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

From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@kernel.org>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	"Brian Gerst" <brgerst@gmail.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Frédéric Weisbecker" <fweisbec@gmail.com>,
	"Peter Zijlstra" <peterz@infradead.org>
Subject: Re: [PATCH 1/4] x86/entry/64: Fix irqflag tracing wrt context tracking
Date: Sat, 7 Nov 2015 12:18:24 +0100	[thread overview]
Message-ID: <20151107111824.GA6137@pd.tnic> (raw)
In-Reply-To: <f3b6b3003f7a2d3304763394160dc7e32ff70d6c.1446849780.git.luto@kernel.org>

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.

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