linux-kernel - Re: [PATCH] x86/xen: Remove use of VLAs

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Message-ID: <18e8c47d-55f7-795a-053a-f667650b43b7@redhat.com>
Date:   Fri, 13 Apr 2018 20:43:02 -0700
From:   Laura Abbott <labbott@...hat.com>
To:     David Brown <david.brown@...aro.org>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] x86/xen: Remove use of VLAs

On 04/13/2018 07:55 PM, David Brown wrote:
> On Fri, Apr 13, 2018 at 03:11:46PM -0700, Laura Abbott wrote:
> 
>> There's an ongoing effort to remove VLAs[1] from the kernel to eventually
>> turn on -Wvla. The few VLAs in use have an upper bound based on a size
>> of 64K. This doesn't produce an excessively large stack so just switch
>> the upper bound.
>>
>> [1] https://lkml.org/lkml/2018/3/7/621
> 
> This comment is more in regards to many of these patches, and not as
> much this one specifically.
> 
> How confident are we in the upper bounds we're setting, and how
> obvious is it in the resulting code so that something does later
> change to overflow these bounds.
> 
> The danger here is that we're converting something a little easier to
> detect (a stack overflow), with something harder to detect
> (overflowing an array on the stack).
> 

Several people have remarked on that and the solution has been to
put in some kind of WARN and/or error check to make it obvious something
needs to be adjusted.

> I guess the question is twofold: how did you determine that 64K was
> the largest 'size' value, and how should reviewers verify this as
> well.  Perhaps this should at least be in the commit text so someone
> tracking down something with this code can find it later.
> 

It's not in the patch context but there's a large comment below:

         /*
          * A GDT can be up to 64k in size, which corresponds to 8192
          * 8-byte entries, or 16 4k pages..
          */

         BUG_ON(size > 65536);


Given the frames was calculated based off the size, that seemed
sufficient.

> David

Thanks,
Laura