Messages in this thread | | | Date | Thu, 11 Feb 2010 17:50:59 +0100 | From | Oleg Nesterov <> | Subject | Re: Race in ptrace. |
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On 02/11, Salman Qazi wrote: > > On Thu, Feb 11, 2010 at 4:56 AM, Oleg Nesterov <oleg@redhat.com> wrote: > > > > But this all is correct, you can't expect PTRACE_SYSCALL can succeed > > is the tracee is running, it must be stopped or traced. > > > > The tracee is running because it was TASK_STOPPED and antagonist() > > sends SIGCONT. > > > > The tracee was TASK_STOPPED because the tracer passes sig = SIGSTOP > > via ptrace(PTRACE_SYSCALL, WSTOPSIG(status). > > > > Where do you see the bug? > > Shouldn't ptrace(PTRACE_SYSCALL, WSTOPSIG(status)...), cause any > future signals to the child be intercepted by the parent?
Not sure I understand your question. Of course the tracee will report any future signals signals, after it has a chance to dequeue a signal.
But if you mean that after, say, ptrace(PTRACE_SYSCALL, SIGTERM) the tracee should report _this_ SIGTERM to the tracer - then no. Well, actually "this depends", but if PTRACE_SYSCALL (or any other req) is called after the tracee reported the signal - no. The signal was already reported.
> > int main(void) > > { > > int stat, ret; > > int pid = fork(); > > > > if (!pid) { > > ptrace(PTRACE_TRACEME, 0, NULL, NULL); > > for (;;) > > ; > > } > > > > sleep(1); // wait for PTRACE_TRACEME > > kill(pid, SIGSTOP); > > > > // the child reports SIGSTOP, it is TASK_TRACED > > assert(pid == wait(&stat) && WIFSTOPPED(stat)); > > > > // the tracee should stop, we pass sig = SIGSTOP > > assert(ptrace(PTRACE_SYSCALL, pid, 0, WSTOPSIG(stat)) == 0); > > > > // the child reports the group stop, it is TASK_STOPPED > > assert(pid == wait(&stat) && WIFSTOPPED(stat)); > > > > // the tracee is STOPPED as requested, not TRACED, > > // SIGCONT wakes it up > > kill(pid, SIGCONT); > > According to the man page, any signals to the > process are supposed to be intercepted by the parent and that is how > one is supposed to be able to control which signals make it to the > child. I am not sure if it makes any difference if the signal > originates at the parent. But in our test case, it doesn't. So, why > doesn't the parent get a notification first?
It does. You can insert another wait() (or just sleep(1)) between kill(SIGCONT) and PTRACE_SYSCALL below, the tracee will stop to report SIGCONT and the tracer will be notified. In this case the following PTRACE_SYSCALL should succeed.
Perhaps I should have mentioned that the code above is racy. It is, I only did it to simplify the explanations.
Oleg.
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