Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init

[Nadav Amit]

From: Peter Zijlstra
Sent: November 5, 2018 at 2:09:25 PM GMT
> To: Nadav Amit <namit@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, H. Peter Anvin <hpa@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxx>, Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Subject: Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init
> 
> 
> On Fri, Nov 02, 2018 at 04:29:41PM -0700, Nadav Amit wrote:
>> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
>> index aac0c1f7e354..367c1d0c20a3 100644
>> --- a/arch/x86/kernel/jump_label.c
>> +++ b/arch/x86/kernel/jump_label.c
>> @@ -52,7 +52,13 @@ static void __ref __jump_label_transform(struct jump_entry *entry,
>> 	jmp.offset = jump_entry_target(entry) -
>> 		     (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
>> 
>> -	if (early_boot_irqs_disabled)
>> +	/*
>> +	 * As long as we are in early boot, we can use text_poke_early(), which
>> +	 * is more efficient: the memory was still not marked as read-only (it
>> +	 * is only marked after poking_init()). This also prevents us from using
>> +	 * text_poke() before poking_init() is called.
>> +	 */
>> +	if (!early_boot_done)
>> 		poker = text_poke_early;
>> 
>> 	if (type == JUMP_LABEL_JMP) {
> 
> It took me a while to untangle init/maze^H^Hin.c... but I think this
> is all we need:
> 
> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
> index aac0c1f7e354..ed5fe274a7d8 100644
> --- a/arch/x86/kernel/jump_label.c
> +++ b/arch/x86/kernel/jump_label.c
> @@ -52,7 +52,12 @@ static void __ref __jump_label_transform(struct jump_entry *entry,
> 	jmp.offset = jump_entry_target(entry) -
> 		     (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
> 
> -	if (early_boot_irqs_disabled)
> +	/*
> +	 * As long as we're UP and not yet marked RO, we can use
> +	 * text_poke_early; SYSTEM_BOOTING guarantees both, as we switch to
> +	 * SYSTEM_SCHEDULING before going either.
> +	 */
> +	if (system_state == SYSTEM_BOOTING)
> 		poker = text_poke_early;
> 
> 	if (type == JUMP_LABEL_JMP) {

Thanks for this change, I will incorporate it.

I wanted to point a small difference from my version. Although this version
ensures we are UP and the kernel is still RW, preemption is possible with
this version. I presume that it should not affect jump-labels, since it
switches between JMP and multi-byte NOPs.

Thanks,
Nadav