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Subject: Re: [PATCH] writeback: prefer struct_size over open coded arithmetic
To:     Len Baker <len.baker@gmx.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Cc:     Kees Cook <keescook@chromium.org>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-hardening@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20210919094630.30668-1-len.baker@gmx.com>
From:   "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Message-ID: <6bc45138-c3a5-9465-40cb-3db714e7706d@embeddedor.com>
Date:   Mon, 20 Sep 2021 19:17:36 -0500
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On 9/19/21 04:46, Len Baker wrote:
> As noted in the "Deprecated Interfaces, Language Features, Attributes,
> and Conventions" documentation [1], size calculations (especially
> multiplication) should not be performed in memory allocator (or similar)
> function arguments due to the risk of them overflowing. This could lead
> to values wrapping around and a smaller allocation being made than the
> caller was expecting. Using those allocations could lead to linear
> overflows of heap memory and other misbehaviors.
> 
> In this case this is not actually dynamic size: all the operands
> involved in the calculation are constant values. However it is best to
> refactor this anyway, just to keep the open-coded math idiom out of
> code.
> 
> So, use the struct_size() helper to do the arithmetic instead of the
> argument "size + count * size" in the kzalloc() function.
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
> 
> Signed-off-by: Len Baker <len.baker@gmx.com>
> ---
>  fs/fs-writeback.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 81ec192ce067..f7abff31e026 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -624,8 +624,8 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
>  	int nr;
>  	bool restart = false;
> 
> -	isw = kzalloc(sizeof(*isw) + WB_MAX_INODES_PER_ISW *
> -		      sizeof(struct inode *), GFP_KERNEL);
> +	isw = kzalloc(struct_size(isw, inodes, WB_MAX_INODES_PER_ISW),
> +		      GFP_KERNEL);


There is another instance at:

 569         isw = kzalloc(sizeof(*isw) + 2 * sizeof(struct inode *), GFP_ATOMIC);
 570         if (!isw)
 571                 return;

If you are finding these with the help of Coccinelle, please mention it
in the changelog text. :)

Thanks
--
Gustavo

>  	if (!isw)
>  		return restart;
> 
> --
> 2.25.1
>