Re: [PATCH 1/1] arm64: Simplify __range_ok

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Mar 20, 2020 at 10:13:52PM -0700, Richard Henderson wrote:
> The general case is not quite as compact as the inline assembly,
> but with a sufficiently advanced compiler it is only 6 insns vs 5.
> 
> The real improvement comes from assuming that limit is never tiny,
> and using __builtin_constant_p to make sure the constant folding
> does not go awry.  This produces a 2 insn sequence even for older
> compilers.

Neat; thanks for putting this together!

Do you happen to have numbers for the impact on a defconfig Image size
(or vmlinux .text size)?

> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  arch/arm64/include/asm/uaccess.h | 31 +++++++++++++------------------
>  1 file changed, 13 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 32fc8061aa76..683727696dc3 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -60,7 +60,8 @@ static inline void set_fs(mm_segment_t fs)
>   */
>  static inline unsigned long __range_ok(const void __user *addr, unsigned long size)
>  {
> -	unsigned long ret, limit = current_thread_info()->addr_limit;
> +	unsigned long limit = current_thread_info()->addr_limit;
> +	unsigned long iaddr;

Trivial: could we move the initialisation here, please?

>  
>  	/*
>  	 * Asynchronous I/O running in a kernel thread does not have the
> @@ -72,24 +73,18 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
>  		addr = untagged_addr(addr);
>  
>  	__chk_user_ptr(addr);
> -	asm volatile(
> -	// A + B <= C + 1 for all A,B,C, in four easy steps:
> -	// 1: X = A + B; X' = X % 2^64
> -	"	adds	%0, %3, %2\n"
> -	// 2: Set C = 0 if X > 2^64, to guarantee X' > C in step 4
> -	"	csel	%1, xzr, %1, hi\n"
> -	// 3: Set X' = ~0 if X >= 2^64. For X == 2^64, this decrements X'
> -	//    to compensate for the carry flag being set in step 4. For
> -	//    X > 2^64, X' merely has to remain nonzero, which it does.
> -	"	csinv	%0, %0, xzr, cc\n"
> -	// 4: For X < 2^64, this gives us X' - C - 1 <= 0, where the -1
> -	//    comes from the carry in being clear. Otherwise, we are
> -	//    testing X' - C == 0, subject to the previous adjustments.
> -	"	sbcs	xzr, %0, %1\n"
> -	"	cset	%0, ls\n"
> -	: "=&r" (ret), "+r" (limit) : "Ir" (size), "0" (addr) : "cc");
>  
> -	return ret;
> +	/*
> +	 * Quite a lot of range checks use sizeof(some_type), and are
> +	 * therefore constant.  If we can assume that limit is never unusably
> +	 * small, then we can rearrange the computation to avoid the need for
> +	 * 65-bit arithmetic.  Arbitrary choice for size limit of 1MiB.
> +	 */
> +	iaddr = (unsigned long)addr;
> +	if (__builtin_constant_p(size) && size > 0 && size < 0x100000)
> +		return iaddr <= limit + 1 - size;

The limit should be either USER_DS or KERNEL_DS, where USER_DS is
smaller than KERNEL_DS, so we could derive a less arbitrary bound from
USER_DS.

Thanks,
Mark.

> +
> +	return (__uint128_t)iaddr + size <= (__uint128_t)limit + 1;
>  }
>  
>  #define access_ok(addr, size)	__range_ok(addr, size)
> -- 
> 2.20.1
> 

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