linux-kernel - Re: [PATCH v2 12/39] x86/ibt,ftrace: Search for __fentry_

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Message-ID: <20220224110130.4f4ae4ac@gandalf.local.home>
Date:   Thu, 24 Feb 2022 11:01:30 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        joao@...rdrivepizza.com, hjl.tools@...il.com, jpoimboe@...hat.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz,
        alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 12/39] x86/ibt,ftrace: Search for __fentry__ location

On Thu, 24 Feb 2022 10:58:47 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 25 Feb 2022 00:55:20 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > >  unsigned long ftrace_location(unsigned long ip)
> > >  {
> > > -	return ftrace_location_range(ip, ip);
> > > +	struct dyn_ftrace *rec;
> > > +	unsigned long offset;
> > > +	unsigned long size;
> > > +
> > > +	rec = lookup_rec(ip, ip);
> > > +	if (!rec) {
> > > +		if (!kallsyms_lookup_size_offset(ip, &size, &offset))
> > > +			goto out;
> > > +
> > > +		if (!offset)    
> > 
> > Isn't this 'if (offset)' ?  
> 
> No, the point to only look for the fentry location if the ip passed in
> points to the start of the function. IOW, +0 offset.
> 

I do agree with Masami that it is confusing. Please add a comment:

		/* Search the entire function if ip is the start of the function */
		if (!offset)
			[..]

-- Steve

> 
> 
> >   
> > > +			rec = lookup_rec(ip - offset, (ip - offset) + size);
> > > +	}
> > > +
> > > +	if (rec)
> > > +		return rec->ip;
> > > +
> > > +out:
> > > +	return 0;