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

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Date:   Thu, 30 May 2019 08:28:58 +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 06:44:07PM +0200, Peter Zijlstra wrote:
> On Wed, May 29, 2019 at 05:25:28PM +0100, Will Deacon wrote:
> 
> > > > > On Wed, May 29, 2019 at 02:05:21PM +0100, Will Deacon wrote:
> > > > > > On Wed, May 29, 2019 at 02:55:57PM +0200, Peter Zijlstra wrote:
> > > > > 
> > > > > > >  	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?
> 
> > Sorry, I'm not trying to catch you out! Just trying to understand what the
> > semantics are supposed to be.
> > 
> > I do find the concept of user_mode(regs) bizarre for the idle task. By the
> > above, we definitely have a bug on arm64 (user_mode(regs) tends to be
> > true for the idle task), and I couldn't figure out how you avoided it on
> > x86. I guess it happens to work because the stack is zero-initialised or
> > something?
> 
> So lets take the whole thing:
> 
> static void perf_sample_regs_user(struct perf_regs *regs_user,
> 				  struct pt_regs *regs,
> 				  struct pt_regs *regs_user_copy)
> {
> 	if (user_mode(regs)) {
> 		regs_user->abi = perf_reg_abi(current);
> 		regs_user->regs = regs;
> 	} 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;
> 		regs_user->regs = NULL;
> 	}
> }
> 
> This is called from the perf-generate-a-sample path, which is typically
> an exception (IRQ/NMI/whatever) or a software/tracepoint thing.

Yes, sorry, fell into the same trap as Mark here and misunderstood your
assertion about user_mode(regs) always needing to be valid. Then I went down
a stupid rabbit hole and dragged you with me. I can't ack a patch twice, so
I'll just go do something else for a bit...

Thanks for your patience!

Will