Re: [PATCH 4/8] ftrace: Store direct called addresses in their ops

[Florent Revest]

On Thu, Feb 2, 2023 at 4:30 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Wed, Feb 01, 2023 at 05:34:16PM +0100, Florent Revest wrote:
> > All direct calls are now registered using the register_ftrace_direct API
> > so each ops can jump to only one direct-called trampoline.
> >
> > By storing the direct called trampoline address directly in the ops we
> > can save one hashmap lookup in the direct call ops and implement arm64
> > direct calls on top of call ops.
> >
> > Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
> > ---
> >  include/linux/ftrace.h | 3 +++
> >  kernel/trace/ftrace.c  | 6 ++++--
> >  2 files changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index a7dbd307c3a4..84f717f8959e 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -321,6 +321,9 @@ struct ftrace_ops {
> >       unsigned long                   trampoline_size;
> >       struct list_head                list;
> >       ftrace_ops_func_t               ops_func;
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> > +     unsigned long                   direct_call;
> > +#endif
> >  #endif
> >  };
> >
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index cb77a0a208c7..b0426de11c45 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -2577,9 +2577,8 @@ 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;
> >
> > -     addr = ftrace_find_rec_direct(ip);
> >       if (!addr)
> >               return;
> >
> > @@ -5375,6 +5374,7 @@ 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);
> >
> > @@ -5445,6 +5445,7 @@ __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)
> > @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> >                       entry->direct = addr;
> >               }
> >       }
> > +     ops->direct_call = addr;
>
> AFAICT we don't synchronize threads when installing the tmp_ops, so IIUC on
> arches with call_ops, there could be a a thread in the middle of ftrace_caller
> which has loaded the ops pointer from the patch-site, but hasn't yet loaded the
> ops::direct_func pointer, and could race with this assignment.
>
> Given that, I think this needs to be:
>
>         WRITE_ONCE(ops->direct_call, addr);
>
> ... in order to avoid the risk of the store being torn, and the ftrace_caller
> trampoline loading a corrupted pointer.

Good point, I'll do that in v2.