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

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Message-ID: <CALCETrXzi1fMBOAijwbPPLAVsAjoyhGEAXGqByp=Peehs8HeQQ@mail.gmail.com>
Date:	Sun, 8 Nov 2015 20:20:31 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.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 Sat, Nov 7, 2015 at 3:18 AM, Borislav Petkov <bp@...en8.de> wrote:
> 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.
>

Done for v2.

--Andy
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