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SubjectRe: [RFC] [RFC] trace: Add kprobe on tracepoint
Hi Tzvetomir,

On Fri, 6 Aug 2021 20:49:28 +0300
"Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com> wrote:

> From: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
>
> The /sys/kernel/debug/tracing/kprobe_events interface is extended with a
> new type of user defined dynamic event, that is attached to an existing
> tracepoint. The user can specify custom format string of the new event,
> select what tracepoint arguments will be printed and how to print them.
>
> The format is similar to the existing kprobe_events interface:
> e:GNAME/ENAME SYSTEM/EVENT [FETCHARGS] - Set an event probe
> -e:GNAME/ENAME - Delete an event probe
>
> Where:
> GNAME - Group name, mandatory
> ENAME - Name of the new event in GNAME, mandatory
> SYSTEM - Name of the system, where the tracepoint is defined, mandatory
> EVENT - Name of the tracepoint event in SYSTEM, mandatory
> FETCHARGS - Arguments:
> <name>=$<field>[:TYPE] - Fetch given filed of the tracepoint and print it as
> given TYPE with given name. Supported types are
> (u8/u16/u32/u64/s8/s16/s32/s64), basic type
> (x8/x16/x32/x64), hexadecimal types
> "string", "ustring" and bitfield.
>
> Example, attach event probe on openat system call and print name of the
> file that will be opened:
> echo "e:esys/eopen syscalls.sys_enter_openat file=\$filename:string" >> kprobe_events
> A new dynamic event is created in events/esys/eopen/ directory. It
> can be deleted with:
> echo "-e:esys/eopen" >> kprobe_events

Hmm, sorry, I rather like to use synthetic event with trigger action,
since this is not a kprobe.
Can you change your idea to use trigger action with synthetic event?

For example, if we have a "trace" action in the trigger action,

echo "eopen char filename[]" >> synthetic_events
echo "trace:eopen,filename.ustring" >> events/syscalls/sys_enter_openat/trigger

A new action is,
trace:SYNTH_EVENT,PARAM(s) [if FILTER]
and
.ustring/.string modifier for the PARAMS.

I think this matches the current dynamic event model, and can extend
programmability of the ftrace, and keeps dynamic events simple.

Thank you,


--
Masami Hiramatsu <mhiramat@kernel.org>

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