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Date:   Tue, 28 May 2019 16:01:03 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     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,
        mpe@ellerman.id.au
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process
Message-ID: <20190528140103.GT2623@hirez.programming.kicks-ass.net>
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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?