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

If args are to be skipped (only care about second, third or later arguments)
then add a NULL to ignore them. For example, if one only wants to record the
third argument of a function, they can perform:

 echo foo(NULL, NULL, u32 arg3) > function_events

Then only the third argument is saved in the function based event.

Signed-off-by: Steven Rostedt (VMware) <rost...@goodmis.org>
---
 Documentation/trace/function-based-events.rst | 28 +++++++++++++++++++++-
 kernel/trace/trace_event_ftrace.c             | 34 ++++++++++++++++++++++++++-
 2 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/Documentation/trace/function-based-events.rst 
b/Documentation/trace/function-based-events.rst
index 6c643ea749e7..b90b52b7061d 100644
--- a/Documentation/trace/function-based-events.rst
+++ b/Documentation/trace/function-based-events.rst
@@ -91,7 +91,7 @@ as follows:
 
  ARGS := ARG | ARG ',' ARGS | ''
 
- ARG := TYPE FIELD | TYPE <name> '=' ADDR | TYPE ADDR | ARG '|' ARG
+ ARG := TYPE FIELD | TYPE <name> '=' ADDR | TYPE ADDR | ARG '|' ARG | 'NULL'
 
  TYPE := ATOM | ATOM '[' <number> ']' | 'unsigned' TYPE
 
@@ -359,3 +359,29 @@ it will be truncated.
  # echo 'link_path_walk(string name)' > function_events
 
 Gives the same result as above, but does not waste buffer space.
+
+
+NULL arguments
+==============
+
+If you are only interested in the second, or later parameter of a function,
+you do not have to record the previous parameters. Just set them as NULL and
+they will not be recorded.
+
+If we only wanted the perm_addr of the net_device of ip_rcv() and not the
+sk_buff, we put a NULL into the first parameter when created the function
+based event.
+
+  # echo 'ip_rcv(NULL, x8[6] perm_addr+558)' > function_events
+
+  # echo 1 > events/functions/ip_rcv/enable
+  # cat trace
+    <idle>-0     [003] ..s3   165.617114: 
__netif_receive_skb_core->ip_rcv(perm_addr=b4,b5,2f,ce,18,65)
+    <idle>-0     [003] ..s3   165.617133: 
__netif_receive_skb_core->ip_rcv(perm_addr=b4,b5,2f,ce,18,65)
+    <idle>-0     [003] ..s3   166.412277: 
__netif_receive_skb_core->ip_rcv(perm_addr=b4,b5,2f,ce,18,65)
+    <idle>-0     [003] ..s3   166.412797: 
__netif_receive_skb_core->ip_rcv(perm_addr=b4,b5,2f,ce,18,65)
+
+
+NULL can appear in any argument, to have them ignored. Note, skipping arguments
+does not give you access to later arguments if they are not supported by the
+architecture. The architecture only supplies the first set of arguments.
diff --git a/kernel/trace/trace_event_ftrace.c 
b/kernel/trace/trace_event_ftrace.c
index 273c5838a8e2..22bcb67ad184 100644
--- a/kernel/trace/trace_event_ftrace.c
+++ b/kernel/trace/trace_event_ftrace.c
@@ -75,6 +75,7 @@ enum func_states {
        FUNC_STATE_ARRAY_END,
        FUNC_STATE_VAR,
        FUNC_STATE_COMMA,
+       FUNC_STATE_NULL,
        FUNC_STATE_END,
        FUNC_STATE_ERROR,
 };
@@ -117,6 +118,7 @@ static struct func_type {
        int             sign;
 } func_types[] = {
        FUNC_TYPES,
+       { "NULL",       0,      0 },
        { NULL,         0,      0 }
 };
 
@@ -125,6 +127,7 @@ static struct func_type {
 
 enum {
        FUNC_TYPES,
+       FUNC_TYPE_NULL,
        FUNC_TYPE_MAX
 };
 
@@ -364,6 +367,8 @@ process_event(struct func_event *fevent, const char *token, 
enum func_states sta
                        fevent->arg_cnt++;
                update_arg = false;
        case FUNC_STATE_PIPE:
+               if (strcmp(token, "NULL") == 0)
+                       return FUNC_STATE_NULL;
                if (strcmp(token, "unsigned") == 0) {
                        unsign = 2;
                        return FUNC_STATE_UNSIGNED;
@@ -513,6 +518,19 @@ process_event(struct func_event *fevent, const char 
*token, enum func_states sta
                fevent->last_arg->indirect = INDIRECT_FLAG;
                return FUNC_STATE_ADDR;
 
+       case FUNC_STATE_NULL:
+               ret = add_arg(fevent, FUNC_TYPE_NULL, 0);
+               if (ret < 0)
+                       break;
+               switch (token[0]) {
+               case ')':
+                       goto end;
+               case ',':
+                       update_arg = true;
+                       return FUNC_STATE_COMMA;
+               }
+               break;
+
        default:
                break;
        }
@@ -689,6 +707,8 @@ static void func_event_trace(struct trace_event_file 
*trace_file,
        entry->parent_ip = parent_ip;
 
        list_for_each_entry(arg, &func_event->args, list) {
+               if (arg->func_type == FUNC_TYPE_NULL)
+                       continue;
                if (arg->arg < nr_args)
                        val = get_arg(arg, args);
                else
@@ -811,6 +831,8 @@ func_event_print(struct trace_iterator *iter, int flags,
        trace_seq_printf(s, "%ps->%ps(",
                         (void *)entry->parent_ip, (void *)entry->ip);
        list_for_each_entry(arg, &func_event->args, list) {
+               if (arg->func_type == FUNC_TYPE_NULL)
+                       continue;
                if (comma)
                        trace_seq_puts(s, ", ");
                comma = true;
@@ -858,6 +880,9 @@ static int func_event_define_fields(struct trace_event_call 
*event_call)
        list_for_each_entry(arg, &fevent->args, list) {
                int size = arg->size;
 
+               if (arg->func_type == FUNC_TYPE_NULL)
+                       continue;
+
                if (arg->array)
                        size *= arg->array;
                ret = trace_define_field(event_call, arg->type,
@@ -960,6 +985,8 @@ static int __set_print_fmt(struct func_event *func_event,
        r = snprintf(buf, len, "%s", fmt_start);
        len = update_len(len, r);
        list_for_each_entry(arg, &func_event->args, list) {
+               if (arg->func_type == FUNC_TYPE_NULL)
+                       continue;
                if (comma) {
                        i = snprintf(buf + r, len, ", ");
                        r += i;
@@ -998,6 +1025,8 @@ static int __set_print_fmt(struct func_event *func_event,
        len = update_len(len, i);
 
        list_for_each_entry(arg, &func_event->args, list) {
+               if (arg->func_type == FUNC_TYPE_NULL)
+                       continue;
                /* Don't iterate for strings */
                if (arg->array && arg->func_type != FUNC_TYPE_char) {
                        for (a = 0; a < arg->array; a++) {
@@ -1148,7 +1177,10 @@ static int func_event_seq_show(struct seq_file *m, void 
*v)
                }
                last_arg = arg->arg;
                comma = true;
-               seq_printf(m, "%s %s", arg->type, arg->name);
+               if (arg->func_type == FUNC_TYPE_NULL)
+                       seq_puts(m, "NULL");
+               else
+                       seq_printf(m, "%s %s", arg->type, arg->name);
                if (arg->arg < 0) {
                        seq_printf(m, "=0x%lx", arg->index);
                } else {
-- 
2.15.1


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