linux-kernel - Re: [PATCH 04/12] powerpc/ftrace: Prepare for -mprofile-kernel

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Message-ID: <1456396627.15739.9.camel@ellerman.id.au>
Date:	Thu, 25 Feb 2016 21:37:07 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Balbir Singh <bsingharora@...il.com>, linuxppc-dev@...abs.org
Cc:	duwe@....de, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	kamalesh@...ux.vnet.ibm.com, pmladek@...e.com, jeyu@...hat.com,
	jkosina@...e.cz, live-patching@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH 04/12] powerpc/ftrace: Prepare for -mprofile-kernel

On Thu, 2016-02-25 at 11:28 +1100, Balbir Singh wrote:
> On 25/02/16 01:28, Michael Ellerman wrote:
> > @@ -300,8 +298,34 @@ __ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> >  	 * The load offset is different depending on the ABI. For simplicity
> >  	 * just mask it out when doing the compare.
> >  	 */
> > -	if ((op[0] != 0x48000008) || ((op[1] & 0xffff0000) != 0xe8410000)) {
> > -		pr_err("Unexpected call sequence: %x %x\n", op[0], op[1]);
> > +	if ((op0 != 0x48000008) || ((op1 & 0xffff0000) != 0xe8410000))
> > +		return 0;
> > +	return 1;
> > +}
> > +#else
> > +static int
> > +expected_nop_sequence(void *ip, unsigned int op0, unsigned int op1)
> > +{
> > +	/* look for patched "NOP" on ppc64 with -mprofile-kernel */
> > +	if (op0 != PPC_INST_NOP)
> > +		return 0;
> > +	return 1;

> With the magic changes, do we care for this? I think it's a bit of an overkill

I don't particularly like it either. However this code doesn't actually use the
magic, it's the reverse case of turning a nop into a call to the stub. So the
magic in the stub doesn't actually make that any safer.

I think we do at least want to check there's a nop there. But without
mprofile-kernel it's not a nop, so we need some check and it does need to be
different between the profiling ABIs. So I think for now this is the
conservative approach.

cheers