linux-kernel - Re: [PATCH] perf: Fix oops when kthread execs user process

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Message-ID: <20190529162528.GB12420@fuggles.cambridge.arm.com>
Date:   Wed, 29 May 2019 17:25:28 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Young Xiao <92siuyang@...il.com>, linux@...linux.org.uk,
        mark.rutland@....com, mingo@...hat.com, bp@...en8.de,
        hpa@...or.com, x86@...nel.org, kan.liang@...ux.intel.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        ravi.bangoria@...ux.vnet.ibm.com, mpe@...erman.id.au,
        acme@...hat.com, eranian@...gle.com, fweisbec@...il.com,
        jolsa@...hat.com
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process

On Wed, May 29, 2019 at 06:19:55PM +0200, Peter Zijlstra wrote:
> On Wed, May 29, 2019 at 03:35:10PM +0100, Will Deacon wrote:
> > On Wed, May 29, 2019 at 03:25:15PM +0200, Peter Zijlstra wrote:
> > > On Wed, May 29, 2019 at 02:05:21PM +0100, Will Deacon wrote:
> > > > On Wed, May 29, 2019 at 02:55:57PM +0200, Peter Zijlstra wrote:
> > > 
> > > > >  	if (user_mode(regs)) {
> > > > 
> > > > Hmm, so it just occurred to me that Mark's observation is that the regs
> > > > can be junk in some cases. In which case, should we be checking for
> > > > kthreads first?
> > > 
> > > task_pt_regs() can return garbage, but @regs is the exception (or
> > > perf_arch_fetch_caller_regs()) regs, and for those user_mode() had
> > > better be correct.
> > 
> > So what should we report for the idle task?
> 
> If an interrupt hits the idle task, @regs would be !user_mode(regs),
> we'll find current->flags & PF_KTHREAD (idle not having passed through
> exec()) and therefore we'll take ABI_NONE for the user regs.
> 
> Or am I not getting it?

Sorry, I'm not trying to catch you out! Just trying to understand what the
semantics are supposed to be.

I do find the concept of user_mode(regs) bizarre for the idle task. By the
above, we definitely have a bug on arm64 (user_mode(regs) tends to be
true for the idle task), and I couldn't figure out how you avoided it on
x86. I guess it happens to work because the stack is zero-initialised or
something?

Will