Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 29 May 2019 17:24:36 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Will Deacon <will.deacon@arm.com>,
        Young Xiao <92siuyang@gmail.com>, linux@armlinux.org.uk,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
        kan.liang@linux.intel.com, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, ravi.bangoria@linux.vnet.ibm.com,
        mpe@ellerman.id.au, acme@redhat.com, eranian@google.com,
        fweisbec@gmail.com, jolsa@redhat.com
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process
Message-ID: <20190529162435.GM31777@lakrids.cambridge.arm.com>
References: <20190528153224.GE20758@fuggles.cambridge.arm.com>
 <20190528173228.GW2623@hirez.programming.kicks-ass.net>
 <20190529091733.GA4485@fuggles.cambridge.arm.com>
 <20190529101042.GN2623@hirez.programming.kicks-ass.net>
 <20190529102022.GC4485@fuggles.cambridge.arm.com>
 <20190529125557.GU2623@hirez.programming.kicks-ass.net>
 <20190529130521.GA11023@fuggles.cambridge.arm.com>
 <20190529132515.GW2623@hirez.programming.kicks-ass.net>
 <20190529143510.GA11154@fuggles.cambridge.arm.com>
 <20190529161955.GZ2623@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190529161955.GZ2623@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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?

If the contents of task_pt_regs(current) is garbage, then the result of
user_mode(task_pt_regs(current)) is also garbage, no?

Thanks,
Mark.