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

From: Gustavo A. R. Silva
Date: Mon Sep 20 2021 - 20:19:55 EST




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@xxxxxxx>
> ---
> 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
>