Messages in this thread | | | From | Andy Lutomirski <> | Subject | Re: [PATCH v9 00/17] Enable FSGSBASE instructions | Date | Tue, 21 Apr 2020 13:21:39 -0700 |
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> On Apr 21, 2020, at 12:56 PM, Andi Kleen <ak@linux.intel.com> wrote: > > >> >> 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, I’m 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.
> > [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]
That’s 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?
> > For rr or criu we're also not aware of any case that could break. > > I honestly don't know what else could be done in due diligence. > > Also just to reiterate merging this would immediately shave off > hundreds of cycles in most context switches. > > -Andi
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