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Date: Fri, 29 Mar 2024 13:56:15 +0100
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Subject: Re: [PATCH 1/2] [RESEND] wifi: carl9170: re-fix fortified-memset
 warning
To: Arnd Bergmann <arnd@kernel.org>, Kees Cook <keescook@chromium.org>,
 Kalle Valo <kvalo@kernel.org>, Johannes Berg <johannes.berg@intel.com>
Cc: linux-hardening@vger.kernel.org, =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vu?=
 =?UTF-8?Q?sen?= <toke@toke.dk>, linux-wireless@vger.kernel.org,
 Arnd Bergmann <arnd@arndb.de>, Colin Ian King <colin.i.king@gmail.com>,
 linux-kernel@vger.kernel.org
References: <20240328135509.3755090-1-arnd@kernel.org>
 <20240328135509.3755090-2-arnd@kernel.org>
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From: Christian Lamparter <chunkeey@gmail.com>
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On 3/28/24 2:55 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> The carl9170_tx_release() function sometimes triggers a fortified-memset
> warning in my randconfig builds:
> 
> In file included from include/linux/string.h:254,
>                   from drivers/net/wireless/ath/carl9170/tx.c:40:
> In function 'fortify_memset_chk',
>      inlined from 'carl9170_tx_release' at drivers/net/wireless/ath/carl9170/tx.c:283:2,
>      inlined from 'kref_put' at include/linux/kref.h:65:3,
>      inlined from 'carl9170_tx_put_skb' at drivers/net/wireless/ath/carl9170/tx.c:342:9:
> include/linux/fortify-string.h:493:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
>    493 |                         __write_overflow_field(p_size_field, size);
> 
> Kees previously tried to avoid this by using memset_after(), but it seems
> this does not fully address the problem. I noticed that the memset_after()
> here is done on a different part of the union (status) than the original
> cast was from (rate_driver_data), which may confuse the compiler.
> 
> Unfortunately, the memset_after() trick does not work on driver_rates[]
> because that is part of an anonymous struct, and I could not get
> struct_group() to do this either. Using two separate memset() calls
> on the two members does address the warning though.
> 
> Fixes: fb5f6a0e8063b ("mac80211: Use memset_after() to clear tx status")
> Link: https://lore.kernel.org/lkml/20230623152443.2296825-1-arnd@kernel.org/
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Sure... though I think AI-supported compilers will in the future nag about
this again. Oh well!

Acked-by: Christian Lamparter <chunkeey@gmail.com>


> ---
> I found this while testing randconfig builds, a .config that shows this
> for me is at https://pastebin.com/yWFKvZYu
> 
> Sorry I failed to follow up to Kees' request for a reproducer when
> I posted this last year.
> ---
>   drivers/net/wireless/ath/carl9170/tx.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/wireless/ath/carl9170/tx.c b/drivers/net/wireless/ath/carl9170/tx.c
> index e902ca80eba7..0226c31a6cae 100644
> --- a/drivers/net/wireless/ath/carl9170/tx.c
> +++ b/drivers/net/wireless/ath/carl9170/tx.c
> @@ -280,7 +280,8 @@ static void carl9170_tx_release(struct kref *ref)
>   	 * carl9170_tx_fill_rateinfo() has filled the rate information
>   	 * before we get to this point.
>   	 */
> -	memset_after(&txinfo->status, 0, rates);
> +	memset(&txinfo->pad, 0, sizeof(txinfo->pad));
> +	memset(&txinfo->rate_driver_data, 0, sizeof(txinfo->rate_driver_data));