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SubjectRe: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init
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From: Andy Lutomirski
Sent: November 5, 2018 at 5:22:32 PM GMT
> To: Peter Zijlstra <peterz@infradead.org>
> Cc: Nadav Amit <namit@vmware.com>, Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org, x86@kernel.org, H. Peter Anvin <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, Andy Lutomirski <luto@kernel.org>, Kees Cook <keescook@chromium.org>, Dave Hansen <dave.hansen@intel.com>, Masami Hiramatsu <mhiramat@kernel.org>
> Subject: Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init
>
>
>
>> On Nov 5, 2018, at 6:09 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>
>>> 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) {
>
> Can we move this logic into text_poke() and get rid of text_poke_early()?

This will negatively affect poking of modules doing module loading, e.g.,
apply_paravirt(). This can be resolved by keeping track when the module is
write-protected and giving a module parameter to text_poke(). Does it worth
the complexity?

> FWIW, alternative patching was, at some point, a significant fraction of
> total boot time in some cases. This was probably mostly due to unnecessary
> sync_core() calls. Although I think this was reported on a VM, and
> sync_core() used to be *extremely* expensive on a VM, but that’s fixed
> now, and it even got backported, I think.
>
> (Hmm. Maybe we can also make jump label patching work in early boot, too!)

It may be possible to resolve the dependencies between poking_init() and the
other *_init(). I first considered doing that, yet, it makes the code very
fragile, and I don’t see the value in getting rid of text_poke_early() from
security or simplicity point of views. Let me know if you think otherwise.

Regards,
Nadav
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