Re: [PATCH 2/2] Fix efi_call

Alex Thorlton
2016-05-16T16:24:55Z

On Thu, May 12, 2016 at 08:48:35AM +0200, Ingo Molnar wrote:
> I suppose the SGI/UV code is the only one using 7 arguments or more? Might make
> sense to point that out in the changelog.

First off, to everybody, sorry for the delayed responses. I've been
AFK for a few days and forgot to set my vacation notice :(

Yes, I believe that's it. I didn't do a full audit, but a quick glance
at the other users of this call showed that nobody else appears to be
using that many args.

> Just curious, how did you find this bug? It's a pretty obscure one, of the
> 'developer tears out hairs from frustruation' type ...

Yes, this one was a real puzzle to figure out. Basically I just stepped
through the assembly code from a known good point to see how we ended up
where we did. I quite a bit of help from the vets around here, as well
as from our simulator that I used to step through our early boot code to
find the problem.

The real hair pulling mostly came from trying to figure out *WHY* we
were putting the return address in this seemingly random spot on the
stack. After thoroughly re-reading assorted Intel (et. al.) docs about
a hundred times, I was able to piece together what I thought was
supposed to be going on here. The solution may be simple, but arriving
there was anything but that :)

- Alex