ftrace: Add MODIFIED flag to show if IPMODIFY or direct was attached

If a function had ever had IPMODIFY or DIRECT attached to it, where this
is how live kernel patching and BPF overrides work, mark them and display
an "M" in the enabled_functions and touched_functions files. This can be
used for debugging. If a function had been modified and later there's a bug
in the code related to that function, this can be used to know if the cause
is possibly from a live kernel patch or a BPF program that changed the
behavior of the code.

Also update the documentation on the enabled_functions and
touched_functions output, as it was missing direct callers and CALL_OPS.
And include this new modify attribute.

Link: https://lore.kernel.org/linux-trace-kernel/20230502213233.004e3ae4@gandalf.local.home

Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Documentation/trace/ftrace.rst

@@ -350,6 +350,19 @@ of ftrace. Here is a list of some of the key files:
 	an 'I' will be displayed on the same line as the function that
 	can be overridden.
 
+	If a non ftrace trampoline is attached (BPF) a 'D' will be displayed.
+	Note, normal ftrace trampolines can also be attached, but only one
+	"direct" trampoline can be attached to a given function at a time.
+
+	Some architectures can not call direct trampolines, but instead have
+	the ftrace ops function located above the function entry point. In
+	such cases an 'O' will be displayed.
+
+	If a function had either the "ip modify" or a "direct" call attached to
+	it in the past, a 'M' will be shown. This flag is never cleared. It is
+	used to know if a function was every modified by the ftrace infrastructure,
+	and can be used for debugging.
+
 	If the architecture supports it, it will also show what callback
 	is being directly called by the function. If the count is greater
 	than 1 it most likely will be ftrace_ops_list_func().
@@ -359,6 +372,18 @@ of ftrace. Here is a list of some of the key files:
 	its address will be printed as well as the function that the
 	trampoline calls.
 
+  touched_functions:
+
+	This file contains all the functions that ever had a function callback
+	to it via the ftrace infrastructure. It has the same format as
+	enabled_functions but shows all functions that have every been
+	traced.
+
+	To see any function that has every been modified by "ip modify" or a
+	direct trampoline, one can perform the following command:
+
+	grep ' M ' /sys/kernel/tracing/touched_functions
+
   function_profile_enabled:
 
 	When set it will enable all functions with either the function

include/linux/ftrace.h

@@ -549,6 +549,7 @@ bool is_ftrace_trampoline(unsigned long addr);
  *  CALL_OPS - the record can use callsite-specific ops
  *  CALL_OPS_EN - the function is set up to use callsite-specific ops
  *  TOUCHED  - A callback was added since boot up
+ *  MODIFIED - The function had IPMODIFY or DIRECT attached to it
  *
  * When a new ftrace_ops is registered and wants a function to save
  * pt_regs, the rec->flags REGS is set. When the function has been
@@ -569,9 +570,10 @@ enum {
 	FTRACE_FL_CALL_OPS	= (1UL << 22),
 	FTRACE_FL_CALL_OPS_EN	= (1UL << 21),
 	FTRACE_FL_TOUCHED	= (1UL << 20),
+	FTRACE_FL_MODIFIED	= (1UL << 19),
 };
 
-#define FTRACE_REF_MAX_SHIFT	20
+#define FTRACE_REF_MAX_SHIFT	19
 #define FTRACE_REF_MAX		((1UL << FTRACE_REF_MAX_SHIFT) - 1)
 
 #define ftrace_rec_count(rec)	((rec)->flags & FTRACE_REF_MAX)

kernel/trace/ftrace.c

@@ -46,7 +46,8 @@
 #include "trace_stat.h"
 
 /* Flags that do not get reset */
-#define FTRACE_NOCLEAR_FLAGS	(FTRACE_FL_DISABLED | FTRACE_FL_TOUCHED)
+#define FTRACE_NOCLEAR_FLAGS	(FTRACE_FL_DISABLED | FTRACE_FL_TOUCHED | \
+				 FTRACE_FL_MODIFIED)
 
 #define FTRACE_INVALID_FUNCTION		"__ftrace_invalid_address__"
 
@@ -2273,6 +2274,10 @@ static int ftrace_check_record(struct dyn_ftrace *rec, bool enable, bool update)
 					rec->flags &= ~FTRACE_FL_TRAMP_EN;
 			}
 
+			/* Keep track of anything that modifies the function */
+			if (rec->flags & (FTRACE_FL_DIRECT | FTRACE_FL_IPMODIFY))
+				rec->flags |= FTRACE_FL_MODIFIED;
+
 			if (flag & FTRACE_FL_DIRECT) {
 				/*
 				 * If there's only one user (direct_ops helper)
@@ -3866,12 +3871,13 @@ static int t_show(struct seq_file *m, void *v)
 	if (iter->flags & (FTRACE_ITER_ENABLED | FTRACE_ITER_TOUCHED)) {
 		struct ftrace_ops *ops;
 
-		seq_printf(m, " (%ld)%s%s%s%s",
+		seq_printf(m, " (%ld)%s%s%s%s%s",
 			   ftrace_rec_count(rec),
 			   rec->flags & FTRACE_FL_REGS ? " R" : "  ",
 			   rec->flags & FTRACE_FL_IPMODIFY ? " I" : "  ",
 			   rec->flags & FTRACE_FL_DIRECT ? " D" : "  ",
-			   rec->flags & FTRACE_FL_CALL_OPS ? " O" : "  ");
+			   rec->flags & FTRACE_FL_CALL_OPS ? " O" : "  ",
+			   rec->flags & FTRACE_FL_MODIFIED ? " M " : "   ");
 		if (rec->flags & FTRACE_FL_TRAMP_EN) {
 			ops = ftrace_find_tramp_ops_any(rec);
 			if (ops) {