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

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Message-ID: <20151107111824.GA6137@pd.tnic>
Date:	Sat, 7 Nov 2015 12:18:24 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/4] x86/entry/64: Fix irqflag tracing wrt context
 tracking

On Fri, Nov 06, 2015 at 03:12:43PM -0800, 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.

I would not mind to have that explanation in the code itself so that
people don't scratch heads why the duplicated TRACE_IRQS_OFF call.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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