linux-kernel - Re: [RFC][PATCH] x86: Verify access

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Date:   Wed, 18 Jan 2017 16:16:43 -0600
From:   David Smith <dsmith@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [RFC][PATCH] x86: Verify access_ok() context

On 01/16/2017 03:14 PM, Thomas Gleixner wrote:
> On Mon, 16 Jan 2017, David Smith wrote:
> 
>> If you call access_ok() with page faulting disabled, you'll still see
>> this new warning.
> 
> And how so? It's just checking for task context. page fault disable/enable
> has absolutely nothing to do with that.

True, task context and page fault disable/enable have nothing to do with each other. However, the access_ok() comment states:

 * Context: User context only. This function may sleep if pagefaults are        
 *          enabled.                                                            

That seems to indicate that the function won't sleep if pagefaults are disabled, and thus there is no need for a CONFIG_DEBUG_ATOMIC_SLEEP warning if pagefaults are disabled.

>> If you put that new access_ok() call in a module that gets
>> loaded/unloaded, you see one warning for every module load, which gets a
>> bit annoying.
> 
> Can you please elaborate where this access_ok() is placed in the module
> code?

It doesn't really matter where you place the access_ok() call in the module code. If you call access_ok() in a module, then that module has its own WARN_ON_ONCE() static variable. If access_ok() was a function exported from the kernel, then there would be only one copy of the WARN_ON_ONCE() static variable.

-- 
David Smith
dsmith@...hat.com
Red Hat
http://www.redhat.com
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