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

Steven Rostedt
2023-03-19T17:54:55Z

On Sun, 19 Mar 2023 16:29:22 +0100
Jiri Olsa <olsajiri@gmail.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?

-- 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);