Re: [tip:x86/mm] x86, mm: Use a bitfield to mask nuisance get_user()warnings

[H. Peter Anvin]

On 02/11/2013 07:33 PM, Linus Torvalds wrote:
> On Mon, Feb 11, 2013 at 5:37 PM, tip-bot for H. Peter Anvin
> <hpa@xxxxxxxxx> wrote:
>>
>> However, we can declare a bitfield using sizeof(), which is legal
>> because sizeof() is a constant expression.  This quiets the warning,
>> although the code generated isn't 100% identical from the baseline
>> before 96477b4 x86-32: Add support for 64bit get_user():
> 
> Christ. This is so ugly that it's almost a work of art.

:)

> Has anybody run this past any gcc developers? And if so, did they run
> away screaming?

I haven't no... H.J., any comments on this patch?

	-hpa


> --- Begin Message ---
> Commit-ID:  b390784dc1649f6e6c5e66e5f53c21e715ccf39b
> Gitweb:     http://git.kernel.org/tip/b390784dc1649f6e6c5e66e5f53c21e715ccf39b
> Author:     H. Peter Anvin <hpa@xxxxxxxxx>
> AuthorDate: Mon, 11 Feb 2013 16:27:28 -0800
> Committer:  H. Peter Anvin <hpa@xxxxxxxxx>
> CommitDate: Mon, 11 Feb 2013 17:26:51 -0800
>
> x86, mm: Use a bitfield to mask nuisance get_user() warnings
>
> Even though it is never executed, gcc wants to warn for casting from
> a large integer to a pointer.  Furthermore, using a variable with
> __typeof__() doesn't work because __typeof__ retains storage
> specifiers (const, restrict, volatile).
>
> However, we can declare a bitfield using sizeof(), which is legal
> because sizeof() is a constant expression.  This quiets the warning,
> although the code generated isn't 100% identical from the baseline
> before 96477b4 x86-32: Add support for 64bit get_user():
>
> [x86-mb is baseline, x86-mm is this commit]
>
>    text      data        bss     filename
> 113716147  15858380   35037184   tip.x86-mb/o.i386-allconfig/vmlinux
> 113716145  15858380   35037184   tip.x86-mm/o.i386-allconfig/vmlinux
>  12989837   3597944   12255232   tip.x86-mb/o.i386-modconfig/vmlinux
>  12989831   3597944   12255232   tip.x86-mm/o.i386-modconfig/vmlinux
>   1462784    237608    1401988   tip.x86-mb/o.i386-noconfig/vmlinux
>   1462837    237608    1401964   tip.x86-mm/o.i386-noconfig/vmlinux
>   7938994    553688    7639040   tip.x86-mb/o.i386-pae/vmlinux
>   7943136    557784    7639040   tip.x86-mm/o.i386-pae/vmlinux
>   7186126    510572    6574080   tip.x86-mb/o.i386/vmlinux
>   7186124    510572    6574080   tip.x86-mm/o.i386/vmlinux
> 103747269  33578856   65888256   tip.x86-mb/o.x86_64-allconfig/vmlinux
> 103746949  33578856   65888256   tip.x86-mm/o.x86_64-allconfig/vmlinux
>  12116695  11035832   20160512   tip.x86-mb/o.x86_64-modconfig/vmlinux
>  12116567  11035832   20160512   tip.x86-mm/o.x86_64-modconfig/vmlinux
>   1700790    380524     511808   tip.x86-mb/o.x86_64-noconfig/vmlinux
>   1700790    380524     511808   tip.x86-mm/o.x86_64-noconfig/vmlinux
>  12413612   1133376    1101824   tip.x86-mb/o.x86_64/vmlinux
>  12413484   1133376    1101824   tip.x86-mm/o.x86_64/vmlinux
>
> Cc: Jamie Lokier <jamie@xxxxxxxxxxxxx>
> Cc: Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/20130209110031.GA17833@xxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxx>
> ---
>  arch/x86/include/asm/uaccess.h | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 1e96326..a8d1265 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -168,31 +168,29 @@ do {						      \
>  #define get_user(x, ptr)						\
>  ({									\
>  	int __ret_gu;							\
> -	unsigned long __val_gu;						\
> -	unsigned long long __val_gu8;					\
> +	struct {							\
> +		unsigned long long __val_n : 8*sizeof(*(ptr));		\
> +	} __val_gu;							\
>  	__chk_user_ptr(ptr);						\
>  	might_fault();							\
>  	switch (sizeof(*(ptr))) {					\
>  	case 1:								\
> -		__get_user_x(1, __ret_gu, __val_gu, ptr);		\
> +		__get_user_x(1, __ret_gu, __val_gu.__val_n, ptr);	\
>  		break;							\
>  	case 2:								\
> -		__get_user_x(2, __ret_gu, __val_gu, ptr);		\
> +		__get_user_x(2, __ret_gu, __val_gu.__val_n, ptr);	\
>  		break;							\
>  	case 4:								\
> -		__get_user_x(4, __ret_gu, __val_gu, ptr);		\
> +		__get_user_x(4, __ret_gu, __val_gu.__val_n, ptr);	\
>  		break;							\
>  	case 8:								\
> -		__get_user_8(__ret_gu, __val_gu8, ptr);			\
> +		__get_user_8(__ret_gu, __val_gu.__val_n, ptr);		\
>  		break;							\
>  	default:							\
> -		__get_user_x(X, __ret_gu, __val_gu, ptr);		\
> +		__get_user_x(X, __ret_gu, __val_gu.__val_n, ptr);	\
>  		break;							\
>  	}								\
> -	if (sizeof(*(ptr)) == 8)					\
> -		(x) = (__typeof__(*(ptr)))__val_gu8;			\
> -	else								\
> -		(x) = (__typeof__(*(ptr)))__val_gu;			\
> +	(x) = (__typeof__(*(ptr)))__val_gu.__val_n;			\
>  	__ret_gu;							\
>  })
>  
>
> --- End Message ---