linux-kernel - Re: [PATCH] crypto: algapi - fix be32_to_cpu macro call in crypto

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Date:   Thu, 17 Nov 2022 02:15:42 +0300
From:   Alexey Khoroshilov <khoroshilov@...ras.ru>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        lvc-project@...uxtesting.org
Subject: Re: [PATCH] crypto: algapi - fix be32_to_cpu macro call in
 crypto_inc()

On 16.11.2022 20:39, Eric Biggers wrote:
> On Wed, Nov 16, 2022 at 04:52:51PM +0300, Alexey Khoroshilov wrote:
>> be32_to_cpu() macro in some cases may be expanded to an expression
>> that evaluates its arguments multiple times. 
> 
> When is that, exactly?


I considered the path
#define be32_to_cpu __be32_to_cpu

#define __be32_to_cpu(x) __swab32((__force __u32)(__be32)(x))

#define __swab32(x)				\
	(__builtin_constant_p((__u32)(x)) ?	\
	___constant_swab32(x) :			\
	__fswab32(x))


#define ___constant_swab32(x) ((__u32)(				\
	(((__u32)(x) & (__u32)0x000000ffUL) << 24) |		\
	(((__u32)(x) & (__u32)0x0000ff00UL) <<  8) |		\
	(((__u32)(x) & (__u32)0x00ff0000UL) >>  8) |		\
	(((__u32)(x) & (__u32)0xff000000UL) >> 24)))


But you are right, it is protected by __builtin_constant_p() that
prevents that for memory access.


> If that's true, then lots of other places in the kernel would need to be fixed
> too.  Try running:
> 
> 	git grep -E '[bl]e(16|32|64)_to_cpu\([^)]+\+\+\)'
> 
> If true, then it would be much better to fix the macros.


And yes, anyway it makes sense to keep for be32_to_cpu()-like function
macro a contract to handle its arguments as a plain function.

Thanks,
Alexey