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Date:   Tue, 28 May 2019 13:41:52 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Young Xiao <92siuyang@...il.com>
Cc:     will.deacon@....com, mark.rutland@....com, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, x86@...nel.org, peterz@...radead.org,
        kan.liang@...ux.intel.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process

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.
> 
> 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.
> 
> Signed-off-by: Young Xiao <92siuyang@...il.com>
> ---
>  arch/arm/kernel/perf_regs.c   | 3 ++-
>  arch/arm64/kernel/perf_regs.c | 3 ++-
>  arch/x86/kernel/perf_regs.c   | 3 ++-
>  3 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm/kernel/perf_regs.c b/arch/arm/kernel/perf_regs.c
> index 05fe92a..78ee29a 100644
> --- a/arch/arm/kernel/perf_regs.c
> +++ b/arch/arm/kernel/perf_regs.c
> @@ -36,5 +36,6 @@ void perf_get_regs_user(struct perf_regs *regs_user,
>  			struct pt_regs *regs_user_copy)
>  {
>  	regs_user->regs = task_pt_regs(current);
> -	regs_user->abi = perf_reg_abi(current);
> +	regs_user->abi = (regs_user->regs) ? perf_reg_abi(current) :
> +			 PERF_SAMPLE_REGS_ABI_NONE;

I'd prefer it if we didn't introduce unnecessary parens - what function
do the parens around "regs_user->regs" serve?
-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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