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From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
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 19:32:28 +0200	[thread overview]
Message-ID: <20190528173228.GW2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190528153224.GE20758@fuggles.cambridge.arm.com>

On Tue, May 28, 2019 at 04:32:24PM +0100, Will Deacon wrote:
> 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.

'funny' thing that, perf_sample_regs_user() seems to assume that
anything with current->mm is in fact a user task, and that assumption is
just plain wrong, consider use_mm().

So I'm thinking the right thing to do here is something like the below;
umh should get PF_KTHREAD cleared when it passes exec(). And this should
also fix the power splat I'm thinking.

---

diff --git a/kernel/events/core.c b/kernel/events/core.c
index abbd4b3b96c2..9929404b6eb9 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5923,7 +5923,7 @@ static void perf_sample_regs_user(struct perf_regs *regs_user,
 	if (user_mode(regs)) {
 		regs_user->abi = perf_reg_abi(current);
 		regs_user->regs = regs;
-	} else if (current->mm) {
+	} else if (!(current->flags & PF_KTHREAD) && current->mm) {
 		perf_get_regs_user(regs_user, regs, regs_user_copy);
 	} else {
 		regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;

WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: mark.rutland@arm.com, Young Xiao <92siuyang@gmail.com>,
	mpe@ellerman.id.au, x86@kernel.org, linux@armlinux.org.uk,
	linux-kernel@vger.kernel.org, mingo@redhat.com, bp@alien8.de,
	hpa@zytor.com, ravi.bangoria@linux.vnet.ibm.com,
	linux-arm-kernel@lists.infradead.org, kan.liang@linux.intel.com
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process
Date: Tue, 28 May 2019 19:32:28 +0200	[thread overview]
Message-ID: <20190528173228.GW2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190528153224.GE20758@fuggles.cambridge.arm.com>

On Tue, May 28, 2019 at 04:32:24PM +0100, Will Deacon wrote:
> 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.

'funny' thing that, perf_sample_regs_user() seems to assume that
anything with current->mm is in fact a user task, and that assumption is
just plain wrong, consider use_mm().

So I'm thinking the right thing to do here is something like the below;
umh should get PF_KTHREAD cleared when it passes exec(). And this should
also fix the power splat I'm thinking.

---

diff --git a/kernel/events/core.c b/kernel/events/core.c
index abbd4b3b96c2..9929404b6eb9 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5923,7 +5923,7 @@ static void perf_sample_regs_user(struct perf_regs *regs_user,
 	if (user_mode(regs)) {
 		regs_user->abi = perf_reg_abi(current);
 		regs_user->regs = regs;
-	} else if (current->mm) {
+	} else if (!(current->flags & PF_KTHREAD) && current->mm) {
 		perf_get_regs_user(regs_user, regs, regs_user_copy);
 	} else {
 		regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;

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  parent reply	other threads:[~2019-05-28 17:32 UTC|newest]

Thread overview: 60+ 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 12:31 ` Young Xiao
2019-05-28 12:41 ` Russell King - ARM Linux admin
2019-05-28 12:41   ` Russell King - ARM Linux admin
2019-05-28 14:01 ` Peter Zijlstra
2019-05-28 14:01   ` Peter Zijlstra
2019-05-28 15:32   ` Will Deacon
2019-05-28 15:32     ` Will Deacon
2019-05-28 16:12     ` Mark Rutland
2019-05-28 16:12       ` Mark Rutland
2019-05-28 17:32     ` Peter Zijlstra [this message]
2019-05-28 17:32       ` Peter Zijlstra
2019-05-29  9:17       ` Will Deacon
2019-05-29  9:17         ` Will Deacon
2019-05-29 10:10         ` Peter Zijlstra
2019-05-29 10:10           ` Peter Zijlstra
2019-05-29 10:20           ` Will Deacon
2019-05-29 10:20             ` Will Deacon
2019-05-29 12:55             ` Peter Zijlstra
2019-05-29 12:55               ` Peter Zijlstra
2019-05-29 13:05               ` Will Deacon
2019-05-29 13:05                 ` Will Deacon
2019-05-29 13:25                 ` Peter Zijlstra
2019-05-29 13:25                   ` Peter Zijlstra
2019-05-29 14:35                   ` Will Deacon
2019-05-29 14:35                     ` Will Deacon
2019-05-29 16:19                     ` Peter Zijlstra
2019-05-29 16:19                       ` Peter Zijlstra
2019-05-29 16:24                       ` Mark Rutland
2019-05-29 16:24                         ` Mark Rutland
2019-05-29 16:38                         ` Mark Rutland
2019-05-29 16:38                           ` Mark Rutland
2019-05-29 17:03                           ` Peter Zijlstra
2019-05-29 17:03                             ` Peter Zijlstra
2019-05-30 10:35                             ` Mark Rutland
2019-05-30 10:35                               ` Mark Rutland
2019-05-29 16:25                       ` Will Deacon
2019-05-29 16:25                         ` Will Deacon
2019-05-29 16:44                         ` Peter Zijlstra
2019-05-29 16:44                           ` Peter Zijlstra
2019-05-30  7:28                           ` Will Deacon
2019-05-30  7:28                             ` Will Deacon
2019-05-30  8:38               ` Ravi Bangoria
2019-05-30  8:38                 ` Ravi Bangoria
2019-05-30 10:27                 ` Ravi Bangoria
2019-05-30 10:27                   ` Ravi Bangoria
2019-05-31 15:37                   ` Will Deacon
2019-05-31 15:37                     ` Will Deacon
2019-06-03 11:23                     ` Will Deacon
2019-06-03 11:23                       ` Will Deacon
2019-06-03 11:48                     ` Peter Zijlstra
2019-06-03 11:48                       ` Peter Zijlstra
2019-06-03 13:30                     ` Michael Ellerman
2019-06-03 13:30                       ` Michael Ellerman
2019-05-29 10:11       ` Mark Rutland
2019-05-29 10:11         ` Mark Rutland
2019-05-29  4:21     ` Michael Ellerman
2019-05-29  4:21       ` Michael Ellerman
2019-05-29  1:44   ` Michael Ellerman
2019-05-29  1:44     ` Michael Ellerman

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