Re: [RFC] Proposal for ptrace improvements

[Denys Vlasenko <vda.linux@googlemail.com>]

On Fri, Mar 4, 2011 at 2:41 PM, Tejun Heo <tj@kernel.org> wrote:
>> And again, I would like to ask you kernel guys to give
>> userspace a way to distinguish this stop from other possible stops.
>>
>> IOW: debugger PTRACE_SYSCALL(0)'ed a job control stopped task.
>> Debugger gets WIFSTOPPED, WSTOPSIG = SIGTRAP.
>> Debugger wants to know: is it a "syscall entry/exit" stop
>> or an "end of job control stop" stop?
>> Preferably without the need to query PTRACE_GETSIGINFO
>> on every SIGTRAP - that'd slow strace down a lot.
>
> There's no question that it should be distinguishible.  I was still
> mostly thinking about siginfo tho.  Is PTRACE_GETSIGINFO such big
> deal?  It's a very simple syscall.  It enters kernel, copyout the
> structure and returns.  Are you interested in adding GET_SIGINFO call
> to the current ptrace and see how much things actually slow down?

I would rather speed strace up than slow it down further, even if slightly.


>> I imagine one way to do it is to #define a PTRACE_EVENT_foo
>> for "end of job control stop" stop and return it in high byte
>> of waitpid status, just like other PTRACE_EVENTs are returned today.
>
> Maybe but I'm not fully sure yet because I think for debuggers the
> group stop status needs to be available for other ptrace traps too, so

By all means, do make this information accessible
with PTRACE_GETSIGINFO too. I have nothing against that.


> it might really not fit there.  I'd like to avoid twisting debugging
> API

It is *already* twisted that way. Other ptrace stops *already*
return extra information in waitpid status: run "man ptrace"
and read PTRACE_SETOPTIONS section. For example:

      "PTRACE_O_TRACEEXEC (since Linux 2.5.46)
             Stop  the  child  at  the next execve(2) call with
             SIGTRAP | PTRACE_EVENT_EXEC << 8."

I don't see why adding another PTRACE_EVENT_foo
is a big deal.

-- 
vda