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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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