Re: [PATCHv4 1/5] x86/mm: split arch_mmap_rnd() on compat/native versions

[Borislav Petkov <bp@alien8.de>]

On Mon, Jan 30, 2017 at 03:04:28PM +0300, Dmitry Safonov wrote:
> I need those arch_{native,compat}_rnd() to compute separately
> random factor for mmap() in compat syscalls for 64-bit binaries
> and vice-versa for native syscall in 32-bit compat binaries.
> They will be used in the following patches.
> 
> Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
> ---
>  arch/x86/mm/mmap.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index d2dc0438d654..42063e787717 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -65,20 +65,27 @@ static int mmap_is_legacy(void)
>  	return sysctl_legacy_va_layout;
>  }
>  
> -unsigned long arch_mmap_rnd(void)
> +#ifdef CONFIG_COMPAT
> +static unsigned long arch_compat_rnd(void)
>  {
> -	unsigned long rnd;
> +	return (get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1))
> +		<< PAGE_SHIFT;
> +}
> +#endif
>  
> -	if (mmap_is_ia32())
> +static unsigned long arch_native_rnd(void)
> +{
> +	return (get_random_long() & ((1UL << mmap_rnd_bits) - 1)) << PAGE_SHIFT;
> +}
> +
> +unsigned long arch_mmap_rnd(void)
> +{
>  #ifdef CONFIG_COMPAT
> -		rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
> -#else
> -		rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
> +	if (mmap_is_ia32())
> +		return arch_compat_rnd();
>  #endif

I can't say that I'm thrilled about the ifdeffery this is adding.

But I can't think of a cleaner approach at a quick glance, though -
that's generic and arch-specific code intertwined muck. Sad face.

-- 
Regards/Gruss,
    Boris.

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