Re: [PATCH 1/4] x86/entry/64: Fix irqflag tracing wrt context tracking

[Thomas Gleixner]

On Fri, 6 Nov 2015, Andy Lutomirski wrote:
> Paolo pointed out that enter_from_user_mode could be called while
> irqflags were traced as though IRQs were on.
> 
> In principle, this could confuse lockdep.  It doesn't cause any
> problems that I've seen in any configuration, but if I build with
> CONFIG_DEBUG_LOCKDEP=y, enable a nohz_full CPU, and add code like:
> 
> 	if (irqs_disabled()) {
> 		spin_lock(&something);
> 		spin_unlock(&something);
> 	}
> 
> to the top of enter_from_user_mode, then lockdep will complain
> without this fix.  It seems that lockdep's irqflags sanity checks
> are too weak to detect this bug without forcing the issue.
> 
> This patch adds one byte to normal kernels, and it's IMO a bit ugly.
> I haven't spotted a better way to do this yet, though.  The issue is
> that we can't do TRACE_IRQS_OFF until after SWAPGS (if needed), but
> we're also supposed to do it before calling C code.
> 
> An alternative approach would be to call trace_hardirqs_off in
> enter_from_user_mode.  That would be less code and would not bloat
> normal kernels at all, but it would be harder to see how the code
> worked.
> 
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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