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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Peter Zijlstra <peterz@infradead.org>,
        Young Xiao <92siuyang@gmail.com>
Cc:     will.deacon@arm.com, linux@armlinux.org.uk, mark.rutland@arm.com,
        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
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process
In-Reply-To: <20190528140103.GT2623@hirez.programming.kicks-ass.net>
References: <1559046689-24091-1-git-send-email-92siuyang@gmail.com> <20190528140103.GT2623@hirez.programming.kicks-ass.net>
Date:   Wed, 29 May 2019 11:44:02 +1000
Message-ID: <87a7f6ox0d.fsf@concordia.ellerman.id.au>
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Sender: linux-kernel-owner@vger.kernel.org
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Peter Zijlstra <peterz@infradead.org> writes:
> On Tue, May 28, 2019 at 08:31:29PM +0800, Young Xiao wrote:
>> When a kthread calls call_usermodehelper() the steps are:
>>   1. allocate current->mm
>>   2. load_elf_binary()
>>   3. populate current->thread.regs
>> 
>> While doing this, interrupts are not disabled. If there is a perf
>> interrupt in the middle of this process (i.e. step 1 has completed
>> but not yet reached to step 3) and if perf tries to read userspace
>> regs, kernel oops.
>> 
>> Fix it by setting abi to PERF_SAMPLE_REGS_ABI_NONE when userspace
>> pt_regs are not set.
>> 
>> See commit bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs
>> user process") for details.
>
> Why the hell do we set current->mm before it is complete? Note that
> normally exec() builds the new mm before attaching it, see exec_mmap()
> in flush_old_exec().
>
> Also, why did those PPC folks 'fix' this in isolation? And why didn't
> you Cc them?

We just assumed it was our bug, 'cause we have plenty of those :)

cheers