add infrastructure for tagging functions as error injectable

Using BPF we can override kprob'ed functions and return arbitrary
values.  Obviously this can be a bit unsafe, so make this feature opt-in
for functions.  Simply tag a function with KPROBE_ERROR_INJECT_SYMBOL in
order to give BPF access to that function for error injection purposes.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/module.h

@@ -475,6 +475,11 @@ struct module {
 	ctor_fn_t *ctors;
 	unsigned int num_ctors;
 #endif
+
+#ifdef CONFIG_BPF_KPROBE_OVERRIDE
+	unsigned int num_kprobe_ei_funcs;
+	unsigned long *kprobe_ei_funcs;
+#endif
 } ____cacheline_aligned __randomize_layout;
 #ifndef MODULE_ARCH_INIT
 #define MODULE_ARCH_INIT {}