linux-kernel - Re: Kprobes: pre-handler with interrupts enabled

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Message-ID: <54EC1487.4020902@rosalab.ru>
Date:	Tue, 24 Feb 2015 09:04:55 +0300
From:	Eugene Shatokhin <eugene.shatokhin@...alab.ru>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Kprobes: pre-handler with interrupts enabled - is it possible?

24.02.2015 06:47, Masami Hiramatsu пишет:
> No, that is not allowed. I mean, you can do anything you want to do
> on your handler (enabling preemption/irq etc.) but the result may be
> not safe (it can crash your kernel, but it's not a kprobes' bug).

Yes, that is why I am asking.

> Actually, enable interrupts on kprobe handlers can cause reentering
> kprobes (by kprobes on interrupt handlers), and currently kprobe skips
> all those reentered kprobes.
> Is it acceptable that some of your kprobe handlers are not fired when
> hitting?

I think, yes. When a software breakpoint hits, my system decodes the 
instruction, finds the address that is about to be accessed and tries to 
place a hardware breakpoint on that memory area.

There are only 4 hardware breakpoints a CPU can use on x86, so if the 
software breakpoint hits too often, the system will not be able to 
process all hits anyway because all HW breakpoints may be already in use.

> Would you mean sleep on your handler??

No, I use mdelay(). It is, in essence, a busy-wait loop as far as I 
know. The delay intervals may vary, the default is 5 jiffies.

Regards,
Eugene

-- 
Eugene Shatokhin, ROSA
www.rosalab.com
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