netdev - Re: [PATCH net-next 1/2] random32: improvements to prandom

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Message-Id: <20140731.222804.1403723639172562683.davem@davemloft.net>
Date:	Thu, 31 Jul 2014 22:28:04 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dborkman@...hat.com
Cc:	netdev@...r.kernel.org, hannes@...essinduktion.org
Subject: Re: [PATCH net-next 1/2] random32: improvements to prandom_bytes

From: Daniel Borkmann <dborkman@...hat.com>
Date: Thu, 31 Jul 2014 22:11:16 +0200

> -		for (j = 0; j < sizeof(u32); j++) {
> -			p[i + j] = random;
> -			random >>= BITS_PER_BYTE;
> -		}
> +	while (bytes > sizeof(u32)) {
> +		put_unaligned(prandom_u32_state(state), (u32 *) ptr);
> +		ptr += sizeof(u32);
> +		bytes -= sizeof(u32);
>  	}
> -	if (i < bytes) {
> -		u32 random = prandom_u32_state(state);
>  
> -		for (; i < bytes; i++) {
> -			p[i] = random;
> -			random >>= BITS_PER_BYTE;
> -		}
> +	if (bytes > 0) {
> +		u32 rem = prandom_u32_state(state);
> +		do {
> +			*ptr++ = (u8) rem;
> +			bytes--;
> +			rem >>= BITS_PER_BYTE;
> +		} while (bytes > 0);
>  	}

This conversion to put_unaligned() is not an equivalent depending
upon the endianness of the cpu.

And this means the random value gets distributed differently into
full words than it will into trailing bytes.

Let's just not mess around with this, ok?

Thanks.
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