hv_kvp_exchg_msg

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Message-ID: <20260115182242.GA3690299@liuwe-devbox-debian-v2.local>
Date: Thu, 15 Jan 2026 18:22:42 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: "K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Long Li <longli@...rosoft.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Hans de Goede <hansg@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 1/2] hyper-v: Mark inner union in hv_kvp_exchg_msg_value
 as packed

On Thu, Jan 15, 2026 at 08:35:44AM +0100, Thomas Weißschuh wrote:
> The unpacked union within a packed struct generates alignment warnings
> on clang for 32-bit ARM:
> 
> ./usr/include/linux/hyperv.h:361:2: error: field  within 'struct hv_kvp_exchg_msg_value'
>   is less aligned than 'union hv_kvp_exchg_msg_value::(anonymous at ./usr/include/linux/hyperv.h:361:2)'
>   and is usually due to 'struct hv_kvp_exchg_msg_value' being packed,
>   which can lead to unaligned accesses [-Werror,-Wunaligned-access]
>      361 |         union {
>          |         ^
> 
> With the recent changes to compile-test the UAPI headers in more cases,
> this warning in combination with CONFIG_WERROR breaks the build.
> 
> Fix the warning.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202512140314.DzDxpIVn-lkp@intel.com/
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Closes: https://lore.kernel.org/linux-kbuild/20260110-uapi-test-disable-headers-arm-clang-unaligned-access-v1-1-b7b0fa541daa@kernel.org/
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Link: https://lore.kernel.org/linux-kbuild/29b2e736-d462-45b7-a0a9-85f8d8a3de56@app.fastmail.com/
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>

Acked-by: Wei Liu (Microsoft) <wei.liu@...nel.org>

> ---
>  include/uapi/linux/hyperv.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/hyperv.h b/include/uapi/linux/hyperv.h
> index aaa502a7bff4..1749b35ab2c2 100644
> --- a/include/uapi/linux/hyperv.h
> +++ b/include/uapi/linux/hyperv.h
> @@ -362,7 +362,7 @@ struct hv_kvp_exchg_msg_value {
>  		__u8 value[HV_KVP_EXCHANGE_MAX_VALUE_SIZE];
>  		__u32 value_u32;
>  		__u64 value_u64;
> -	};
> +	} __attribute__((packed));
>  } __attribute__((packed));
>  
>  struct hv_kvp_msg_enumerate {
> 
> -- 
> 2.52.0
> 
>