Subject: Re: [PATCH] powerpc: Simplify access_ok()

From: David Laight <[email-masked]>
Date: Sun, 22 Mar 2026 11:03:36 +0000

On Tue, 17 Mar 2026 19:07:04 +0100
"Christophe Leroy (CS GROUP)" <[email-masked]> wrote:

> With the implementation of masked user access, we always have a memory
> gap between user memory space and kernel memory space, so use it to
> simplify access_ok() by relying on access fault in case of an access
> in the gap.
> 
> Most of the time the size is known at build time.
> 
> On powerpc64, the kernel space starts at 0x8000000000000000 which is
> always more than two times TASK_USER_MAX so when the size is known at
> build time and lower than TASK_USER_MAX, only the address needs to be
> verified. If not, a binary or of address and size must be lower than
> TASK_USER_MAX. As TASK_USER_MAX is a power of 2, just check that
> there is no bit set outside of TASK_USER_MAX - 1 mask.
> 
> On powerpc32, there is a garanteed gap of 128KB so when the size is
> known at build time and not greater than 128KB, just check that the
> address is below TASK_SIZE. Otherwise use the original formula.

Given that the whole thing relies on the kernel code 'obeying the rules'
is it enough to require that the accesses will be 'moderately sequential'?
Provided there are no jumps greater than 128k the length can be ignored.

I think Linus thought about doing that for x86-64.

I can't imagine that happening unless there is code that probes the end of
the user buffer before starting a transfer - and that is pretty pointless.

There are places that skip a few bytes (or just access in the wrong order)
but it is likely to be alignment padding, and code should be doing the
access_ok() check for each fragment - not on the entire buffer.

> 
> Signed-off-by: Christophe Leroy (CS GROUP) <[email-masked]>
> ---
>  arch/powerpc/include/asm/uaccess.h | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index 570b3d91e2e4..ec210ae62be7 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -15,8 +15,34 @@
>  #define TASK_SIZE_MAX		TASK_SIZE_USER64
>  #endif
>  
> +#define __access_ok __access_ok
> +
>  #include <asm-generic/access_ok.h>
>  
> +/*
> + * On powerpc64, TASK_SIZE_MAX is 0x0010000000000000 then even if both ptr and size
> + * are TASK_SIZE_MAX we are still inside the memory gap. So make it simple.
> + */
> +static __always_inline int __access_ok(const void __user *ptr, unsigned long size)
> +{
> +	unsigned long addr = (unsigned long)ptr;
> +
> +	if (IS_ENABLED(CONFIG_PPC64)) {
> +		BUILD_BUG_ON(!is_power_of_2(TASK_SIZE_MAX));
> +		BUILD_BUG_ON(TASK_SIZE_MAX > 0x0010000000000000);
> +
> +		if (__builtin_constant_p(size))
> +			return size <= TASK_SIZE_MAX && !(addr & ~(TASK_SIZE_MAX - 1));
> +		else
> +			return !((size | addr) & ~(TASK_SIZE_MAX - 1));

The compiler may know an upper bound for 'size' even when it isn't a constant.
It might be 32bit or from 'size = is_compat_foo ? 16 : 24', so:
		if (statically_true(size < TASK_SIZE_MAX)
			return !(addr & ~(TASK_SIZE_MAX - 1);
		return !((size | addr) & ~(TASK_SIZE_MAX - 1));

> +	} else {
> +		if (__builtin_constant_p(size) && size < SZ_128K)

Again the compiler may know an upper bound even if the value isn't constant:
		if (statically_true(size < SZ_128K)

	David

> +			return addr < TASK_SIZE;
> +		else
> +			return size <= TASK_SIZE && addr <= TASK_SIZE - size);
> +	}
> +}
> +
>  /*
>   * These are the main single-value transfer routines.  They automatically
>   * use the right size if we just have the right pointer type.