linux-kernel - Re: [PATCH 5/7] ftrace: Store direct called addresses in their ops

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Date:   Sun, 19 Mar 2023 19:54:56 +0100
From:   Jiri Olsa <olsajiri@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Jiri Olsa <olsajiri@...il.com>,
        Florent Revest <revest@...omium.org>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        mhiramat@...nel.org, mark.rutland@....com, ast@...nel.org,
        daniel@...earbox.net, kpsingh@...nel.org
Subject: Re: [PATCH 5/7] ftrace: Store direct called addresses in their ops

On Sun, Mar 19, 2023 at 01:54:43PM -0400, Steven Rostedt wrote:
> On Sun, 19 Mar 2023 16:29:22 +0100
> Jiri Olsa <olsajiri@...il.com> wrote:
> 
> > > +++ b/kernel/trace/ftrace.c
> > > @@ -2582,9 +2582,8 @@ ftrace_add_rec_direct(unsigned long ip, unsigned long addr,
> > >  static void call_direct_funcs(unsigned long ip, unsigned long pip,
> > >  			      struct ftrace_ops *ops, struct ftrace_regs *fregs)
> > >  {
> > > -	unsigned long addr;
> > > +	unsigned long addr = ops->direct_call;  
> > 
> > nice, should it be read with READ_ONCE ?
> 
> Is there a "read tearing" too?

don't know, saw the comment in __modify_ftrace_direct and got curious
why it's not in here.. feel free to ignore, I'll look it up

jirka

> 
> -- Steve
> 
> > 
> > jirka
> > 
> > >  
> > > -	addr = ftrace_find_rec_direct(ip);
> > >  	if (!addr)
> > >  		return;
> > >  
> > > @@ -5380,6 +5379,7 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> > >  	ops->func = call_direct_funcs;
> > >  	ops->flags = MULTI_FLAGS;
> > >  	ops->trampoline = FTRACE_REGS_ADDR;
> > > +	ops->direct_call = addr;
> > >  
> > >  	err = register_ftrace_function_nolock(ops);
> > >  
> > > @@ -5454,6 +5454,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> > >  	/* Enable the tmp_ops to have the same functions as the direct ops */
> > >  	ftrace_ops_init(&tmp_ops);
> > >  	tmp_ops.func_hash = ops->func_hash;
> > > +	tmp_ops.direct_call = addr;
> > >  
> > >  	err = register_ftrace_function_nolock(&tmp_ops);
> > >  	if (err)
> > > @@ -5475,6 +5476,8 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> > >  			entry->direct = addr;
> > >  		}
> > >  	}
> > > +	/* Prevent store tearing if a trampoline concurrently accesses the value */
> > > +	WRITE_ONCE(ops->direct_call, addr);
> > >  
> > >  	mutex_unlock(&ftrace_lock);