Re: [PATCH -tip v5 07/10] kprobes/x86: Support kprobes jump optimization on x86

[Masami Hiramatsu <mhiramat@redhat.com>]

Jason Baron wrote:
[...]
>> +/*
>> + * Cross-modifying kernel text with stop_machine().
>> + * This code originally comes from immediate value.
>> + * This does _not_ protect against NMI and MCE. However,
>> + * since kprobes can't probe NMI/MCE handler, it is OK for kprobes.
>> + */
>> +static atomic_t stop_machine_first;
>> +static int wrote_text;
>> +
>> +struct text_poke_param {
>> +	void *addr;
>> +	const void *opcode;
>> +	size_t len;
>> +};
>> +
>> +static int __kprobes stop_machine_multibyte_poke(void *data)
>> +{
>> +	struct text_poke_param *tpp = data;
>> +
>> +	if (atomic_dec_and_test(&stop_machine_first)) {
>> +		text_poke(tpp->addr, tpp->opcode, tpp->len);
>> +		smp_wmb();	/* Make sure other cpus see that this has run */
>> +		wrote_text = 1;
>> +	} else {
>> +		while (!wrote_text)
>> +			smp_rmb();
>> +		sync_core();
>> +	}
>> +
>> +	flush_icache_range((unsigned long)tpp->addr,
>> +			   (unsigned long)tpp->addr + tpp->len);
>> +	return 0;
>> +}
>> +
>> +static void *__kprobes __multibyte_poke(void *addr, const void *opcode,
>> +					size_t len)
>> +{
>> +	struct text_poke_param tpp;
>> +
>> +	tpp.addr = addr;
>> +	tpp.opcode = opcode;
>> +	tpp.len = len;
>> +	atomic_set(&stop_machine_first, 1);
>> +	wrote_text = 0;
>> +	stop_machine(stop_machine_multibyte_poke, (void *)&tpp, NULL);
>> +	return addr;
>> +}
>
> As you know, I'd like to have the jump label optimization for
> tracepoints, make use of this '__multibyte_poke()' interface. So perhaps
> it can be moved to arch/x86/kernel/alternative.c. This is where 'text_poke()'
> and friends currently live.

Hmm, maybe current text_poke needs to have singlebyte_poke() wrapper
for avoiding confusion.

> Also, with multiple users we don't want to trample over each others code
> patching. Thus, if each sub-system could register some type of
> 'is_reserved()' callback, and then we can call all these call backs from
> the '__multibyte_poke()' routine before we do any patching to make sure
> that we aren't trampling on each others code. After a successful
> patching, each sub-system can update its reserved set of code as
> appropriate. I can code a prototype here, if this makes sense.

Hmm, we have to implement it carefully, because here kprobes already
inserted int3 and optprobe rewrites the int3 again. If is_reserved()
returns 1 and multibyte_poke returns error, we can't optimize it anymore.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com