Re: [PATCH 1/2] carl9170: re-fix fortified-memset warning

[Kees Cook <keescook@chromium.org>]

On Fri, Jun 23, 2023 at 05:23:59PM +0200, 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")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  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 6bb9aa2bfe654..88ef6e023f826 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));

This is "accidentally" equivalent, which makes me nervous. It was
designed to clear everything after "rates", regardless of padding, etc.

What I don't get is why the warning is being emitted. It boils down to
an expansion of this:

        memset(__ptr + offsetofend(typeof(*(obj)), member), __val,
               sizeof(*(obj)) - offsetofend(typeof(*(obj)), member));

into:

	memset(&txinfo->status + offsetofend(struct ieee80211_tx_info, rates),
	       0, sizeof(txinfo->status - offsetofend(struct ieee80211_tx_info, rates)))

Is offsetofend() broken?

-- 
Kees Cook