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    SubjectRe: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE_ATTACH
    On 02/14, Oleg Nesterov wrote:
    >
    > On 02/14, Denys Vlasenko wrote:
    > >
    > > On Mon, Feb 14, 2011 at 6:39 PM, Oleg Nesterov <oleg@redhat.com> wrote:
    > > > On 02/14, Denys Vlasenko wrote:
    > > >>
    > > >> On Mon, Feb 14, 2011 at 4:31 PM, Oleg Nesterov <oleg@redhat.com> wrote:
    > > >> > On 02/13, Denys Vlasenko wrote:
    > > >> >>
    > > >> >> $ 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)
    > > >>
    > > >> Ah, I see. Is there any way debugger can distinguish between these two
    > > >> different stops?
    > > >
    > > > IIRC, the (only?) way to distinguish is to check last_siginfo != NULL
    > > > via ptrace(PTRACE_GETSIGINFO).
    > >
    > > What do you think strace needs to do when it sees second SIGSTOP
    > > (meaning "in theory", not "on current kernel which may be buggy")?
    > >
    > > ptrace(PTRACE_SYSCALL, $PID, 0x1, 0)?
    >
    > proably this, or even ptrace(PTRACE_SYSCALL, $PID, 0x1, SIGSTOP).
    > I think.
    >
    > (assuming that ptrace_resume() respects TASK_STOPPED)

    Oh, but I forgot to mention... there is another problem, _any_
    ptrace request when the tracee is stopped turns it into TASK_TRACED.

    Oleg.

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