linux-kernel - Re: [PATCH] arm64: reenable interrupt when handling ptrace breakpoint

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Message-ID: <20151221170028.GT23092@arm.com>
Date:	Mon, 21 Dec 2015 17:00:29 +0000
From:	Will Deacon <will.deacon@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"Shi, Yang" <yang.shi@...aro.org>, Catalin.Marinas@....com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH] arm64: reenable interrupt when handling ptrace breakpoint

On Mon, Dec 21, 2015 at 05:51:22PM +0100, Thomas Gleixner wrote:
> On Mon, 21 Dec 2015, Will Deacon wrote:
> > +static void send_user_sigtrap(int si_code)
> > +{
> > +	struct pt_regs *regs = current_pt_regs();
> > +	siginfo_t info = {
> > +		.si_signo	= SIGTRAP,
> > +		.si_errno	= 0,
> > +		.si_code	= si_code,
> > +		.si_addr	= (void __user *)instruction_pointer(regs),
> > +	};
> > +
> > +	if (WARN_ON(!user_mode(regs)))
> > +		return;
> > +
> > +	preempt_disable();
> 
> That doesn't work on RT either. force_sig_info() takes task->sighand->siglock,
> which is a 'sleeping' spinlock on RT.

Ah, I missed that :/

> Why would we need to disable preemption here at all? What's the problem of
> being preempted or even migrated?

There *might* not be a problem, I'm just really nervous about changing
the behaviour on the debug path and subtly changing how ptrace behaves.

My worry was that you could somehow get back into the tracer, and it
could remove a software breakpoint in the knowledge that it wouldn't
see any future (spurious) SIGTRAPs for that location.

Without a concrete example, however, I guess I'll bite the bullet and
enable irqs across the call to force_sig_info, since there is clearly a
real issue here on RT.

Will
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