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Date:   Wed, 17 Mar 2021 18:11:29 +0100
From:   Jann Horn <jannh@...gle.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        intel-wired-lan@...ts.osuosl.org,
        Network Development <netdev@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] ixgbe: Fix out-of-bounds warning in ixgbe_host_interface_command()

On Wed, Mar 17, 2021 at 8:43 AM Gustavo A. R. Silva
<gustavoars@...nel.org> wrote:
> Fix the following out-of-bounds warning by replacing the one-element
> array in an anonymous union with a pointer:
>
>   CC [M]  drivers/net/ethernet/intel/ixgbe/ixgbe_common.o
> drivers/net/ethernet/intel/ixgbe/ixgbe_common.c: In function ‘ixgbe_host_interface_command’:
> drivers/net/ethernet/intel/ixgbe/ixgbe_common.c:3729:13: warning: array subscript 1 is above array bounds of ‘u32[1]’ {aka ‘unsigned int[1]’} [-Warray-bounds]
>  3729 |   bp->u32arr[bi] = IXGBE_READ_REG_ARRAY(hw, IXGBE_FLEX_MNG, bi);
>       |   ~~~~~~~~~~^~~~
> drivers/net/ethernet/intel/ixgbe/ixgbe_common.c:3682:7: note: while referencing ‘u32arr’
>  3682 |   u32 u32arr[1];
>       |       ^~~~~~
>
> This helps with the ongoing efforts to globally enable -Warray-bounds.
>
> Notice that, the usual approach to fix these sorts of issues is to
> replace the one-element array with a flexible-array member. However,
> flexible arrays should not be used in unions. That, together with the
> fact that the array notation is not being affected in any ways, is why
> the pointer approach was chosen in this case.
>
> Link: https://github.com/KSPP/linux/issues/109
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_common.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_common.c
> index 62ddb452f862..bff3dc1af702 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_common.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_common.c
> @@ -3679,7 +3679,7 @@ s32 ixgbe_host_interface_command(struct ixgbe_hw *hw, void *buffer,
>         u32 hdr_size = sizeof(struct ixgbe_hic_hdr);
>         union {
>                 struct ixgbe_hic_hdr hdr;
> -               u32 u32arr[1];
> +               u32 *u32arr;
>         } *bp = buffer;
>         u16 buf_len, dword_len;
>         s32 status;

This looks bogus. An array is inline, a pointer points elsewhere -
they're not interchangeable.

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