ftrace: Add a helper function to modify_ftrace_direct() to allow arch optimization

If a direct ftrace callback is at a location that does not have any other
ftrace helpers attached to it, it is possible to simply just change the
text to call the new caller (if the architecture supports it). But this
requires special architecture code. Currently, modify_ftrace_direct() uses a
trick to add a stub ftrace callback to the location forcing it to call the
ftrace iterator. Then it can change the direct helper to call the new
function in C, and then remove the stub. Removing the stub will have the
location now call the new location that the direct helper is using.

The new helper function does the registering the stub trick, but is a weak
function, allowing an architecture to override it to do something a bit more
direct.

Link: https://lore.kernel.org/r/20191115215125.mbqv7taqnx376yed@ast-mbp.dhcp.thefacebook.com

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

1 files changed, 19 insertions, 2 deletions

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 73eb2e93593f..dfaa37e1943d 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -246,12 +246,24 @@ static inline void ftrace_free_init_mem(void) { }
 static inline void ftrace_free_mem(struct module *mod, void *start, void *end) { }
 #endif /* CONFIG_FUNCTION_TRACER */
 
+struct ftrace_func_entry {
+	struct hlist_node hlist;
+	unsigned long ip;
+	unsigned long direct; /* for direct lookup only */
+};
+
+struct dyn_ftrace;
+
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 extern int ftrace_direct_func_count;
 int register_ftrace_direct(unsigned long ip, unsigned long addr);
 int unregister_ftrace_direct(unsigned long ip, unsigned long addr);
 int modify_ftrace_direct(unsigned long ip, unsigned long old_addr, unsigned long new_addr);
 struct ftrace_direct_func *ftrace_find_direct_func(unsigned long addr);
+int ftrace_modify_direct_caller(struct ftrace_func_entry *entry,
+				struct dyn_ftrace *rec,
+				unsigned long old_addr,
+				unsigned long new_addr);
 #else
 # define ftrace_direct_func_count 0
 static inline int register_ftrace_direct(unsigned long ip, unsigned long addr)
@@ -271,6 +283,13 @@ static inline struct ftrace_direct_func *ftrace_find_direct_func(unsigned long a
 {
 	return NULL;
 }
+static inline int ftrace_modify_direct_caller(struct ftrace_func_entry *entry,
+					      struct dyn_ftrace *rec,
+					      unsigned long old_addr,
+					      unsigned long new_addr)
+{
+	return -ENODEV;
+}
 #endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
 
 #ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
@@ -343,8 +362,6 @@ static inline void stack_tracer_enable(void) { }
 int ftrace_arch_code_modify_prepare(void);
 int ftrace_arch_code_modify_post_process(void);
 
-struct dyn_ftrace;
-
 enum ftrace_bug_type {
 	FTRACE_BUG_UNKNOWN,
 	FTRACE_BUG_INIT,