From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

Add a "string" type that will create a dynamic length string for the
event, this is the same as the __string() field in normal TRACE_EVENTS.

[ missing 'static' found by Fengguang Wu's kbuild test robot ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 Documentation/trace/function-based-events.rst |  19 ++-
 kernel/trace/trace_event_ftrace.c             | 183 +++++++++++++++++++++++---
 2 files changed, 181 insertions(+), 21 deletions(-)

diff --git a/Documentation/trace/function-based-events.rst b/Documentation/trace/function-based-events.rst
index 99ae77cd59e6..6c643ea749e7 100644
--- a/Documentation/trace/function-based-events.rst
+++ b/Documentation/trace/function-based-events.rst

@@ -99,7 +99,7 @@ as follows:
          's8' | 's16' | 's32' | 's64' |
          'x8' | 'x16' | 'x32' | 'x64' |
          'char' | 'short' | 'int' | 'long' | 'size_t' |
-	 'symbol'
+	 'symbol' | 'string'
 
  FIELD := <name> | <name> INDEX | <name> OFFSET | <name> OFFSET INDEX
 

@@ -342,3 +342,20 @@ the format "%s". If a nul is found, the output will stop. Use another type
       bash-1470  [003] ...2   980.678715: path_openat->link_path_walk(name=/lib64/ld-linux-x86-64.so.2)
       bash-1470  [003] ...2   980.678721: path_openat->link_path_walk(name=ld-2.24.so)
       bash-1470  [003] ...2   980.678978: path_lookupat->link_path_walk(name=/etc/ld.so.preload)
+
+
+Dynamic strings
+===============
+
+Static strings are fine, but they can waste a lot of memory in the ring buffer.
+The above allocated 64 bytes for a character array, but most of the output was
+less than 20 characters. Not wanting to truncate strings or waste space on
+the ring buffer, the dynamic string can help.
+
+Use the "string" type for strings that have a large range in size. The max
+size that will be recorded is 512 bytes. If a string is larger than that, then
+it will be truncated.
+
+ # echo 'link_path_walk(string name)' > function_events
+
+Gives the same result as above, but does not waste buffer space.

diff --git a/kernel/trace/trace_event_ftrace.c b/kernel/trace/trace_event_ftrace.c
index dd24b840329d..273c5838a8e2 100644
--- a/kernel/trace/trace_event_ftrace.c
+++ b/kernel/trace/trace_event_ftrace.c

@@ -39,6 +39,7 @@ struct func_event {
 	struct func_arg			*last_arg;
 	int				arg_cnt;
 	int				arg_offset;
+	int				has_strings;
 };
 
 struct func_file {

@@ -83,6 +84,8 @@ typedef u32 x32;
 typedef u16 x16;
 typedef u8 x8;
 typedef void * symbol;
+/* 2 byte offset, 2 byte length */
+typedef u32 string;
 
 #define TYPE_TUPLE(type)			\
 	{ #type, sizeof(type), is_signed_type(type) }

@@ -105,7 +108,8 @@ typedef void * symbol;
 	TYPE_TUPLE(u8),				\
 	TYPE_TUPLE(s8),				\
 	TYPE_TUPLE(x8),				\
-	TYPE_TUPLE(symbol)
+	TYPE_TUPLE(symbol),			\
+	TYPE_TUPLE(string)
 
 static struct func_type {
 	char		*name;

@@ -124,6 +128,16 @@ enum {
 	FUNC_TYPE_MAX
 };
 
+#define MAX_STR		512
+
+/* Two contexts, normal and NMI, hence the " * 2" */
+struct func_string {
+	char		buf[MAX_STR * 2];
+};
+
+static struct func_string __percpu *str_buffer;
+static int nr_strings;

+
 /**
  * arch_get_func_args - retrieve function arguments via pt_regs
  * @regs: The registers at the moment the function is called

@@ -163,6 +177,23 @@ int __weak arch_get_func_args(struct pt_regs *regs,
 	return 0;
 }
 
+static void free_arg(struct func_arg *arg)
+{
+	list_del(&arg->list);
+	if (arg->func_type == FUNC_TYPE_string) {
+		nr_strings--;
+		if (WARN_ON(nr_strings < 0))
+			nr_strings = 0;
+		if (!nr_strings) {
+			free_percpu(str_buffer);
+			str_buffer = NULL;
+		}
+	}
+	kfree(arg->name);
+	kfree(arg->type);
+	kfree(arg);
+}
+
 static void free_func_event(struct func_event *func_event)
 {
 	struct func_arg *arg, *n;

@@ -171,10 +202,7 @@ static void free_func_event(struct func_event *func_event)
 		return;
 
 	list_for_each_entry_safe(arg, n, &func_event->args, list) {
-		list_del(&arg->list);
-		kfree(arg->name);
-		kfree(arg->type);
-		kfree(arg);
+		free_arg(arg);
 	}
 	ftrace_free_filter(&func_event->ops);
 	kfree(func_event->call.print_fmt);

@@ -255,6 +283,17 @@ static int add_arg(struct func_event *fevent, int ftype, int unsign)
 	list_add_tail(&arg->list, &fevent->args);
 	fevent->last_arg = arg;
 
+	if (ftype == FUNC_TYPE_string) {
+		fevent->has_strings++;
+		nr_strings++;
+		if (nr_strings == 1) {
+			str_buffer = alloc_percpu(struct func_string);
+			if (!str_buffer) {
+				free_arg(arg);
+				return -ENOMEM;
+			}
+		}
+	}
 	return 0;
 }