Re: [PATCH v10 07/11] tracing/probes: Add $args meta argument for all function args

On Mon, 15 May 2023 07:57:01 -0400
Steven Rostedt [off-list ref] wrote:

On Sun, 14 May 2023 23:12:11 +0900
"Masami Hiramatsu (Google)" [off-list ref] wrote:

quoted

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Add the '$args' meta fetch argument for function-entry probe events. This
will be expanded to the all arguments of the function and the tracepoint
using BTF function argument information.

e.g.
 #  echo 'p vfs_read $args' >> dynamic_events
 #  echo 'f vfs_write $args' >> dynamic_events
 #  echo 't sched_overutilized_tp $args' >> dynamic_events
 # cat dynamic_events
p:kprobes/p_vfs_read_0 vfs_read file=file buf=buf count=count pos=pos
f:fprobes/vfs_write__entry vfs_write file=file buf=buf count=count pos=pos
t:tracepoints/sched_overutilized_tp sched_overutilized_tp rd=rd overutilized=overutilized

NOTE: This is not like other $-vars, you can not use this $args as a
part of fetch args, e.g. specifying name "foo=$args" and using it in
dereferences "+0($args)" will lead a parse error.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
Changes in v10:
 - Change $$args to $args so that user can use $$ for current task's pid.

I hate coming up with new apis, because you never know if what you pick is
correct ;-) And then you are stuck with whatever you decided on. :-/

I know I suggested $args, but since it is special, should we call it
 "$arg*" ?

That way it follows bash wildcard semantics?

 #  echo 'p vfs_read $arg*' >> dynamic_events

I think that is more along the lines of what people would expect.

What do you think?

Good idea!

BTW, user will expect that $arg* will be expanded to available "$argN".
But this $args does different thing at this point, it is expanded
to something equivalent to "<BTF-name>=$argN:<BTF-type>".
So, to give more consistency, I need one more step: when user gives
only "$argN" on BTF supported kernel, it will be translated to
"<BTF-name>=$argN:<BTF-type>". Then, I think it is natural to use '$arg*'.

Thank you,

-- Steve

quoted

Changes in v6:
 - update patch description.
---
 kernel/trace/trace_fprobe.c |   21 ++++++++-
 kernel/trace/trace_kprobe.c |   23 ++++++++--
 kernel/trace/trace_probe.c  |   98 +++++++++++++++++++++++++++++++++++++++++++
 kernel/trace/trace_probe.h  |   10 ++++
 4 files changed, 144 insertions(+), 8 deletions(-)

-- 
Masami Hiramatsu (Google) [off-list ref]