linux-kernel - Re: 4.4-rc5 Setting hardware breakpoint in int_ret_from_sys

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Message-ID: <CAO6TR8URP2zA1CXzQtFsHaiKHyD1-e83H+Zj=mpEoR8LTJX7Bw@mail.gmail.com>
Date:	Wed, 16 Dec 2015 17:42:51 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: 4.4-rc5 Setting hardware breakpoint in int_ret_from_sys_call
 causes triple fault/reboot

On 12/16/15, Andy Lutomirski <luto@...capital.net> wrote:
> On Wed, Dec 16, 2015 at 4:31 PM, Jeff Merkey <linux.mdb@...il.com> wrote:
>> On 12/16/15, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Dec 16, 2015 3:12 PM, "Jeff Merkey" <linux.mdb@...il.com> wrote:
>>>>
>>>> Setting a hardware breakpoint at the
>>>>
>>>> rex64 sysret
>>>>
>>>> instruction at the end of int_ret_from_sys_call causes the system to
>>>> triple fault
>>>> and reboot when the breakpoint is triggered.  Appears to be related
>>>> the same problem
>>>> as the lockup.
>>>>
>>>> This function can be stepped over and traced through with the TRAP
>>>> FLAG set so long as a hardware breakpoint is set somewhere in the
>>>> function.  Otherwise upon exist the system hard hangs.  If you break
>>>> exactly on that instruction -- reboot.   If you break a few
>>>> instructions before it and single step through the call it works.  If
>>>> you step through the call with no breakpoint the system hard hangs.
>>>> Same behavior as when you try to step from inside an nmi handler.
>>>> Looks related.
>>>
>>> You're probably encountering the user mode RSP when SYSRET happens.
>>>
>>> --Andy
>>>
>>
>> Hi Andy,
>>
>> Could be, but I am getting a double fault message with an error code
>> of 0 that then scrolls off the screen when the triple fault hits.  It
>> flashes too quickly to get the function address -- wish I had a logic
>> analyzer with an inverse assembler -- would already be there.    A
>> usermode RSP would I assume clear TRAP flag and that does not explain
>> why it works if I set a breakpoint right above the instruction then
>> step over it, which I can without the triple fault.
>>
>> Easy to reproduce, download the mdb debugger for 4.3.3 and apply it to
>> 4.4-rc5, modprobe mdb, echo a > /proc/sysrq_trigger, u
>> int_ret_from_syscall (scroll til you get to the swapgs then rex64
>> sysret, set a hardware breakpoint  at that address , i.e. b
>> ffffffff81673ae1 (or whatever address the swapgs instruction is at),
>> then step through with t a few times (should just return after rex64
>> sysret since it returns to user space).  The set a breakpoint at the
>> rex64 sysret instruction, b <address>, let it break at the
>> instruction, then hit g for go and watch the fireworks -- it will try
>> to print a double fault message then reboot.
>>
>> I handle the whole user RSP thing, I just return if I see regs set to
>> user space.  This looks like some sort of problem in the exception
>> handlers.
>
> It's kernel regs but user RSP.
>
> --Andy
>

right, I handle that case and I have handled that case since about
2001.  Used to before all the change I could just step from userspace
to kernel space with mdb.  Have not been able to do that for while
since Linus fixed the VM in about 2002.

So I handle that case.

Jeff
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