linux-kernel - Re: [PATCH v3 bpf-next 5/7] x86/alternative: introduce text_poke

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Date:   Tue, 11 Jan 2022 13:13:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <song@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, kernel-team@...com, x86@...nel.org
Subject: Re: [PATCH v3 bpf-next 5/7] x86/alternative: introduce text_poke_jit

On Wed, Jan 05, 2022 at 06:25:31PM -0800, Song Liu wrote:

> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 23fb4d51a5da..02c35725cc62 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1102,6 +1102,34 @@ void *text_poke_kgdb(void *addr, const void *opcode, size_t len)
>  	return __text_poke(addr, opcode, len);
>  }
>  
> +/**
> + * text_poke_jit - Update instructions on a live kernel by jit engine
> + * @addr: address to modify
> + * @opcode: source of the copy
> + * @len: length to copy, could be more than 2x PAGE_SIZE
> + *
> + * Only module memory taking jit text (e.g. for bpf) should be patched.
> + */

Maybe:

	text_poke_copy() - Copy instructions into (an unused part of) RX memory
	@args...

	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 life.

or something along those lines?

> +void *text_poke_jit(void *addr, const void *opcode, size_t len)
> +{
> +	unsigned long start = (unsigned long)addr;
> +	size_t patched = 0;
> +
> +	if (WARN_ON_ONCE(core_kernel_text(start)))
> +		return NULL;
> +
> +	while (patched < len) {
> +		unsigned long ptr = start + patched;
> +		size_t s;
> +
> +		s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), len - patched);
> +
> +		__text_poke((void *)ptr, opcode + patched, s);
> +		patched += s;
> +	}
> +	return addr;
> +}
> +
>  static void do_sync_core(void *info)
>  {
>  	sync_core();
> -- 
> 2.30.2
>