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    SubjectRe: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE_ATTACH
    On Mon, Feb 14, 2011 at 6:51 PM, Oleg Nesterov <oleg@redhat.com> wrote:
    > On 02/14, 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...)
    >> >> 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?
    >> >>
    >> >> 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!
    >> >>
    >> >> PTRACE_CONT in this situation would do the same.
    >> >
    >> > This can be fixed by updating strace, right?  strace can look at the
    >> > wait(2) exit code and if the tracee stopped for group stop, wait for
    >> > the tracee to be continued instead of issuing PTRACE_SYSCALL.
    >
    > Ah, I seem to understand the confusion, let me repeat...
    >
    >> But tracee didn't stop _yet_.
    >
    > This depends on "_yet_". strace does ptrace(SYSCALL, SIGSTOP) twice.
    > The first time it does this after the tracee reports the signal, and
    > the tracee stopps.
    >
    >> Signal is not delivered _yet_, debugger
    >> can decide at this point whether to deliver it:
    >> ptrace(PTRACE_SYSCALL, $PID, 0x1, SIGSTOP)
    >> or ignore:
    >> ptrace(PTRACE_SYSCALL, $PID, 0x1, 0)
    >>
    >> strace has to deliver SIGSTOP if it wants to make program run exactly
    >> as it would run without strace. So it tries to do so.
    >> Currently, ptrace machinery doesn't react as strace, its user, expects it to.
    >
    > It does, see above. But then the tracee actually stopps, and report
    > this to the tracer. However, strace handles this case as if this was
    > another signal=SIGSTOP, so it does ptrace(SYSCALL, SIGSTOP) again.
    >
    > SIGSTOP has no effect, but PTRACE_SYSCALL wakeups the tracee.

    I performed a small experiment. You are right, SIGSTOP here
    is ignored, and PTRACE_SYSCALL wakes the tracee up:
    replacing SIGSTOP with 0 doesn't change anything.

    I tried to simply not do ptrace(PTRACE_SYSCALL, ..., 0) at all.
    Behavior changes, but it is still wrong. Now tracee doesn't wake up
    on SIGCONT. Here is the run of modified strace:

    # strace -tt -s99 -oLOG ./strace sleep 55
    execve("/bin/sleep", ["sleep", "55"], [/* 48 vars */]) = 0
    brk(0) = 0x22a9000
    ...
    nanosleep({55, 0}, NULL) = ? ERESTART_RESTARTBLOCK (To
    be restarted)
    <-- kill -STOP 25339
    --- SIGSTOP (Stopped (signal)) @ 0 (0) --- STOP: si_signo:19 si_code:0
    si_status:0 si_value:(nil)
    --- SIGSTOP (Stopped (signal)) @ 0 (0) --- STOP:
    ptrace(PTRACE_GETSIGINFO) failed
    ...does not exit for minutes...
    <-- kill -CONT 25339
    ...still nothing, it is stopped, does not exit for minutes...
    <-- kill -KILL 25339
    +++ killed by SIGKILL +++


    Here is what patched strace saw and did:

    19:41:09.601764 wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGSTOP}],
    __WALL, NULL) = 25339
    19:41:09.601914 rt_sigprocmask(SIG_BLOCK, [HUP INT QUIT PIPE TERM], NULL, 8) = 0
    19:41:09.602081 ptrace(PTRACE_GETSIGINFO, 25339, 0, {si_signo=SIGSTOP,
    si_code=SI_USER, si_pid=10105, si_uid=0, si_value={int=0, ptr=0}}) = 0
    19:41:09.602273 write(2, "--- SIGSTOP (Stopped (signal)) @ 0 (0) ---
    STOP: si_signo:19 si_code:0 si_status:0 si_value:(nil) \n", 99) = 99
    19:41:09.602456 ptrace(PTRACE_SYSCALL, 25339, 0x1, SIGSTOP) = 0
    19:41:09.602582 --- SIGCHLD (Child exited) @ 0 (0) ---
    19:41:09.602652 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0

    19:41:09.602792 wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGSTOP}],
    __WALL, NULL) = 25339
    19:41:09.602927 rt_sigprocmask(SIG_BLOCK, [HUP INT QUIT PIPE TERM], NULL, 8) = 0
    19:41:09.603081 ptrace(PTRACE_GETSIGINFO, 25339, 0, 0x7fff436fc730) =
    -1 EINVAL (Invalid argument)
    19:41:09.603231 write(2, "--- SIGSTOP (Stopped (signal)) @ 0 (0) ---
    STOP: ptrace(PTRACE_GETSIGINFO) failed \n", 83) = 83
    19:41:09.603369 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
    <<<< the change is here. Unpatched strace would do
    ptrace(PTRACE_SYSCALL, 25339, 0x1, SIGSTOP) >>>

    19:41:09.603511 wait4(-1, [{WIFSIGNALED(s) && WTERMSIG(s) ==
    SIGKILL}], __WALL, NULL) = 25339
    <<<< SIGCONT is not visible! >>>>>
    19:47:00.836723 --- SIGCHLD (Child exited) @ 0 (0) ---
    19:47:00.836804 rt_sigprocmask(SIG_BLOCK, [HUP INT QUIT PIPE TERM], NULL, 8) = 0
    19:47:00.837010 write(2, "+++ killed by SIGKILL +++\n", 26) = 26
    19:47:00.837212 rt_sigaction(SIGKILL, {SIG_DFL, [KILL],
    SA_RESTORER|SA_RESTART, 0x7f5df12d5970}, {0x7fff436f0043, ~[HUP INT
    BUS USR2 PIPE ALRM TTIN XCPU PROF WINCH IO PWR RTMIN RT_16 RT_17 RT_18
    RT
    19:47:00.837458 gettid() = 25338
    19:47:00.837596 tgkill(25338, 25338, SIGKILL <unfinished ...>
    19:47:00.837831 +++ killed by SIGKILL +++

    As you see, SIGCONT was completely invisible to debugger.

    --
    vda
    --
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