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

Namhyung Kim
2018-02-08T10:59:24Z

On Fri, Feb 02, 2018 at 06:05:07PM -0500, Steven Rostedt wrote:
> 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;

So this also works without the indirect, and just add the immediate to
the value.

Thanks,
Namhyung

>
> @@ -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
>
>