Re: [PATCH] perf: Fix oops when kthread execs user process

From: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>,
	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, acme@redhat.com, eranian@google.com,
	fweisbec@gmail.com, jolsa@redhat.com,
	ravi.bangoria@linux.ibm.com
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process
Date: Thu, 30 May 2019 14:08:46 +0530	[thread overview]
Message-ID: <efcd5cf4-3501-f3b6-bf47-145a9ef19a53@linux.ibm.com> (raw)
In-Reply-To: <20190529125557.GU2623@hirez.programming.kicks-ass.net>

On 5/29/19 6:25 PM, Peter Zijlstra wrote:
> On Wed, May 29, 2019 at 11:20:22AM +0100, Will Deacon wrote:
>> Anyway, you can add my ack to your patch, but I bet we can remove that mm
>> check :D
> 
> I've ended up with the below. Ravi, can you test if that does indeed
> obsolete your PPC patch?

Checking.

> 
> ---
> Subject: perf: Fix perf_sample_regs_user()
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Wed May 29 14:37:24 CEST 2019
> 
> perf_sample_regs_user() uses 'current->mm' to test for the presence of
> userspace, but this is insufficient, consider use_mm().
> 
> A better test is: '!(current->flags & PF_KTHREAD)', exec() clears
> PF_KTHREAD after it sets the new ->mm but before it drops to userspace
> for the first time.

This looks correct. I'll give it a try.

> 
> Possibly obsoletes: bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs user process")
> 
> Reported-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
> Reported-by: Young Xiao <92siuyang@gmail.com>
> Cc: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
> Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Stephane Eranian <eranian@google.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Acked-by: Will Deacon <will.deacon@arm.com>
> Fixes: 4018994f3d87 ("perf: Add ability to attach user level registers dump to sample")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/events/core.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5923,7 +5923,7 @@ static void perf_sample_regs_user(struct
>  	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)) {
>  		perf_get_regs_user(regs_user, regs, regs_user_copy);
>  	} else {
>  		regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;
> 