by Mark Rutland

[permalink] [raw]

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

On Mon, Mar 20, 2023 at 05:31:55PM -0400, Steven Rostedt wrote:
> On Mon, 20 Mar 2023 18:45:08 +0100
> Florent Revest <[email protected]> wrote:
>
> > On Sun, Mar 19, 2023 at 7:55 PM Jiri Olsa <[email protected]> wrote:
> > >
> > > On Sun, Mar 19, 2023 at 01:54:43PM -0400, Steven Rostedt wrote:
> > > > On Sun, 19 Mar 2023 16:29:22 +0100
> > > > Jiri Olsa <[email protected]> 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
> >
> > Mhh, that's a good question. Based on my current understanding, it
> > seems that it should have a READ_ONCE, indeed. However, I'd like Mark
> > to confirm/deny this. :)
> >
> > If this should be a READ_ONCE, I can send a v2 series with this fixed.
>
> After re-reading: https://lwn.net/Articles/793253/
>
> I think we should add the READ_ONCE() (also with a comment).

I think so, too.

AFAICT there's nothing that prevents __modify_ftrace_direct() and
call_direct_funcs() from concurrently accessing ftrace_ops::direct_call, so we
need READ_ONCE() in call_direct_funcs() to prevent load tearing and other
issues mentioned in the article linked above.

The existing code has a similar pattern where __modify_ftrace_direct() and
ftrace_find_rec_direct() access ftrace_func_entry::direct concurrently. Do we
want a preparatory patch fixing that for stable?

Thanks,
Mark.

2023-03-21 10:40:43

by Jiri Olsa

[permalink] [raw]

Subject: Re: [PATCH 1/7] ftrace: Let unregister_ftrace_direct_multi() call ftrace_free_filter()

On Tue, Mar 21, 2023 at 09:47:17AM +0000, Mark Rutland wrote:
> On Thu, Mar 16, 2023 at 06:38:05PM +0100, Florent Revest wrote:
> > A common pattern when using the ftrace_direct_multi API is to unregister
> > the ops and also immediately free its filter. We've noticed it's very
> > easy for users to miss calling ftrace_free_filter().
> >
> > This adds a "free_filters" argument to unregister_ftrace_direct_multi()
> > to both remind the user they should free filters and also to make their
> > life easier.
> >
> > Suggested-by: Steven Rostedt <[email protected]>
> > Signed-off-by: Florent Revest <[email protected]>
> > ---
> > include/linux/ftrace.h | 6 ++++--
> > kernel/bpf/trampoline.c | 2 +-
> > kernel/trace/ftrace.c | 6 +++++-
> > samples/ftrace/ftrace-direct-multi-modify.c | 3 +--
> > samples/ftrace/ftrace-direct-multi.c | 3 +--
> > 5 files changed, 12 insertions(+), 8 deletions(-)
>
> This looks good to me; I see that the BPF code frees the filter in
> bpf_trampoline_put(), so it not doing so via unregister_ftrace_direct_multi()
> looks fine. FWIW:
>
> Acked-by: Mark Rutland <[email protected]>

yes, I was going to ack the next version ;-)

thanks,
jirka

>
> Mark.
>
> >
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index 366c730beaa3..5b68ee874bc1 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -407,7 +407,8 @@ int ftrace_modify_direct_caller(struct ftrace_func_entry *entry,
> > unsigned long new_addr);
> > unsigned long ftrace_find_rec_direct(unsigned long ip);
> > int register_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr);
> > -int unregister_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr);
> > +int unregister_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr,
> > + bool free_filters);
> > int modify_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr);
> > int modify_ftrace_direct_multi_nolock(struct ftrace_ops *ops, unsigned long addr);
> >
> > @@ -446,7 +447,8 @@ static inline int register_ftrace_direct_multi(struct ftrace_ops *ops, unsigned
> > {
> > return -ENODEV;
> > }
> > -static inline int unregister_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr)
> > +static inline int unregister_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr,
> > + bool free_filters)
> > {
> > return -ENODEV;
> > }
> > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > index d0ed7d6f5eec..88bc23f1e10a 100644
> > --- a/kernel/bpf/trampoline.c
> > +++ b/kernel/bpf/trampoline.c
> > @@ -198,7 +198,7 @@ static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
> > int ret;
> >
> > if (tr->func.ftrace_managed)
> > - ret = unregister_ftrace_direct_multi(tr->fops, (long)old_addr);
> > + ret = unregister_ftrace_direct_multi(tr->fops, (long)old_addr, false);
> > else
> > ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, NULL);
> >
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index 29baa97d0d53..fa379cf91fdb 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -5804,7 +5804,8 @@ EXPORT_SYMBOL_GPL(register_ftrace_direct_multi);
> > * 0 on success
> > * -EINVAL - The @ops object was not properly registered.
> > */
> > -int unregister_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr)
> > +int unregister_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr,
> > + bool free_filters)
> > {
> > struct ftrace_hash *hash = ops->func_hash->filter_hash;
> > int err;
> > @@ -5822,6 +5823,9 @@ int unregister_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr)
> > /* cleanup for possible another register call */
> > ops->func = NULL;
> > ops->trampoline = 0;
> > +
> > + if (free_filters)
> > + ftrace_free_filter(ops);
> > return err;
> > }
> > EXPORT_SYMBOL_GPL(unregister_ftrace_direct_multi);
> > diff --git a/samples/ftrace/ftrace-direct-multi-modify.c b/samples/ftrace/ftrace-direct-multi-modify.c
> > index b58c594efb51..196b43971cb5 100644
> > --- a/samples/ftrace/ftrace-direct-multi-modify.c
> > +++ b/samples/ftrace/ftrace-direct-multi-modify.c
> > @@ -151,8 +151,7 @@ static int __init ftrace_direct_multi_init(void)
> > static void __exit ftrace_direct_multi_exit(void)
> > {
> > kthread_stop(simple_tsk);
> > - unregister_ftrace_direct_multi(&direct, my_tramp);
> > - ftrace_free_filter(&direct);
> > + unregister_ftrace_direct_multi(&direct, my_tramp, true);
> > }
> >
> > module_init(ftrace_direct_multi_init);
> > diff --git a/samples/ftrace/ftrace-direct-multi.c b/samples/ftrace/ftrace-direct-multi.c
> > index c27cf130c319..ea0e88ee5e43 100644
> > --- a/samples/ftrace/ftrace-direct-multi.c
> > +++ b/samples/ftrace/ftrace-direct-multi.c
> > @@ -78,8 +78,7 @@ static int __init ftrace_direct_multi_init(void)
> >
> > static void __exit ftrace_direct_multi_exit(void)
> > {
> > - unregister_ftrace_direct_multi(&direct, (unsigned long) my_tramp);
> > - ftrace_free_filter(&direct);
> > + unregister_ftrace_direct_multi(&direct, (unsigned long) my_tramp, true);
> > }
> >
> > module_init(ftrace_direct_multi_init);
> > --
> > 2.40.0.rc2.332.ga46443480c-goog
> >