netdev - Re: [PATCH 1/3] s390/vmlogrdr: Remove function pointer cast

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Message-ID: <20240418095438.6056-A-hca@linux.ibm.com>
Date: Thu, 18 Apr 2024 11:54:38 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: akpm@...ux-foundation.org, arnd@...db.de, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        wintera@...ux.ibm.com, twinkler@...ux.ibm.com,
        linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        llvm@...ts.linux.dev, patches@...ts.linux.dev
Subject: Re: [PATCH 1/3] s390/vmlogrdr: Remove function pointer cast

On Wed, Apr 17, 2024 at 11:24:35AM -0700, Nathan Chancellor wrote:
> Clang warns (or errors with CONFIG_WERROR) after enabling
> -Wcast-function-type-strict by default:
> 
>   drivers/s390/char/vmlogrdr.c:746:18: error: cast from 'void (*)(const void *)' to 'void (*)(struct device *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
>     746 |                 dev->release = (void (*)(struct device *))kfree;
>         |                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   1 error generated.
> 
> Add a standalone function to fix the warning properly, which addresses
> the root of the warning that these casts are not safe for kCFI. The
> comment is not really relevant after this change, so remove it.
> 
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> ---
>  drivers/s390/char/vmlogrdr.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)

> @@ -736,14 +740,7 @@ static int vmlogrdr_register_device(struct vmlogrdr_priv_t *priv)
>  		dev->driver = &vmlogrdr_driver;
>  		dev->groups = vmlogrdr_attr_groups;
>  		dev_set_drvdata(dev, priv);
> -		/*
> -		 * The release function could be called after the
> -		 * module has been unloaded. It's _only_ task is to
> -		 * free the struct. Therefore, we specify kfree()
> -		 * directly here. (Probably a little bit obfuscating
> -		 * but legitime ...).
> -		 */

Why is the comment not relevant after this change? Or better: why is it not
valid before this change, which is why the code was introduced a very long
time ago? Any reference?

I've seen the warning since quite some time, but didn't change the code
before sure that this doesn't introduce the bug described in the comment.