Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

Miroslav Benes
2020-10-29T13:51:15Z

On Wed, 28 Oct 2020, Steven Rostedt wrote:

> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
>
> If a ftrace callback does not supply its own recursion protection and
> does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
> make a helper trampoline to do so before calling the callback instead of
> just calling the callback directly.
>
> The default for ftrace_ops is going to assume recursion protection unless
> otherwise specified.

Hm, I've always thought that we did not need any kind of recursion
protection for our callback. It is marked as notrace and it does not call
anything traceable. In fact, it does not call anything. I even have a note
in my todo list to mark the callback as RECURSION_SAFE :)

At the same time, it probably does not hurt and the patch is still better
than what we have now without RECURSION_SAFE if I understand the patch set
correctly.

> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Jiri Kosina <jikos@kernel.org>
> Cc: Miroslav Benes <mbenes@suse.cz>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Joe Lawrence <joe.lawrence@redhat.com>
> Cc: live-patching@vger.kernel.org
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> kernel/livepatch/patch.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> index b552cf2d85f8..6c0164d24bbd 100644
> --- a/kernel/livepatch/patch.c
> +++ b/kernel/livepatch/patch.c
> @@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> struct klp_ops *ops;
> struct klp_func *func;
> int patch_state;
> + int bit;
>
> ops = container_of(fops, struct klp_ops, fops);
>
> + bit = ftrace_test_recursion_trylock();
> + if (bit < 0)
> + return;

This means that the original function will be called in case of recursion.
That's probably fair, but I'm wondering if we should at least WARN about
it.

Thanks
Miroslav