linux-kernel - Re: [PATCH v3 1/7] Fix "x86/alternatives: Lockdep-enforce text_mutex in text

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Date:   Sun, 4 Nov 2018 21:58:20 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Nadav Amit <namit@...are.com>
cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jiri Kosina <jkosina@...e.cz>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v3 1/7] Fix "x86/alternatives: Lockdep-enforce text_mutex
 in text_poke*()"

On Fri, 2 Nov 2018, Nadav Amit wrote:

> text_mutex is expected to be held before text_poke() is called, but we
> cannot add a lockdep assertion since kgdb does not take it, and instead
> *supposedly* ensures the lock is not taken and will not be acquired by
> any other core while text_poke() is running.
> 
> The reason for the "supposedly" comment is that it is not entirely clear
> that this would be the case if gdb_do_roundup is zero.
> 
> Add a comment to clarify this behavior, and restore the assertions as
> they were before the recent commit.

It restores nothing. It just removes the assertion.

> This partially reverts commit 9222f606506c ("x86/alternatives:
> Lockdep-enforce text_mutex in text_poke*()")

That opens up the same can of worms again, which took us a while to close.

Can we please instead split out the text_poke() code into a helper function
and have two callers:

    text_poke() which contains the assert

    text_poke_kgdb() which does not

Thanks,

	tglx