linux-kernel - Re: [PATCH bpf-next 2/5] x86/alternative: introduce text_poke

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Message-ID: <20220518170934.GG10117@worktop.programming.kicks-ass.net>
Date:   Wed, 18 May 2022 19:09:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <song@...nel.org>
Cc:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org, ast@...nel.org,
        daniel@...earbox.net, mcgrof@...nel.org,
        torvalds@...ux-foundation.org, rick.p.edgecombe@...el.com,
        kernel-team@...com
Subject: Re: [PATCH bpf-next 2/5] x86/alternative: introduce text_poke_set

On Sun, May 15, 2022 at 10:40:48PM -0700, Song Liu wrote:
> Introduce a memset like API for text_poke. This will be used to fill the
> unused RX memory with illegal instructions.

FWIW, you're going to use it to set INT3 (0xCC), that's not an illegal
instruction. INTO (0xCE) would be an illegal instruction (in 64bit
mode).


> +	return addr;
> +}
> +
> +/**
> + * text_poke_set - memset into (an unused part of) RX memory
> + * @addr: address to modify
> + * @c: the byte to fill the area with
> + * @len: length to copy, could be more than 2x PAGE_SIZE
> + *
> + * Not safe against concurrent execution; useful for JITs to dump
> + * new code blocks into unused regions of RX memory. Can be used in
> + * conjunction with synchronize_rcu_tasks() to wait for existing
> + * execution to quiesce after having made sure no existing functions
> + * pointers are live.

That comment suffers from copy-pasta and needs an update because it
clearly isn't correct.

> + */

Other than that, seems fine.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>