[PATCH 09/18] tracing: Add indexing of arguments for function based events

Steven Rostedt
2018-02-02T23:10:20Z

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

Currently reading of 8 byte words can only happen 8 bytes aligned from the
argument. But there may be cases that they are 4 bytes aligned. To make the
capturing of arguments more flexible, add a plus '+' operator that can index
the variable at arbitrary indexes to get any location.

u64 arg+4[3]

Will get an 8 byte word at index 28 (3 * 8 + 4)

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
Documentation/trace/function-based-events.rst | 24 +++++++++++++++++++++++-
kernel/trace/trace_event_ftrace.c | 18 ++++++++++++++++++
2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/Documentation/trace/function-based-events.rst b/Documentation/trace/function-based-events.rst
index 72e3e7730d63..bdb28f433bfb 100644
--- a/Documentation/trace/function-based-events.rst
+++ b/Documentation/trace/function-based-events.rst
@@ -100,10 +100,12 @@ as follows:
'x8' | 'x16' | 'x32' | 'x64' |
'char' | 'short' | 'int' | 'long' | 'size_t'

- FIELD := <name> | <name> INDEX
+ FIELD := <name> | <name> INDEX | <name> OFFSET | <name> OFFSET INDEX

INDEX := '[' <number> ']'

+ OFFSET := '+' <number>
+
Where <name> is a unique string starting with an alphabetic character
and consists only of letters and numbers and underscores.

@@ -221,3 +223,23 @@ format:
print fmt: "%pS->%pS(skb=%u)", REC->__ip, REC->__parent_ip, REC->skb

It is now printed with a "%u".
+
+
+Offsets
+=======
+
+After the name of the variable, brackets '[' number ']' will index the value of
+the argument by the number given times the size of the field.
+
+ int field[5] will dereference the value of the argument 20 bytes away (4 * 5)
+ as sizeof(int) is 4.
+
+If there's a case where the type is of 8 bytes in size but is not 8 bytes
+alligned in the structure, an offset may be required.
+
+ For example: x64 param+4[2]
+
+The above will take the parameter value, add it by 4, then index it by two
+8 byte words. It's the same in C as: (u64 *)((void *)param + 4)[2]
+
+ Note: "int skb[32]" is the same as "int skb+4[31]".
diff --git a/kernel/trace/trace_event_ftrace.c b/kernel/trace/trace_event_ftrace.c
index 9548b93eb8cd..4c23fa18453d 100644
--- a/kernel/trace/trace_event_ftrace.c
+++ b/kernel/trace/trace_event_ftrace.c
@@ -19,6 +19,7 @@ struct func_arg {
char *type;
char *name;
long indirect;
+ long index;
short offset;
short size;
s8 arg;
@@ -62,6 +63,7 @@ enum func_states {
FUNC_STATE_INDIRECT,
FUNC_STATE_UNSIGNED,
FUNC_STATE_PIPE,
+ FUNC_STATE_PLUS,
FUNC_STATE_TYPE,
FUNC_STATE_VAR,
FUNC_STATE_COMMA,
@@ -182,6 +184,7 @@ static char *next_token(char **ptr, char *last)
*str == ']' ||
*str == ',' ||
*str == '|' ||
+ *str == '+' ||
*str == ')')
break;
}
@@ -323,6 +326,15 @@ process_event(struct func_event *fevent, const char *token, enum func_states sta
}
break;

+ case FUNC_STATE_PLUS:
+ if (WARN_ON(!fevent->last_arg))
+ break;
+ ret = kstrtol(token, 0, &val);
+ if (ret)
+ break;
+ fevent->last_arg->index += val;
+ return FUNC_STATE_VAR;
+
case FUNC_STATE_VAR:
switch (token[0]) {
case ')':
@@ -331,6 +343,8 @@ process_event(struct func_event *fevent, const char *token, enum func_states sta
return FUNC_STATE_COMMA;
case '|':
return FUNC_STATE_PIPE;
+ case '+':
+ return FUNC_STATE_PLUS;
case '[':
return FUNC_STATE_BRACKET;
}
@@ -347,6 +361,8 @@ static long long get_arg(struct func_arg *arg, unsigned long val)
char buf[8];
int ret;

+ val += arg->index;
+
if (!arg->indirect)
return val;

@@ -779,6 +795,8 @@ 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->index)
+ seq_printf(m, "+%ld", arg->index);
if (arg->indirect && arg->size)
seq_printf(m, "[%ld]",
(arg->indirect ^ INDIRECT_FLAG) / arg->size);
--
2.15.1