linux-kernel - Re: [RFC PATCH 1/6] x86/alternative: assert text

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Message-ID: <DFC5DCBA-A572-4603-82C8-77A6C1303630@vmware.com>
Date:   Wed, 29 Aug 2018 22:56:23 +0000
From:   Nadav Amit <namit@...are.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
CC:     Masami Hiramatsu <mhiramat@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH 1/6] x86/alternative: assert text_mutex is taken

at 2:00 PM, Sean Christopherson <sean.j.christopherson@...el.com> wrote:

> On Wed, Aug 29, 2018 at 08:44:47PM +0000, Nadav Amit wrote:
>> at 1:13 PM, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
>> 
>>> On Wed, Aug 29, 2018 at 07:36:22PM +0000, Nadav Amit wrote:
>>>> at 10:11 AM, Nadav Amit <namit@...are.com> wrote:
>>>> 
>>>>> at 1:59 AM, Masami Hiramatsu <mhiramat@...nel.org> wrote:
>>>>> 
>>>>>> On Wed, 29 Aug 2018 01:11:42 -0700
>>>>>> Nadav Amit <namit@...are.com> wrote:
>>>>>> 
>>>>>>> Use lockdep to ensure that text_mutex is taken when text_poke() is
>>>>>>> called.
>>>>>>> 
>>>>>>> Actually it is not always taken, specifically when it is called by kgdb,
>>>>>>> so take the lock in these cases.
>>>>>> 
>>>>>> Can we really take a mutex in kgdb context?
>>>>>> 
>>>>>> kgdb_arch_remove_breakpoint
>>>>>> <- dbg_deactivate_sw_breakpoints
>>>>>> <- kgdb_reenter_check
>>>>>>    <- kgdb_handle_exception
>>>>>>       <- __kgdb_notify
>>>>>>         <- kgdb_ll_trap
>>>>>>           <- do_int3
>>>>>>         <- kgdb_notify
>>>>>>           <- die notifier
>>>>>> 
>>>>>> kgdb_arch_set_breakpoint
>>>>>> <- dbg_activate_sw_breakpoints
>>>>>> <- kgdb_reenter_check
>>>>>>    <- kgdb_handle_exception
>>>>>>        ...
>>>>>> 
>>>>>> Both seems called in exception context, so we can not take a mutex lock.
>>>>>> I think kgdb needs a special path.
>>>>> 
>>>>> You are correct, but I don’t want a special path. Presumably text_mutex is
>>>>> guaranteed not to be taken according to the code.
>>>>> 
>>>>> So I guess the only concern is lockdep. Do you see any problem if I change
>>>>> mutex_lock() into mutex_trylock()? It should always succeed, and I can add a
>>>>> warning and a failure path if it fails for some reason.
>>>> 
>>>> Err.. This will not work. I think I will drop this patch, since I cannot
>>>> find a proper yet simple assertion. Creating special path just for the
>>>> assertion seems wrong.
>>> 
>>> It's probably worth expanding the comment for text_poke() to call out
>>> the kgdb case and reference kgdb_arch_{set,remove}_breakpoint(), whose
>>> code and comments make it explicitly clear why its safe for them to
>>> call text_poke() without acquiring the lock.  Might prevent someone
>>> from going down this path again in the future.
>> 
>> I thought that the whole point of the patch was to avoid comments, and
>> instead enforce the right behavior. I don’t understand well enough kgdb
>> code, so I cannot attest it does the right thing. What happens if
>> kgdb_do_roundup==0?
> 
> As is, the comment is wrong because there are obviously cases where
> text_poke() is called without text_mutex being held.  I can't attest
> to the kgdb code either.  My thought was to document the exception so
> that if someone does want to try and enforce the right behavior they
> can dive right into the problem instead of having to learn of the kgdb
> gotcha the hard way.  Maybe a FIXME is the right approach?

Ok. I’ll add a FIXME comment as you propose, but this does not deserve a
separate patch. I’ll squash it into patch 5.