fprobe: Introduce FPROBE_FL_KPROBE_SHARED flag for fprobe

Introduce FPROBE_FL_KPROBE_SHARED flag for sharing fprobe callback with
kprobes safely from the viewpoint of recursion.

Since the recursion safety of the fprobe (and ftrace) is a bit different
from the kprobes, this may cause an issue if user wants to run the same
code from the fprobe and the kprobes.

The kprobes has per-cpu 'current_kprobe' variable which protects the
kprobe handler from recursion in any case. On the other hand, the fprobe
uses only ftrace_test_recursion_trylock(), which will allow interrupt
context calls another (or same) fprobe during the fprobe user handler is
running.

This is not a matter in cases if the common callback shared among the
kprobes and the fprobe has its own recursion detection, or it can handle
the recursion in the different contexts (normal/interrupt/NMI.)
But if it relies on the 'current_kprobe' recursion lock, it has to check
kprobe_running() and use kprobe_busy_*() APIs.

Fprobe has FPROBE_FL_KPROBE_SHARED flag to do this. If your common callback
code will be shared with kprobes, please set FPROBE_FL_KPROBE_SHARED
*before* registering the fprobe, like;

 fprobe.flags = FPROBE_FL_KPROBE_SHARED;

 register_fprobe(&fprobe, "func*", NULL);

This will protect your common callback from the nested call.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Tested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/164735293127.1084943.15687374237275817599.stgit@devnote2

include/linux/fprobe.h

@@ -34,13 +34,25 @@ struct fprobe {
 	void (*exit_handler)(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs);
 };
 
+/* This fprobe is soft-disabled. */
 #define FPROBE_FL_DISABLED	1
 
+/*
+ * This fprobe handler will be shared with kprobes.
+ * This flag must be set before registering.
+ */
+#define FPROBE_FL_KPROBE_SHARED	2
+
 static inline bool fprobe_disabled(struct fprobe *fp)
 {
 	return (fp) ? fp->flags & FPROBE_FL_DISABLED : false;
 }
 
+static inline bool fprobe_shared_with_kprobes(struct fprobe *fp)
+{
+	return (fp) ? fp->flags & FPROBE_FL_KPROBE_SHARED : false;
+}
+
 #ifdef CONFIG_FPROBE
 int register_fprobe(struct fprobe *fp, const char *filter, const char *notfilter);
 int register_fprobe_ips(struct fprobe *fp, unsigned long *addrs, int num);

include/linux/kprobes.h

@@ -427,6 +427,9 @@ static inline struct kprobe *kprobe_running(void)
 {
 	return NULL;
 }
+#define kprobe_busy_begin()	do {} while (0)
+#define kprobe_busy_end()	do {} while (0)
+
 static inline int register_kprobe(struct kprobe *p)
 {
 	return -EOPNOTSUPP;

kernel/trace/fprobe.c

@@ -56,6 +56,20 @@ out:
 }
 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);
+
+	if (unlikely(kprobe_running())) {
+		fp->nmissed++;
+		return;
+	}
+	kprobe_busy_begin();
+	fprobe_handler(ip, parent_ip, ops, fregs);
+	kprobe_busy_end();
+}
+
 static void fprobe_exit_handler(struct rethook_node *rh, void *data,
 				struct pt_regs *regs)
 {
@@ -110,6 +124,9 @@ error:
 static void fprobe_init(struct fprobe *fp)
 {
 	fp->nmissed = 0;
+	if (fprobe_shared_with_kprobes(fp))
+		fp->ops.func = fprobe_kprobe_handler;
+	else
 		fp->ops.func = fprobe_handler;
 	fp->ops.flags |= FTRACE_OPS_FL_SAVE_REGS;
 }