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Date:   Tue, 17 Nov 2020 17:19:04 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Matt Mullins <mmullins@mmlx.us>,
        Ingo Molnar <mingo@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Dmitry Vyukov <dvyukov@google.com>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH] tracepoint: Do not fail unregistering a probe due to
 memory allocation
Message-ID: <20201117171904.2d455699@gandalf.local.home>
In-Reply-To: <202011171330.94C6BA7E93@keescook>
References: <20201116175107.02db396d@gandalf.local.home>
        <202011171330.94C6BA7E93@keescook>
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On Tue, 17 Nov 2020 13:33:42 -0800
Kees Cook <keescook@chromium.org> wrote:

> As I think got discussed in the thread, what you had here wouldn't work
> in a CFI build if the function prototype of the call site and the
> function don't match. (Though I can't tell if .func() is ever called?)
> 
> i.e. .func's prototype must match tp_stub_func()'s.
> 


Hmm, I wonder how you handle tracepoints? This is called here:

include/linux/tracepoint.h:


#define DEFINE_TRACE_FN(_name, _reg, _unreg, proto, args)		\
	static const char __tpstrtab_##_name[]				\
	__section("__tracepoints_strings") = #_name;			\
	extern struct static_call_key STATIC_CALL_KEY(tp_func_##_name);	\
	int __traceiter_##_name(void *__data, proto);			\
	struct tracepoint __tracepoint_##_name	__used			\
	__section("__tracepoints") = {					\
		.name = __tpstrtab_##_name,				\
		.key = STATIC_KEY_INIT_FALSE,				\
		.static_call_key = &STATIC_CALL_KEY(tp_func_##_name),	\
		.static_call_tramp = STATIC_CALL_TRAMP_ADDR(tp_func_##_name), \
		.iterator = &__traceiter_##_name,			\
		.regfunc = _reg,					\
		.unregfunc = _unreg,					\
		.funcs = NULL };					\
	__TRACEPOINT_ENTRY(_name);					\
	int __traceiter_##_name(void *__data, proto)			\
	{								\
		struct tracepoint_func *it_func_ptr;			\
		void *it_func;						\
									\
		it_func_ptr =						\
			rcu_dereference_raw((&__tracepoint_##_name)->funcs); \
		do {							\
			it_func = (it_func_ptr)->func;			\
			__data = (it_func_ptr)->data;			\

			((void(*)(void *, proto))(it_func))(__data, args); \

			^^^^ called above ^^^^

Where args is unique for every tracepoint, but func is simply a void
pointer.

-- Steve


		} while ((++it_func_ptr)->func);			\
		return 0;						\
	}								\