Current implementation calls kprobe related functions before doing
ftrace recursion check in fprobe_kprobe_handler, which opens door
to kernel crash due to stack recursion if preempt_count_{add, sub}
is traceable in kprobe_busy_{begin, end}.

Things goes like this without this patch quoted from Steven:
"
fprobe_kprobe_handler() {
   kprobe_busy_begin() {
      preempt_disable() {
         preempt_count_add() {  <-- trace
            fprobe_kprobe_handler() {
                [ wash, rinse, repeat, CRASH!!! ]
"

By refactoring the common part out of fprobe_kprobe_handler and
fprobe_handler and call ftrace recursion detection at the very beginning,
the whole fprobe_kprobe_handler is free from recursion.

Signed-off-by: Ze Gao <redacted>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/linux-trace-kernel/20230516071830.8190-3-zegao@tencent.com (local)
---
 kernel/trace/fprobe.c | 59 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 44 insertions(+), 15 deletions(-)

diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 9abb3905bc8e..097c740799ba 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c

@@ -20,30 +20,22 @@ struct fprobe_rethook_node {
        char data[];
 };

-static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
-                          struct ftrace_ops *ops, struct ftrace_regs *fregs)
+static inline void __fprobe_handler(unsigned long ip, unsigned long
+               parent_ip, struct ftrace_ops *ops, struct ftrace_regs *fregs)
 {
        struct fprobe_rethook_node *fpr;
        struct rethook_node *rh = NULL;
        struct fprobe *fp;
        void *entry_data = NULL;
-       int bit, ret;
+       int ret;

        fp = container_of(ops, struct fprobe, ops);
-       if (fprobe_disabled(fp))
-               return;
-
-       bit = ftrace_test_recursion_trylock(ip, parent_ip);
-       if (bit < 0) {
-               fp->nmissed++;
-               return;
-       }

        if (fp->exit_handler) {
                rh = rethook_try_get(fp->rethook);
                if (!rh) {
                        fp->nmissed++;
-                       goto out;
+                       return;
                }
                fpr = container_of(rh, struct fprobe_rethook_node, node);
                fpr->entry_ip = ip;

@@ -61,23 +53,60 @@ static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
                else
                        rethook_hook(rh, ftrace_get_regs(fregs), true);
        }
-out:
+}
+
+static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
+               struct ftrace_ops *ops, struct ftrace_regs *fregs)
+{
+       struct fprobe *fp;
+       int bit;
+
+       fp = container_of(ops, struct fprobe, ops);
+       if (fprobe_disabled(fp))
+               return;
+
+       /* recursion detection has to go before any traceable function and
+        * all functions before this point should be marked as notrace
+        */
+       bit = ftrace_test_recursion_trylock(ip, parent_ip);
+       if (bit < 0) {
+               fp->nmissed++;
+               return;
+       }
+       __fprobe_handler(ip, parent_ip, ops, fregs);
        ftrace_test_recursion_unlock(bit);
+
 }
 NOKPROBE_SYMBOL(fprobe_handler);

 static void fprobe_kprobe_handler(unsigned long ip, unsigned long parent_ip,
                                  struct ftrace_ops *ops, struct ftrace_regs *fregs)
 {
-       struct fprobe *fp = container_of(ops, struct fprobe, ops);
+       struct fprobe *fp;
+       int bit;
+
+       fp = container_of(ops, struct fprobe, ops);
+       if (fprobe_disabled(fp))
+               return;
+
+       /* recursion detection has to go before any traceable function and
+        * all functions called before this point should be marked as notrace
+        */
+       bit = ftrace_test_recursion_trylock(ip, parent_ip);
+       if (bit < 0) {
+               fp->nmissed++;
+               return;
+       }

        if (unlikely(kprobe_running())) {
                fp->nmissed++;