Subject: Re: [PATCH 13/20] maccess: always use strict semantics for probe_kernel_read

From: Christoph Hellwig <[email-masked]>
Date: Wed, 20 May 2020 13:13:24 +0200

On Wed, May 20, 2020 at 08:11:26PM +0900, Masami Hiramatsu wrote:
> > -		ret = probe_kernel_read(&c, (u8 *)addr + len, 1);
> > +		if (IS_ENABLED(CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE) &&
> > +		    (unsigned long)addr < TASK_SIZE) {
> > +			ret = probe_user_read(&c,
> > +				(__force u8 __user *)addr + len, 1);
> > +		} else {
> > +			ret = probe_kernel_read(&c, (u8 *)addr + len, 1);
> > +		}
> >  		len++;
> >  	} while (c && ret == 0 && len < MAX_STRING_SIZE);
> 
> To avoid redundant check in the loop, we can use strnlen_user_nofault() out of
> the loop. Something like below.

Yes, I've done something very similar in response to Linus' comment (just
using an ifdef instead).