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The patch >> makes the following change in libdtrace/dt_proc.c dt_proc_control(): >> >> if ((Psystem_daemon(dpr->dpr_pid, dtp->dt_useruid, >> dtp->dt_sysslice) > 0) || >> - ((tracer_pid != 0) && >> - (tracer_pid != getpid())) || >> + (tracer_pid == getpid()) || >> - (dpr->dpr_pid == getpid())) >> + (tgid == getpid())) >> noninvasive = 2; >> >> We change a >> (tracer_pid != getpid()) >> into a >> (tracer_pid == getpid()) >> >> Changing that == back into a != makes tst.multitrace.sh work again. >> >> Signed-off-by: Eugene Loh > > Reviewed-by: Kris Van Hees It's better than what I was doing (which was definitely wrong), but it could still be improved. >> diff --git a/libdtrace/dt_proc.c b/libdtrace/dt_proc.c >> index a052abbac..7c3eb2a24 100644 >> --- a/libdtrace/dt_proc.c >> +++ b/libdtrace/dt_proc.c >> @@ -954,7 +954,7 @@ dt_proc_control(void *arg) >> >> if ((Psystem_daemon(dpr->dpr_pid, dtp->dt_useruid, >> dtp->dt_sysslice) > 0) || >> - (tracer_pid == getpid()) || >> + (tracer_pid != getpid()) || >> (tgid == getpid())) >> noninvasive = 2; What the previous code was saying was "if the traced process is a system daemon, or is being debugged by someone else, or is ourself, do not do an invasive grab"; in the one case because stopping systemd or the system logger, even briefly, seems a like a bad idea, and in the other two cases because you can't be debugged by two people at once and because stopping yourself is an obvious deadlock. We want to check if it's ourself in two ways: firstly, if the tracer PID is our own PID, we are obviously ourself (this is the process-monitoring thread itself); secondly, if the tracer PID's task group leader is our own thread's task group leader, this is the main thread asking. Unfortunately that's not what the original code was doing (the tracer_pid thing was indeed wrong, we were saying "if we are debugged by ourself, go noninvasive, but not if we're debugged by someone else"), so your change was good as far as it goes... but if the process-monitoring thread ever decides to monitor itself (perfectly possible with suffiently-wildcardy probes and a ustack() or something), we'll deadlock -- and unfortunately the tracer_pid is 0 when the process is *not* being traced, so what that change is doing is going noninvasive for *everyone, always*. That's what the original's ((tracer_pid != 0) && (tracer_pid != getpid())) || was all about: if you flip the latter conditional away from ==, you *must* put the != 0 back in. Something we're also not covering but should is that we want to fail when asked to do long-lived grabs of ourself or a process being debugged by someone else, or for that matter PID 1, *always*: long-lived grabs mean we rely on getting process death notifications, etc, and noninvasive grabs can't do that, so we *should* fail. Also, we should define "ourself" more widely: we should really refuse ptrace grabs on *any* DTrace thread, because the amount of proxying and callbacks back and forth makes deadlocks horribly likely if, say, the main thread gets invasively grabbed. Fix coming shortly (before you get back and read this, assuming you're guzzling turkey now like you should be). -- NULL && (void)