Re: 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[]'}

[Kees Cook]

On Tue, Sep 19, 2023 at 07:27:26PM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   2cf0f715623872823a72e451243bbf555d10d032
> commit: df8fc4e934c12b906d08050d7779f292b9c5c6b5 kbuild: Enable -fstrict-flex-arrays=3
> date:   4 months ago
> config: loongarch-allmodconfig (https://download.01.org/0day-ci/archive/20230919/202309191958.UBw1cjXk-lkp@xxxxxxxxx/config)
> compiler: loongarch64-linux-gcc (GCC) 13.2.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20230919/202309191958.UBw1cjXk-lkp@xxxxxxxxx/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202309191958.UBw1cjXk-lkp@xxxxxxxxx/
> 
> All warnings (new ones prefixed by >>):
> 
>    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];
>          |                         ^~~~~~~~~

The .config has:
CONFIG_PAGE_SIZE_16KB=y
which makes PAGE_SHIFT == 14

#ifdef CONFIG_PAGE_SIZE_16KB
#define PAGE_SHIFT      14

ETH_JUMBO_MTU is:

#define ETH_JUMBO_MTU	9000

which forces "ETH_JUMBO_MTU >> PAGE_SHIFT" to be 0.

I think the right fix would be:

dma_addr_t      frag_addr[ETH_JUMBO_MTU >> PAGE_SHIFT ?: 1]

Thoughts?

-Kees

-- 
Kees Cook