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Subject[PATCH 16/20 v2] tracing: Add NULL to skip args for function based events
From: "Steven Rostedt (VMware)" <rostedt@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) <rostedt@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 50d0c4d32596..673e2d963319 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
};

@@ -366,6 +369,8 @@ 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;
@@ -515,6 +520,19 @@ 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;
}
@@ -691,6 +709,8 @@ 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
@@ -817,6 +837,8 @@ 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;
@@ -864,6 +886,9 @@ 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,
@@ -966,6 +991,8 @@ 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;
@@ -1004,6 +1031,8 @@ 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++) {
@@ -1150,7 +1179,10 @@ 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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