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

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Date:   Wed, 29 May 2019 14:05:21 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Young Xiao <92siuyang@...il.com>, linux@...linux.org.uk,
        mark.rutland@....com, mingo@...hat.com, bp@...en8.de,
        hpa@...or.com, x86@...nel.org, kan.liang@...ux.intel.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        ravi.bangoria@...ux.vnet.ibm.com, mpe@...erman.id.au,
        acme@...hat.com, eranian@...gle.com, fweisbec@...il.com,
        jolsa@...hat.com
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process

On Wed, May 29, 2019 at 02:55:57PM +0200, 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?
> 
> ---
> Subject: perf: Fix perf_sample_regs_user()
> From: Peter Zijlstra <peterz@...radead.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.
> 
> Possibly obsoletes: bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs user process")
> 
> Reported-by: Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
> Reported-by: Young Xiao <92siuyang@...il.com>
> Cc: Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
> Cc: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Jiri Olsa <jolsa@...hat.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Stephane Eranian <eranian@...gle.com>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Acked-by: Will Deacon <will.deacon@....com>
> Fixes: 4018994f3d87 ("perf: Add ability to attach user level registers dump to sample")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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)) {

Hmm, so it just occurred to me that Mark's observation is that the regs
can be junk in some cases. In which case, should we be checking for
kthreads first?

Will