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

[Christian Lamparter]

On 6/23/23 17:23, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> 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@xxxxxxxx>


Wait! I want to point out this funny thing is happening in ath too!

https://lore.kernel.org/linux-wireless/TYAP286MB03154F9AAFD4C35BEEDE4A99BC4CA@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#mf1b8919a000fe661803c17073f48b3c410888541

And that patch got NACK by Jiri Slaby because like me he suspects that
this is a compiler bug.

so, what's going wrong with fortified there?

Thanks,
Christian