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From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Young Xiao <92siuyang@gmail.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
Date: Tue, 28 May 2019 16:32:24 +0100	[thread overview]
Message-ID: <20190528153224.GE20758@fuggles.cambridge.arm.com> (raw)
In-Reply-To: <20190528140103.GT2623@hirez.programming.kicks-ass.net>

On Tue, May 28, 2019 at 04:01:03PM +0200, Peter Zijlstra wrote:
> 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.

This seems to be because pt_regs(current) gives NULL for kthreads on Power.

> > 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().

From the initial report [1], it doesn't look like the mm isn't initialised,
but rather than we're dereferencing a NULL pt_regs pointer somehow for the
current task (see previous comment). I don't see how that can happen on
arm64, given that we put the pt_regs on the kernel stack which is allocated
during fork.

Will

[1] https://git.kernel.org/linus/bf05fc25f268

  reply	other threads:[~2019-05-28 15:32 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-28 12:31 [PATCH] perf: Fix oops when kthread execs user process Young Xiao
2019-05-28 12:41 ` Russell King - ARM Linux admin
2019-05-28 14:01 ` Peter Zijlstra
2019-05-28 15:32   ` Will Deacon [this message]
2019-05-28 16:12     ` Mark Rutland
2019-05-28 17:32     ` Peter Zijlstra
2019-05-29  9:17       ` Will Deacon
2019-05-29 10:10         ` Peter Zijlstra
2019-05-29 10:20           ` Will Deacon
2019-05-29 12:55             ` Peter Zijlstra
2019-05-29 13:05               ` Will Deacon
2019-05-29 13:25                 ` Peter Zijlstra
2019-05-29 14:35                   ` Will Deacon
2019-05-29 16:19                     ` Peter Zijlstra
2019-05-29 16:24                       ` Mark Rutland
2019-05-29 16:38                         ` Mark Rutland
2019-05-29 17:03                           ` Peter Zijlstra
2019-05-30 10:35                             ` Mark Rutland
2019-05-29 16:25                       ` Will Deacon
2019-05-29 16:44                         ` Peter Zijlstra
2019-05-30  7:28                           ` Will Deacon
2019-05-30  8:38               ` Ravi Bangoria
2019-05-30 10:27                 ` Ravi Bangoria
2019-05-31 15:37                   ` Will Deacon
2019-06-03 11:23                     ` Will Deacon
2019-06-03 11:48                     ` Peter Zijlstra
2019-06-03 13:30                     ` Michael Ellerman
2019-05-29 10:11       ` Mark Rutland
2019-05-29  4:21     ` Michael Ellerman
2019-05-29  1:44   ` Michael Ellerman

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