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

[Laura Abbott]

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