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Date:   Sun, 29 Mar 2020 19:16:27 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Laight <David.Laight@aculab.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [RFC][PATCH 01/22] x86 user stack frame reads: switch to
 explicit __get_user()
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On Sun, Mar 29, 2020 at 10:56:59AM -0700, Linus Torvalds wrote:

> But, if you have lots of performance-critical get_user() calls, just use
> 
>      if (user_access_begin(..))
>         goto efault;
> 
>      .. multiple "unsafe_get_user(x,ptr,efault);" ..
> 
>      user_access_end();
>      ...
> 
>   efault:
>      user_access_end();
>      return -EFAULT;
> 
> and be done with it.

Except that you'd better make that
	if (!user_access_begin(...))
		return -EFAULT;