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 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. >> >>  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
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