Subject: Re: [PATCH] sky2: Make sure there is at least one frag_addr available

From: Stephen Hemminger <[email-masked]>
Date: Wed, 20 Sep 2023 20:37:38 -0700

On Wed, 20 Sep 2023 13:25:13 -0700
Kees Cook <[email-masked]> wrote:

> In the likely pathological case of building sky2 with 16k PAGE_SIZE,
> make sure there is at least 1 frag_addr in struct rx_ring_info:
> 
>    In file included from include/linux/skbuff.h:28,
>                     from include/net/net_namespace.h:43,
>                     from include/linux/netdevice.h:38,
>                     from drivers/net/ethernet/marvell/sky2.c:18:
>    drivers/net/ethernet/marvell/sky2.c: In function 'sky2_rx_unmap_skb':
>    include/linux/dma-mapping.h:416:36: warning: array subscript i is outside array bounds of 'dma_addr_t[0]' {aka 'long long unsigned int[]'} [-Warray-bounds=]
>      416 | #define dma_unmap_page(d, a, s, r) dma_unmap_page_attrs(d, a, s, r, 0)
>          |                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>    drivers/net/ethernet/marvell/sky2.c:1257:17: note: in expansion of macro 'dma_unmap_page'
>     1257 |                 dma_unmap_page(&pdev->dev, re->frag_addr[i],
>          |                 ^~~~~~~~~~~~~~
>    In file included from drivers/net/ethernet/marvell/sky2.c:41:
>    drivers/net/ethernet/marvell/sky2.h:2198:25: note: while referencing 'frag_addr'
>     2198 |         dma_addr_t      frag_addr[ETH_JUMBO_MTU >> PAGE_SHIFT];
>          |                         ^~~~~~~~~
> 
> With CONFIG_PAGE_SIZE_16KB=y, PAGE_SHIFT == 14, so:
> 
>   #define ETH_JUMBO_MTU   9000
> 
> causes "ETH_JUMBO_MTU >> PAGE_SHIFT" to be 0. Use "?: 1" to solve this build warning.
> 
> Cc: Mirko Lindner <[email-masked]>
> Cc: Stephen Hemminger <[email-masked]>
> Cc: "David S. Miller" <[email-masked]>
> Cc: Eric Dumazet <[email-masked]>
> Cc: Jakub Kicinski <[email-masked]>
> Cc: Paolo Abeni <[email-masked]>
> Cc: [email-masked]
> Reported-by: kernel test robot <[email-masked]>
> Closes: https://lore.kernel.org/oe-kbuild-all/[email-masked]/
> Signed-off-by: Kees Cook <[email-masked]>
> ---

With page size of 16K the frag_addr[] array would never be used, so the original
code was correct that size should be 0. But the compiler now gets upset with 0
size arrays thinking this is some flex array leftover? Or can't figure out that
in this case an rx skb with fragments would never be created.

The workaround is fine, but could you add an explanatory comment?