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Date:   Wed, 20 May 2020 13:13:24 +0200
From:   Christoph Hellwig <hch@....de>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, x86@...nel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-parisc@...r.kernel.org, linux-um@...ts.infradead.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/20] maccess: always use strict semantics for
 probe_kernel_read

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).

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