Re: [PATCH 7/7] powerpc/kprobes: Allow probing on any ftrace address

From: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
To: Masami Hiramatsu <mhiramat@kernel.org>, Joe Perches <joe@perches.com>
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	Ingo Molnar <mingo@kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH 7/7] powerpc/kprobes: Allow probing on any ftrace address
Date: Wed, 26 Jun 2019 15:09:56 +0530	[thread overview]
Message-ID: <1561541820.15ifr1qex2.naveen@linux.ibm.com> (raw)
In-Reply-To: <20190621235034.acc00fc3e2b2c7e89caa1fd5@kernel.org>

Masami Hiramatsu wrote:
> On Tue, 18 Jun 2019 20:17:06 +0530
> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
> 
>> With KPROBES_ON_FTRACE, kprobe is allowed to be inserted on instructions
>> that branch to _mcount (referred to as ftrace location). With
>> -mprofile-kernel, we now include the preceding 'mflr r0' as being part
>> of the ftrace location.
>> 
>> However, by default, probing on an instruction that is not actually the
>> branch to _mcount() is prohibited, as that is considered to not be at an
>> instruction boundary. This is not the case on powerpc, so allow the same
>> by overriding arch_check_ftrace_location()
>> 
>> In addition, we update kprobe_ftrace_handler() to detect this scenarios
>> and to pass the proper nip to the pre and post probe handlers.
>> 
>> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>> ---
>>  arch/powerpc/kernel/kprobes-ftrace.c | 30 ++++++++++++++++++++++++++++
>>  1 file changed, 30 insertions(+)
>> 
>> diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
>> index 972cb28174b2..6a0bd3c16cb6 100644
>> --- a/arch/powerpc/kernel/kprobes-ftrace.c
>> +++ b/arch/powerpc/kernel/kprobes-ftrace.c
>> @@ -12,14 +12,34 @@
>>  #include <linux/preempt.h>
>>  #include <linux/ftrace.h>
>>  
>> +/*
>> + * With -mprofile-kernel, we patch two instructions -- the branch to _mcount
>> + * as well as the preceding 'mflr r0'. Both these instructions are claimed
>> + * by ftrace and we should allow probing on either instruction.
>> + */
>> +int arch_check_ftrace_location(struct kprobe *p)
>> +{
>> +	if (ftrace_location((unsigned long)p->addr))
>> +		p->flags |= KPROBE_FLAG_FTRACE;
>> +	return 0;
>> +}
>> +
>>  /* Ftrace callback handler for kprobes */
>>  void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
>>  			   struct ftrace_ops *ops, struct pt_regs *regs)
>>  {
>>  	struct kprobe *p;
>> +	int mflr_kprobe = 0;
>>  	struct kprobe_ctlblk *kcb;
>>  
>>  	p = get_kprobe((kprobe_opcode_t *)nip);
>> +	if (unlikely(!p)) {
> 
> Hmm, is this really unlikely? If we put a kprobe on the second instruction address,
> we will see p == NULL always.
> 
>> +		p = get_kprobe((kprobe_opcode_t *)(nip - MCOUNT_INSN_SIZE));
>> +		if (!p)
> 
> Here will be unlikely, because we can not find kprobe at both of nip and
> nip - MCOUNT_INSN_SIZE.
> 
>> +			return;
>> +		mflr_kprobe = 1;
>> +	}
>> +
>>  	if (unlikely(!p) || kprobe_disabled(p))
> 
> "unlikely(!p)" is not needed here.

...

Joe Perches wrote:
> On Fri, 2019-06-21 at 23:50 +0900, Masami Hiramatsu wrote:
>> On Tue, 18 Jun 2019 20:17:06 +0530
>> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
> 
> trivia:
> 
>> > diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
> []
>> > @@ -57,6 +82,11 @@ NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>> >  
>> >  int arch_prepare_kprobe_ftrace(struct kprobe *p)
>> >  {
>> > +	if ((unsigned long)p->addr & 0x03) {
>> > +		printk("Attempt to register kprobe at an unaligned address\n");
> 
> Please use the appropriate KERN_<LEVEL> or pr_<level>
> 

All good points. Thanks for the review.

- Naveen