Messages in this thread | | | Date | Thu, 23 Jul 2015 23:18:43 +0200 | From | Willy Tarreau <> | Subject | Re: Dealing with the NMI mess |
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On Thu, Jul 23, 2015 at 02:13:16PM -0700, Linus Torvalds wrote: > On Thu, Jul 23, 2015 at 1:52 PM, Willy Tarreau <w@1wt.eu> wrote: > > > > What's the worst case that can happen with RF cleared when returing > > to user space ? > > Not a good idea. We are fine breaking breakpoints on the kernel ("use > the tracing infrastructure instead"). Breaking it in user space is not > really an option.
But that wouldn't disable the breakpoint, just make it strike again, so the user would not be hurt.
> And we really don't need to. We'd only use 'ret' when returning to > kernel code. And not even for the usual case, only for the "interrupts > are off" case. If somebody tries to put a breakpoint on something > that is used in an irq-off situation, they are doing something very > specialized, and we cna tell them: "sorry, we had to break your use > case because it's crazy any other way". > > Those kind of people are by definition not "users". They are mucking > with kernel internals. Breaking them is not a regression. > > Btw, we should still ask Intel for that "fast iret that doesn't > re-enable NMI". So for possible future CPU's we might let people do > crazy things again.
I'm just thinking that there should be an option for this : task switching. You can store the EFLAGS in the TSS, so by preparing a dummy task with everything needed to emulate iret, we might be able to do it without the iret instruction. Or is this a stupid idea ? At least now I've well understood that ugliness is not an excuse for not proposing something :-)
Willy
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