linux-kernel - Re: [PATCH][next] bcachefs: Use DECLARE_FLEX_ARRAY() helper and fix multiple -Warray-bounds warnings

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Message-ID: <202311061607.FFD1D68@keescook>
Date:   Mon, 6 Nov 2023 16:08:00 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Kent Overstreet <kent.overstreet@...ux.dev>,
        Brian Foster <bfoster@...hat.com>,
        linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] bcachefs: Use DECLARE_FLEX_ARRAY() helper and fix
 multiple -Warray-bounds warnings

On Mon, Nov 06, 2023 at 04:27:02PM -0600, Gustavo A. R. Silva wrote:
> Transform zero-length array `s` into a proper flexible-array
> member in `struct snapshot_table` via the DECLARE_FLEX_ARRAY()
> helper; and fix tons of the following -Warray-bounds warnings:
> 
> fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
> fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
> fs/bcachefs/snapshot.c:135:70: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
> fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
> fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
> fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
> 
> This helps with the ongoing efforts to globally enable -Warray-bounds.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  fs/bcachefs/subvolume_types.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/bcachefs/subvolume_types.h b/fs/bcachefs/subvolume_types.h
> index 86833445af20..2d2e66a4e468 100644
> --- a/fs/bcachefs/subvolume_types.h
> +++ b/fs/bcachefs/subvolume_types.h
> @@ -20,7 +20,7 @@ struct snapshot_t {
>  };
>  
>  struct snapshot_table {
> -	struct snapshot_t	s[0];
> +	DECLARE_FLEX_ARRAY(struct snapshot_t, s);
>  };

Yup, this is the current way forward for 0-to-flex transformations in
unions or alone in structs (until it's supported correctly by GCC and
Clang, which is in progress).

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

-- 
Kees Cook