Re: [PATCH 2/2] mfd: omap-usb-tll: use struct_size to allocate tll

[Lee Jones <lee@kernel.org>]

On Thu, 20 Jun 2024, Javier Carrasco wrote:

> Use the struct_size macro to calculate the size of the tll, which
> includes a trailing flexible array.
> 
> Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
> 
> ---
> The memory allocation used to be carried out in two steps:
> 
> tll = devm_kzalloc(dev, sizeof(struct usbtll_omap), GFP_KERNEL);
> tll->ch_clk = devm_kzalloc(dev, sizeof(struct clk *) * tll->nch,
>                            GFP_KERNEL);
> 
> Until commit 16c2004d9e4d ("mfd: omap-usb-tll: Allocate driver data at once")
> turned that into the current allocation:
> 
> tll = devm_kzalloc(dev, sizeof(*tll) + sizeof(tll->ch_clk[nch]),
>                    GFP_KERNEL);
> 
> That has surprised me at first glance because I would have expected
> sizeof(tll->ch_clk[nch]) to return the size of a single pointer, not
> being equivalent to 'sizeof(struct clk *) * nch'.
> 
> I might be missing/misunderstanding something here because the commit
> is not new, and the error should be noticeable. Moreover, I don't have
> real hardware to test it. Hence why I didn't mark this patch as a fix.
> 
> I would be pleased to get feedback about this (why it is right as it is,
> or if that is actually a bug).

You don't need this H/W to test this our for yourself.

Mock-up the structs in a user-space C-program and print out the sizes.

Please report them all to justify the patch.

> ---
>  drivers/mfd/omap-usb-tll.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/mfd/omap-usb-tll.c b/drivers/mfd/omap-usb-tll.c
> index a091e5b0f21d..5f25ac514ff2 100644
> --- a/drivers/mfd/omap-usb-tll.c
> +++ b/drivers/mfd/omap-usb-tll.c
> @@ -230,8 +230,7 @@ static int usbtll_omap_probe(struct platform_device *pdev)
>  		break;
>  	}
>  
> -	tll = devm_kzalloc(dev, sizeof(*tll) + sizeof(tll->ch_clk[nch]),
> -			   GFP_KERNEL);
> +	tll = devm_kzalloc(dev, struct_size(tll, ch_clk, nch), GFP_KERNEL);
>  	if (!tll) {
>  		pm_runtime_put_sync(dev);
>  		pm_runtime_disable(dev);
> 
> -- 
> 2.40.1
> 

-- 
Lee Jones [李琼斯]