[PATCH 06/18] tracing: Add indirect offset to args of ftrace based events

Steven Rostedt
2018-02-02T23:05:04Z

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

Add '[' ']' syntex to allow to get values indirectly from the arguments.
For example:

echo replenish_dl_entity(s64 dl_se[4]) > function_events

Will get the 4th long long word from the first parameter like an array.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
Documentation/trace/function-based-events.rst | 32 +++++++++++-
kernel/trace/trace_event_ftrace.c | 73 +++++++++++++++++++++++++--
2 files changed, 101 insertions(+), 4 deletions(-)

diff --git a/Documentation/trace/function-based-events.rst b/Documentation/trace/function-based-events.rst
index f27a0c4e829c..7d67229e8e88 100644
--- a/Documentation/trace/function-based-events.rst
+++ b/Documentation/trace/function-based-events.rst
@@ -100,11 +100,15 @@ as follows:
'x8' | 'x16' | 'x32' | 'x64' |
'char' | 'short' | 'int' | 'long' | 'size_t'

- FIELD := <name>
+ FIELD := <name> | <name> INDEX
+
+ INDEX := '[' <number> ']'

Where <name> is a unique string starting with an alphabetic character
and consists only of letters and numbers and underscores.

+ Where <number> is a number that can be read by kstrtol() (hex, decimal, etc).
+

Simple arguments
================
@@ -128,3 +132,29 @@ If we are only interested in the first argument (skb):

We use "x64" in order to make sure that the data is displayed in hex.
This is on a x86_64 machine, and we know the pointer sizes are 8 bytes.
+
+
+Indexing
+========
+
+The pointers of the skb and the dev isn't that interesting. But if we want the
+length "len" field of skb, we could index it with an index operator '[' and ']'.
+
+Using gdb, we can find the offset of 'len' from the sk_buff type:
+
+ $ gdb vmlinux
+ (gdb) printf "%d\n", &((struct sk_buff *)0)->len
+128
+
+As 128 / 4 (length of int) is 32, we can see the length of the skb with:
+
+ # echo 'ip_rcv(int skb[32], x64 dev)' > function_events
+
+ # echo 1 > events/functions/ip_rcv/enable
+ # cat trace
+ <idle>-0 [003] ..s3 280.167137: __netif_receive_skb_core->ip_rcv(skb=52, dev=ffff8801092f9400)
+ <idle>-0 [003] ..s3 280.167152: __netif_receive_skb_core->ip_rcv(skb=52, dev=ffff8801092f9400)
+ <idle>-0 [003] ..s3 280.806629: __netif_receive_skb_core->ip_rcv(skb=88, dev=ffff8801092f9400)
+ <idle>-0 [003] ..s3 280.807023: __netif_receive_skb_core->ip_rcv(skb=52, dev=ffff8801092f9400)
+
+Now we see the length of the sk_buff per event.
diff --git a/kernel/trace/trace_event_ftrace.c b/kernel/trace/trace_event_ftrace.c
index aa19c8af9d34..5d37498d1c6b 100644
--- a/kernel/trace/trace_event_ftrace.c
+++ b/kernel/trace/trace_event_ftrace.c
@@ -10,13 +10,15 @@

#include "trace.h"

-#define FUNC_EVENT_SYSTEM "functions"
-#define WRITE_BUFSIZE 4096
+#define FUNC_EVENT_SYSTEM "functions"
+#define WRITE_BUFSIZE 4096
+#define INDIRECT_FLAG 0x10000000

struct func_arg {
struct list_head list;
char *type;
char *name;
+ long indirect;
short offset;
short size;
char arg;
@@ -55,6 +57,9 @@ enum func_states {
FUNC_STATE_INIT,
FUNC_STATE_FUNC,
FUNC_STATE_PARAM,
+ FUNC_STATE_BRACKET,
+ FUNC_STATE_BRACKET_END,
+ FUNC_STATE_INDIRECT,
FUNC_STATE_TYPE,
FUNC_STATE_VAR,
FUNC_STATE_COMMA,
@@ -171,6 +176,8 @@ static char *next_token(char **ptr, char *last)

for (str = arg; *str; str++) {
if (*str == '(' ||
+ *str == '[' ||
+ *str == ']' ||
*str == ',' ||
*str == ')')
break;
@@ -223,6 +230,7 @@ static int add_arg(struct func_event *fevent, int ftype)
static enum func_states
process_event(struct func_event *fevent, const char *token, enum func_states state)
{
+ long val;
int ret;
int i;

@@ -269,12 +277,37 @@ process_event(struct func_event *fevent, const char *token, enum func_states sta
break;
return FUNC_STATE_VAR;

+ case FUNC_STATE_BRACKET:
+ WARN_ON(!fevent->last_arg);
+ ret = kstrtol(token, 0, &val);
+ if (ret)
+ break;
+ val *= fevent->last_arg->size;
+ fevent->last_arg->indirect = val ^ INDIRECT_FLAG;
+ return FUNC_STATE_INDIRECT;
+
+ case FUNC_STATE_INDIRECT:
+ if (token[0] != ']')
+ break;
+ return FUNC_STATE_BRACKET_END;
+
+ case FUNC_STATE_BRACKET_END:
+ switch (token[0]) {
+ case ')':
+ return FUNC_STATE_END;
+ case ',':
+ return FUNC_STATE_COMMA;
+ }
+ break;
+
case FUNC_STATE_VAR:
switch (token[0]) {
case ')':
return FUNC_STATE_END;
case ',':
return FUNC_STATE_COMMA;
+ case '[':
+ return FUNC_STATE_BRACKET;
}
break;

@@ -284,6 +317,37 @@ process_event(struct func_event *fevent, const char *token, enum func_states sta
return FUNC_STATE_ERROR;
}

+static long long get_arg(struct func_arg *arg, unsigned long val)
+{
+ char buf[8];
+ int ret;
+
+ if (!arg->indirect)
+ return val;
+
+ val = val + (arg->indirect ^ INDIRECT_FLAG);
+
+ ret = probe_kernel_read(buf, (void *)val, arg->size);
+ if (ret)
+ return 0;
+
+ switch (arg->size) {
+ case 8:
+ val = *(unsigned long long *)buf;
+ break;
+ case 4:
+ val = *(unsigned int *)buf;
+ break;
+ case 2:
+ val = *(unsigned short *)buf;
+ break;
+ case 1:
+ val = *(unsigned char *)buf;
+ break;
+ }
+ return val;
+}
+
static void func_event_trace(struct trace_event_file *trace_file,
struct func_event *func_event,
unsigned long ip, unsigned long parent_ip,
@@ -323,7 +387,7 @@ static void func_event_trace(struct trace_event_file *trace_file,

list_for_each_entry(arg, &func_event->args, list) {
if (i < nr_args)
- val = args[i];
+ val = get_arg(arg, args[i]);
else
val = 0;
memcpy(&entry->data[arg->offset], &val, arg->size);
@@ -685,6 +749,9 @@ static int func_event_seq_show(struct seq_file *m, void *v)
seq_puts(m, ", ");
comma = true;
seq_printf(m, "%s %s", arg->type, arg->name);
+ if (arg->indirect && arg->size)
+ seq_printf(m, "[%ld]",
+ (arg->indirect ^ INDIRECT_FLAG) / arg->size);
}
seq_puts(m, ")\n");

--
2.15.1