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

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Date:   Wed, 29 Aug 2018 19:36:22 +0000
From:   Nadav Amit <namit@...are.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
CC:     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 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.