linux-kernel - Re: [PATCH 08/15] x86/alternatives: Teach text_poke

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Message-ID: <20190611111410.366f4ced@gandalf.local.home>
Date:   Tue, 11 Jun 2019 11:14:10 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Nadav Amit <namit@...are.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate
 instructions

On Wed, 05 Jun 2019 15:08:01 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> -void text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
> +void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate)
>  {
>  	unsigned char int3 = 0xcc;
>  
> -	bp_int3_handler = handler;
> +	bp_int3_opcode = emulate ?: opcode;
>  	bp_int3_addr = (u8 *)addr + sizeof(int3);
>  	bp_patching_in_progress = true;
>  
>  	lockdep_assert_held(&text_mutex);
>  
>  	/*
> +	 * poke_int3_handler() relies on @opcode being a 5 byte instruction;
> +	 * notably a JMP, CALL or NOP5_ATOMIC.
> +	 */
> +	BUG_ON(len != 5);

If we have a bug on here, why bother with passing in len at all? Just
force it to be 5.

We could make it a WARN_ON() and return without doing anything.

This also prevents us from ever changing two byte jmps.

-- Steve

> +
> +	/*
>  	 * Corresponding read barrier in int3 notifier for making sure the
> -	 * in_progress and handler are correctly ordered wrt. patching.
> +	 * in_progress and opcode are correctly ordered wrt. patching.
>  	 */
>  	smp_wmb();
>  
> -