Re: [PATCH v9 00/17] Enable FSGSBASE instructions

[Andy Lutomirski]

On Tue, Apr 21, 2020 at 1:51 PM Sasha Levin <sashal@xxxxxxxxxx> wrote:
>
> On Tue, Apr 21, 2020 at 01:21:39PM -0700, Andy Lutomirski wrote:
> >
> >
> >> On Apr 21, 2020, at 12:56 PM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> >>
> >> ï
> >>>
> >>> Andi's point is that there is no known user it breaks, and the Intel
> >>> folks did some digging into potential users who might be affected by
> >>> this, including 'rr' brought up by Andy, and concluded that there won't
> >>> be breakage as a result of this patchset:
> >>>
> >>>    https://mail.mozilla.org/pipermail/rr-dev/2018-March/000616.html
> >>>
> >>> Sure, if you poke at it you could see a behavior change, but is there
> >>> an actual user that will be affected by it? I suspect not.
> >>
> >> Actually we don't know of any behavior changes caused by the kernel
> >> with selectors.
> >>
> >> The application can change itself of course, but only if it uses the
> >> new instructions, which no current application does.
> >
> >If you use ptrace to change the gs selector, the behavior is different on a patched kernel.
> >
> >Again, Iâm not saying that the change is problematic. But I will say that the fact that anyone involved in this series keeps ignoring this fact makes me quite uncomfortable with the patch set.
>
> That's what I referred to with "poke at it". While the behavior may be
> different, I fail to find anyone who cares.
>
> >>
> >> [This was different in the original patch kit long ago which could
> >> change behavior on context switch for programs with out of sync selectors,
> >> but this has been long fixed]
> >
> >Thatâs the issue I was referring to.
> >
> >>
> >> A debugger can also change behavior, but we're not aware of any case
> >> that it would break.
> >
> >How hard did you look?
>
> Come on, how does one respond to this?
>
> Is there a real use case affected by this? If so, point it out and I'll
> be happy to go test it. This was already done (per your previous
> request) for gdb and rr.
>

gdb and rr are certainly a good start.  If patches show up, I'll take a look.