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

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Date:   Wed, 29 Aug 2018 20:44:47 +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 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?