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

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Date:   Mon, 5 Nov 2018 18:14:24 +0000
From:   Nadav Amit <namit@...are.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <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*()"

From: Thomas Gleixner
Sent: November 4, 2018 at 8:58:20 PM GMT
> 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.

Sorry - wrong commit log. There were no other assertions before. 

> 
>> 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.

I’m surprised. This patch only removes one assertion that was added two
months ago.

> 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

Sure. I will send another version once I realize how to deal with the other
concerns that Peter and Andy raised.

Regards,
Nadav