linux-kernel - RE: [PATCH v2 02/18] uaccess: fix nios2 and microblaze get_user

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Date:   Wed, 16 Feb 2022 13:35:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
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Subject: RE: [PATCH v2 02/18] uaccess: fix nios2 and microblaze get_user_8()

From: Arnd Bergmann
> Sent: 16 February 2022 13:13
> 
> These two architectures implement 8-byte get_user() through
> a memcpy() into a four-byte variable, which won't fit.
> 
> Use a temporary 64-bit variable instead here, and use a double
> cast the way that risc-v and openrisc do to avoid compile-time
> warnings.
> 
...
>  	case 4:								\
> -		__get_user_asm("lw", (ptr), __gu_val, __gu_err);	\
> +		__get_user_asm("lw", (ptr), x, __gu_err);		\
>  		break;							\
> -	case 8:								\
> -		__gu_err = __copy_from_user(&__gu_val, ptr, 8);		\
> -		if (__gu_err)						\
> -			__gu_err = -EFAULT;				\
> +	case 8: {							\
> +		__u64 __x = 0;						\
> +		__gu_err = raw_copy_from_user(&__x, ptr, 8) ?		\
> +							-EFAULT : 0;	\
> +		(x) = (typeof(x))(typeof((x) - (x)))__x;		\
>  		break;							\

Wouldn't it be better to just fetch two 32bit values:
Something like (for LE - nios2 is definitely LE:
		__u32 val_lo, val_hi;
		__get_user_asm("lw", (ptr), val_lo, __gu_err);
		__get_user_asm("lw", (ptr) + 4, val_hi, __gu_err);
		x = val_lo | val_hi << 32;
		break;

	David

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