linux-kernel - RE: trace: trace_kprobe.c always shows interrupts off

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Date:	Wed, 18 Nov 2015 06:51:12 +0000
From:	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>
To:	"'Steven Rostedt'" <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: RE: trace: trace_kprobe.c always shows interrupts off

Hi Mathieu, Steven,

From: Steven Rostedt [mailto:rostedt@...dmis.org]
>
>On Tue, 17 Nov 2015 16:35:35 +0000 (UTC)
>Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
>
>> Hi,
>>
>> I notice that trace_kprobe.c does local_save_flags() within
>> __kprobe_trace_func(), which is called (at least on x86) with
>> interrupts always disabled. This is then used as interrupt on/off
>> state within the recorded event, which is misleading.
>
>Kinda. The kprobe itself has interrupts disabled, so it's only a white
>lie.
>
>>
>> I also don't understand why arch/x86/kernel/kprobes/ftrace.c
>> pre handler disables interrupts, considering the following
>> comment above arch/x86/kernel/kprobes/core.c: kprobe_int3_handler()
>>
>>  * Interrupts are disabled on entry as trap3 is an interrupt gate and they
>>  * remain disabled throughout this function.
>
>I think you answered your own question. The key piece you may be
>missing is that kprobe_ftrace_handler() is not called from a trap, but
>from a function traced callback, which does not disable interrupts.

right, since the user can not forcibly change any kprobes to
jump, those handlers should be ran under the same environment.

>> A struct pt_regs is received by this function, but I don't see
>> any way to get the state of irq enable/disable from struct pt_regs
>> across architectures.
>>
>> Any thoughts on how to fix this ?
>
>Create a cross arch: flags = regs_irq_save(regs) function.
>

Good idea! Anyway, we can start on x86, in other arch we can
prepare a dummy function to return current interrupt state(as
we are doing now).

Thank you,

----
Masami Hiramatsu 

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