[PATCH bpf-next v2 1/6] ftrace: introduce FTRACE_OPS_FL_JMP

[Menglong Dong <menglong8.dong@gmail.com>]

For now, the "nop" will be replaced with a "call" instruction when a
function is hooked by the ftrace. However, sometimes the "call" can break
the RSB and introduce extra overhead. Therefore, introduce the flag
FTRACE_OPS_FL_JMP, which indicate that the ftrace_ops should be called
with a "jmp" instead of "call". For now, it is only used by the direct
call case.

When a direct ftrace_ops is marked with FTRACE_OPS_FL_JMP, the last bit of
the ops->direct_call will be set to 1. Therefore, we can tell if we should
use "jmp" for the callback in ftrace_call_replace().

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
---
 include/linux/ftrace.h | 33 +++++++++++++++++++++++++++++++++
 kernel/trace/Kconfig   | 12 ++++++++++++
 kernel/trace/ftrace.c  |  9 ++++++++-
 3 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 07f8c309e432..015dd1049bea 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -359,6 +359,7 @@ enum {
 	FTRACE_OPS_FL_DIRECT			= BIT(17),
 	FTRACE_OPS_FL_SUBOP			= BIT(18),
 	FTRACE_OPS_FL_GRAPH			= BIT(19),
+	FTRACE_OPS_FL_JMP			= BIT(20),
 };
 
 #ifndef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
@@ -577,6 +578,38 @@ static inline void arch_ftrace_set_direct_caller(struct ftrace_regs *fregs,
 						 unsigned long addr) { }
 #endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_JMP
+static inline bool ftrace_is_jmp(unsigned long addr)
+{
+	return addr & 1;
+}
+
+static inline unsigned long ftrace_jmp_set(unsigned long addr)
+{
+	return addr | 1UL;
+}
+
+static inline unsigned long ftrace_jmp_get(unsigned long addr)
+{
+	return addr & ~1UL;
+}
+#else
+static inline bool ftrace_is_jmp(unsigned long addr)
+{
+	return false;
+}
+
+static inline unsigned long ftrace_jmp_set(unsigned long addr)
+{
+	return addr;
+}
+
+static inline unsigned long ftrace_jmp_get(unsigned long addr)
+{
+	return addr;
+}
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_JMP */
+
 #ifdef CONFIG_STACK_TRACER
 
 int stack_trace_sysctl(const struct ctl_table *table, int write, void *buffer,
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index d2c79da81e4f..4661b9e606e0 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -80,6 +80,12 @@ config HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
 	  If the architecture generates __patchable_function_entries sections
 	  but does not want them included in the ftrace locations.
 
+config HAVE_DYNAMIC_FTRACE_WITH_JMP
+	bool
+	help
+	  If the architecture supports to replace the __fentry__ with a
+	  "jmp" instruction.
+
 config HAVE_SYSCALL_TRACEPOINTS
 	bool
 	help
@@ -330,6 +336,12 @@ config DYNAMIC_FTRACE_WITH_ARGS
 	depends on DYNAMIC_FTRACE
 	depends on HAVE_DYNAMIC_FTRACE_WITH_ARGS
 
+config DYNAMIC_FTRACE_WITH_JMP
+	def_bool y
+	depends on DYNAMIC_FTRACE
+	depends on DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+	depends on HAVE_DYNAMIC_FTRACE_WITH_JMP
+
 config FPROBE
 	bool "Kernel Function Probe (fprobe)"
 	depends on HAVE_FUNCTION_GRAPH_FREGS && HAVE_FTRACE_GRAPH_FUNC
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 59cfacb8a5bb..a6c060a4f50b 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5951,7 +5951,8 @@ static void remove_direct_functions_hash(struct ftrace_hash *hash, unsigned long
 	for (i = 0; i < size; i++) {
 		hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
 			del = __ftrace_lookup_ip(direct_functions, entry->ip);
-			if (del && del->direct == addr) {
+			if (del && ftrace_jmp_get(del->direct) ==
+				   ftrace_jmp_get(addr)) {
 				remove_hash_entry(direct_functions, del);
 				kfree(del);
 			}
@@ -6018,6 +6019,9 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
 
 	mutex_lock(&direct_mutex);
 
+	if (ops->flags & FTRACE_OPS_FL_JMP)
+		addr = ftrace_jmp_set(addr);
+
 	/* Make sure requested entries are not already registered.. */
 	size = 1 << hash->size_bits;
 	for (i = 0; i < size; i++) {
@@ -6138,6 +6142,9 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
 
 	lockdep_assert_held_once(&direct_mutex);
 
+	if (ops->flags & FTRACE_OPS_FL_JMP)
+		addr = ftrace_jmp_set(addr);
+
 	/* Enable the tmp_ops to have the same functions as the direct ops */
 	ftrace_ops_init(&tmp_ops);
 	tmp_ops.func_hash = ops->func_hash;
-- 
2.51.2