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    SubjectRe: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE_ATTACH
    On 02/13, Denys Vlasenko wrote:
    >
    > On Wednesday 09 February 2011 15:18, Tejun Heo wrote:
    > >
    > > > > and I'm not really sure whether that's something worth achieving
    > > > > at the cost of debugging capabilities especially when we don't _have_
    > > > > to lose them.
    > > >
    > > > But we do not? I mean, at least this is not worse than the current
    > > > behaviour.
    > >
    > > I think it's worse. With your changes, debuggers can't diddle the
    > > tasks behind group stop's back which the current users already expect.
    >
    > But this "diddling behind group stop's back" is exactly the current
    > problem with stop signals.
    >
    > Here I try to stop a ptraced process:
    >
    > $ strace -tt sleep 30
    > 23:02:15.619262 execve("/bin/sleep", ["sleep", "30"], [/* 30 vars */]) = 0
    > ...
    > 23:02:15.622112 nanosleep({30, 0}, NULL) = ? ERESTART_RESTARTBLOCK (To be restarted)
    > 23:02:23.781165 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
    > 23:02:23.781251 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
    > (I forgot again why we see it twice. Another quirk I guess...)

    (this is correct, the tracee reports the signal=SIGSTOP, then
    it reports it actually stopps with exit_code=SIGSTOP)

    > 23:02:23.781310 restart_syscall(<... resuming interrupted call ...>) = 0
    > 23:02:45.622433 close(1) = 0
    > 23:02:45.622743 close(2) = 0
    > 23:02:45.622885 exit_group(0) = ?
    >
    > Why sleep didn't stop?

    Yes. And I think this all should be fixed.

    Although, depending on how we change the kernel, strace may need the
    fixes too.

    > Because PTRACE_SYSCALL brought the task out of group stop at once,
    > even though strace did try hard to not do so:
    >
    > ptrace(PTRACE_SYSCALL, $PID, 0x1, SIGSTOP) <-- note SIGSTOP!

    Yes.

    (just to clarify, data=SIGSTOP has no effect when the tracee reports
    from do_signal_stop. iow, when it reports i-am-stopped)

    But otherwise I agree, and that was my point too.

    Oleg.



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