Re: [PATCH v2] writeback: prefer struct_size over open coded arithmetic

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

On Sat, Sep 25, 2021 at 01:43:08PM +0200, 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 these are not actually dynamic sizes: all the operands
> involved in the calculation are constant values. However it is better to
> refactor them 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() functions.
> 
> This code was detected with the help of Coccinelle and audited and fixed
> manually.
> 
> [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>

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks
--
Gustavo

> ---
> Changelog v1 -> v2
> - Rebase against v5.15-rc2
> - Refactor another instance in the same file (Gustavo A. R. Silva).
> - Update the commit changelog to inform that this code was detected
>   using a Coccinelle script (Gustavo A. R. Silva).
> 
>  fs/fs-writeback.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 81ec192ce067..5eb0ada7468c 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -566,7 +566,7 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  	if (atomic_read(&isw_nr_in_flight) > WB_FRN_MAX_IN_FLIGHT)
>  		return;
> 
> -	isw = kzalloc(sizeof(*isw) + 2 * sizeof(struct inode *), GFP_ATOMIC);
> +	isw = kzalloc(struct_size(isw, inodes, 2), GFP_ATOMIC);
>  	if (!isw)
>  		return;
> 
> @@ -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);
>  	if (!isw)
>  		return restart;
> 
> --
> 2.25.1
>