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

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Message-ID: <1F547CEE-B5D9-42A0-8093-2C5555BACE26@vmware.com>
Date:   Wed, 29 Aug 2018 17:11:53 +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 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.