linux-kernel - Re: [PATCH V3 2/6] arm64: kgdb_step_brk

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Date:   Wed, 2 Nov 2016 15:44:36 +0530
From:   Pratyush Anand <panand@...hat.com>
To:     Sandeepa Prabhu <sandeepa.prabhu@...adcom.com>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        open list <linux-kernel@...r.kernel.org>,
        William Cohen <wcohen@...hat.com>, oleg@...hat.com,
        dave.long@...hat.com, Steve Capper <steve.capper@...aro.org>,
        srikar@...ux.vnet.ibm.com, vijaya.kumar@...iumnetworks.com
Subject: Re: [PATCH V3 2/6] arm64: kgdb_step_brk_fn: ignore other's exception



On Wednesday 02 November 2016 03:30 PM, Sandeepa Prabhu wrote:
>
>
> On Wed, Nov 2, 2016 at 2:40 PM, Pratyush Anand <panand@...hat.com
> <mailto:panand@...hat.com>> wrote:
>
>     ARM64 step exception does not have any syndrome information. So, it is
>     responsibility of exception handler to take care that they handle it
>     only if exception was raised for them.
>
>     Since kgdb_step_brk_fn() always returns 0, therefore we might have
>     problem
>     when we will have other step handler registered as well.
>
>     This patch fixes kgdb_step_brk_fn() to return error in case of step
>     handler
>     was not meant for kgdb.
>
>     Signed-off-by: Pratyush Anand <panand@...hat.com
>     <mailto:panand@...hat.com>>
>     ---
>      arch/arm64/kernel/kgdb.c | 3 +++
>      1 file changed, 3 insertions(+)
>
>     diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
>     index e017a9493b92..d217c9e95b06 100644
>     --- a/arch/arm64/kernel/kgdb.c
>     +++ b/arch/arm64/kernel/kgdb.c
>     @@ -247,6 +247,9 @@ NOKPROBE_SYMBOL(kgdb_compiled_brk_fn);
>
>      static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr)
>      {
>     +       if (!kgdb_single_step)
>     +               return DBG_HOOK_ERROR;
>     +
>
> This is needed. So, single stepping in kprobes working all these days
> because kprobes handler was registered earlier to kgdb handler!

Actually kprobe_single_step_handler() is not called through 
call_step_hook(), so it is always safe.

We had discussed here (https://lkml.org/lkml/2016/9/7/6) that why we can 
not register kprobe_single_step_handler() via register_set_hook()
and only invoke call_step_hook().

~Pratyush