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

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Date:   Wed, 18 May 2022 18:34:18 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Song Liu <song@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Luis Chamberlain <mcgrof@...nel.org>,
        "Torvalds, Linus" <torvalds@...ux-foundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 2/5] x86/alternative: introduce text_poke_set



> On May 18, 2022, at 10:09 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> 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).

Hmm… we have been using INT3 as illegal/invalid/special instructions in 
the JIT. I guess they are equally good for this job?

> 
> 
>> +	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.

Will fix in the next version. 

> 
>> + */
> 
> Other than that, seems fine.
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Thanks,
Song