linux-kernel - Re: [PATCH ftrace/core 2/2] ftrace, kprobes: Support IPMODIFY flag to find IP modify conflict

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Message-ID: <87tx7rao6p.fsf@sejong.aot.lge.com>
Date:	Wed, 11 Jun 2014 16:41:02 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH ftrace/core  2/2] ftrace, kprobes: Support IPMODIFY flag to find IP modify conflict

Hi Masami,

On Wed, 11 Jun 2014 10:28:01 +0900, Masami Hiramatsu wrote:
> (2014/06/10 22:53), Namhyung Kim wrote:
>> Hi Masami,
>> 
>> 2014-06-10 (화), 10:50 +0000, Masami Hiramatsu:
>>> Introduce FTRACE_OPS_FL_IPMODIFY to avoid conflict among
>>> ftrace users who may modify regs->ip to change the execution
>>> path. This also adds the flag to kprobe_ftrace_ops, since
>>> ftrace-based kprobes already modifies regs->ip. Thus, if
>>> another user modifies the regs->ip on the same function entry,
>>> one of them will be broken. So both should add IPMODIFY flag
>>> and make sure that ftrace_set_filter_ip() succeeds.
>>>
>>> Note that currently conflicts of IPMODIFY are detected on the
>>> filter hash. It does NOT care about the notrace hash. This means
>>> that if you set filter hash all functions and notrace(mask)
>>> some of them, the IPMODIFY flag will be applied to all
>>> functions.
>>>
>> 
>> [SNIP]
>>> +static int __ftrace_hash_update_ipmodify(struct ftrace_ops *ops,
>>> +					 struct ftrace_hash *old_hash,
>>> +					 struct ftrace_hash *new_hash)
>>> +{
>>> +	struct ftrace_page *pg;
>>> +	struct dyn_ftrace *rec, *end = NULL;
>>> +	int in_old, in_new;
>>> +
>>> +	/* Only update if the ops has been registered */
>>> +	if (!(ops->flags & FTRACE_OPS_FL_ENABLED))
>>> +		return 0;
>>> +
>>> +	if (!(ops->flags & FTRACE_OPS_FL_SAVE_REGS) ||
>>> +	    !(ops->flags & FTRACE_OPS_FL_IPMODIFY))
>>> +		return 0;
>>> +
>>> +	/* Update rec->flags */
>>> +	do_for_each_ftrace_rec(pg, rec) {
>>> +		/* We need to update only differences of filter_hash */
>>> +		in_old = !old_hash || ftrace_lookup_ip(old_hash, rec->ip);
>>> +		in_new = !new_hash || ftrace_lookup_ip(new_hash, rec->ip);
>> 
>> Why not use ftrace_hash_empty() here instead of checking NULL? 
>
> Ah, a trick is here. Since an empty filter_hash must hit all, we can not
> enable/disable filter_hash if we use ftrace_hash_empty() here.
>
> To enabling the new_hash, old_hash must be EMPTY_HASH which means in_old
> always be false. To disabling, new_hash is EMPTY_HASH too.
> Please see ftrace_hash_ipmodify_enable/disable/update().

I'm confused. 8-p  I guess what you want to do is checking records in
either of the filter_hash, right?  If so, what about this?

	in_old = !ftrace_hash_empty(old_hash) && ftrace_lookup_ip(old_hash, rec->ip);
	in_new = !ftrace_hash_empty(new_hash) && ftrace_lookup_ip(new_hash, rec->ip);


>
>> Also
>> return value of ftrace_lookup_ip is not boolean..  maybe you need to
>> add !! or convert type of the in_{old,new} to bool.
>
> Yeah, I see. And there is '||' (logical OR) which evaluates the result
> as boolean. :)

Argh... you're right! :)

>
>> 
>> 
>>> +		if (in_old == in_new)
>>> +			continue;
>>> +
>>> +		if (in_new) {
>>> +			/* New entries must ensure no others are using it */
>>> +			if (rec->flags & FTRACE_FL_IPMODIFY)
>>> +				goto rollback;
>>> +			rec->flags |= FTRACE_FL_IPMODIFY;
>>> +		} else /* Removed entry */
>>> +			rec->flags &= ~FTRACE_FL_IPMODIFY;
>>> +	} while_for_each_ftrace_rec();
>>> +
>>> +	return 0;
>>> +
>>> +rollback:
>>> +	end = rec;
>>> +
>>> +	/* Roll back what we did above */
>>> +	do_for_each_ftrace_rec(pg, rec) {
>>> +		if (rec == end)
>>> +			goto err_out;
>>> +
>>> +		in_old = !old_hash || ftrace_lookup_ip(old_hash, rec->ip);
>>> +		in_new = !new_hash || ftrace_lookup_ip(new_hash, rec->ip);
>>> +		if (in_old == in_new)
>>> +			continue;
>>> +
>>> +		if (in_new)
>>> +			rec->flags &= ~FTRACE_FL_IPMODIFY;
>>> +		else
>>> +			rec->flags |= FTRACE_FL_IPMODIFY;
>>> +	} while_for_each_ftrace_rec();
>>> +
>>> +err_out:
>>> +	return -EBUSY;
>>> +}
>>> +
>>> +static int ftrace_hash_ipmodify_enable(struct ftrace_ops *ops)
>>> +{
>>> +	struct ftrace_hash *hash = ops->filter_hash;
>>> +
>>> +	if (ftrace_hash_empty(hash))
>>> +		hash = NULL;
>>> +
>>> +	return __ftrace_hash_update_ipmodify(ops, EMPTY_HASH, hash);
>>> +}
>> 
>> Please see above comment.  You can pass an empty hash as is, or pass
>> NULL as second arg.  The same goes to below...
>
> As I said above, that is by design :). EMPTY_HASH means it hits nothing,
> NULL means it hits all.

But doesn't it make unrelated records also get the flag updated?  I'm
curious when new_hash can be empty on _enable() case..

Thanks,
Namhyung
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