Re: [PATCH] net: avoid build bug in skb extension length calculation

["Arnd Bergmann" <arnd@arndb.de>]

On Mon, Dec 18, 2023, at 17:06, Thomas Weißschuh wrote:
> GCC seems to incorrectly fail to evaluate skb_ext_total_length() at
> compile time under certain conditions.
>
> The issue even occurs if all values in skb_ext_type_len[] are "0",
> ruling out the possibility of an actual overflow.
>
> As the patch has been in mainline since v6.6 without triggering the
> problem it seems to be a very uncommon occurrence.
>
> As the issue only occurs when -fno-tree-loop-im is specified as part of
> CFLAGS_GCOV, disable the BUILD_BUG_ON() only when building with coverage
> reporting enabled.
>
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: 
> https://lore.kernel.org/oe-kbuild-all/202312171924.4FozI5FG-lkp@intel.com/
> Suggested-by: Arnd Bergmann <arnd@arndb.de>
> Link: 
> https://lore.kernel.org/lkml/487cfd35-fe68-416f-9bfd-6bb417f98304@app.fastmail.com/
> Fixes: 5d21d0a65b57 ("net: generalize calculation of skb extensions 
> length")
> Cc:  <stable@vger.kernel.org>
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Acked-by: Arnd Bergmann <arnd@arndb.de>

> ---
>  net/core/skbuff.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 83af8aaeb893..94cc40a6f797 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4825,7 +4825,9 @@ static __always_inline unsigned int 
> skb_ext_total_length(void)
>  static void skb_extensions_init(void)
>  {
>  	BUILD_BUG_ON(SKB_EXT_NUM >= 8);
> +#if !IS_ENABLED(CONFIG_KCOV_INSTRUMENT_ALL)
>  	BUILD_BUG_ON(skb_ext_total_length() > 255);
> +#endif

The way I would write this is

BUILD_BUG_ON(!IS_ENABLED(CONFIG_KCOV_INSTRUMENT_ALL) &&
             skb_ext_total_length() > 255);

but of course the effect is the same.

     Arnd