Re: [PATCH tracing/kprobes v2 1/5] tracing/kprobes: Rename specialvariables syntax

From: Masami Hiramatsu
Date: Sun Oct 04 2009 - 01:22:28 EST




Frederic Weisbecker wrote:
> On Fri, Oct 02, 2009 at 05:48:42PM -0400, Masami Hiramatsu wrote:
>> Add $ prefix to the special variables(e.g. sa, rv) of kprobe-tracer.
>> This resolves consistency issue between kprobe_events and perf-kprobe.
>>
>> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
>> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
>> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
>> Cc: Mike Galbraith <efault@xxxxxx>
>> Cc: Paul Mackerras <paulus@xxxxxxxxx>
>> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
>> Cc: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
>> Cc: Jim Keniston <jkenisto@xxxxxxxxxx>
>> Cc: Frank Ch. Eigler <fche@xxxxxxxxxx>
>> ---
>>
>> Documentation/trace/kprobetrace.txt | 10 +++---
>> kernel/trace/trace_kprobe.c | 60 ++++++++++++++++++++++-------------
>> 2 files changed, 42 insertions(+), 28 deletions(-)
>>
>> diff --git a/Documentation/trace/kprobetrace.txt b/Documentation/trace/kprobetrace.txt
>> index 9b8f7c6..40caef0 100644
>> --- a/Documentation/trace/kprobetrace.txt
>> +++ b/Documentation/trace/kprobetrace.txt
>> @@ -36,13 +36,13 @@ Synopsis of kprobe_events
>>
>> FETCHARGS : Arguments. Each probe can have up to 128 args.
>> %REG : Fetch register REG
>> - sN : Fetch Nth entry of stack (N >= 0)
>> - sa : Fetch stack address.
>> @ADDR : Fetch memory at ADDR (ADDR should be in kernel)
>> @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
>> - aN : Fetch function argument. (N >= 0)(*)
>> - rv : Fetch return value.(**)
>> - ra : Fetch return address.(**)
>> + $sN : Fetch Nth entry of stack (N >= 0)
>> + $sa : Fetch stack address.
>> + $aN : Fetch function argument. (N >= 0)(*)
>> + $rv : Fetch return value.(**)
>> + $ra : Fetch return address.(**)
>
>
>
> I feel uncomfortable with that, because of bash scripts that
> may use it and always need to escape it with antislashes or
> use single quotes for it to not be replaced by a random variable
> value. If one uses double quotes without antislashes to protect
> the command line, the result is unpredictable, depending of
> the current set of variables...
>
> May be we can use # instead of $ ? That's a kind of pity because
> $ suggested a variable whereas # suggests a constant, we are then
> losing this self-explainable characteristic for kprobes
> "specific variable fetchs". But that will work without ambiguity.

Hmm, # is widely used for comment, including some kernel pseudo
file interfaces, kprobe_events too. Comments are useful if a
probe list is restored from a file.

For accessing local variables, kprobe-tracer needs to support *at least*
below variables:
- Registers
- Stack address (if a register points stack address, this isn't needed)

Below special vars are complementary aliases.
- Function arguments
- Return value
- Return address

and I'd like perf-probe to have a transparent syntax with kprobe-tracer.

This means, if we can remove special vars except registers, or rename it
non-conflictable name with registers, we just need to separate name spaces
of
- Regsiters
- Local variables

Here, local variables will support fields of data structs, and it will
use '->' expression. Since '>' means redirection in bash, local variables
need to be *escaped* in this case. Thus, I think we can use '$' prefix
for it. (I'm OK, because this is similar syntax to systemtap:-).

So, if you don't like %regs, $svars and locals, we can use regs and $locals :-)

Thank you,


--
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@xxxxxxxxxx

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