linux-kernel - Re: [RFC PATCH 1/4] net: introduce get_optlen() and put

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Message-ID: <Z+vZRcbvh6r1fnZL@gmail.com>
Date: Tue, 1 Apr 2025 05:17:09 -0700
From: Breno Leitao <leitao@...ian.org>
To: Stefan Metzmacher <metze@...ba.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
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Subject: Re: [RFC PATCH 1/4] net: introduce get_optlen() and put_optlen()
 helpers

Hello Stefan,

On Mon, Mar 31, 2025 at 10:10:53PM +0200, Stefan Metzmacher wrote:
> --- a/include/linux/sockptr.h
> +++ b/include/linux/sockptr.h
> @@ -169,4 +169,26 @@ static inline int check_zeroed_sockptr(sockptr_t src, size_t offset,
>  	return memchr_inv(src.kernel + offset, 0, size) == NULL;
>  }
>  
> +#define __check_optlen_t(__optlen)				\
> +({								\
> +	int __user *__ptr __maybe_unused = __optlen; 		\
> +	BUILD_BUG_ON(sizeof(*(__ptr)) != sizeof(int));		\
> +})

I am a bit confused about this macro. I understand that this macro's
goal is to check that __optlen is a pointer to an integer, otherwise
failed to build.

It is unclear to me if that is what it does. Let's suppose that __optlen
is not an integer pointer. Then:

> int __user *__ptr __maybe_unused = __optlen;

This will generate a compile failure/warning due invalid casting,
depending on -Wincompatible-pointer-types.

> BUILD_BUG_ON(sizeof(*(__ptr)) != sizeof(int));

Then this comparison will always false, since __ptr is a pointer to int,
and you are comparing the size of its content with the sizeof(int).