Messages in this thread | | | Date | Wed, 10 Sep 2008 17:11:37 +0200 | From | Pierre Morel <> | Subject | Re: [PATCH 1/1] system call notification with self_ptrace |
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Hello,
Oleg Nesterov wrote: > On 09/08, Pierre Morel wrote: > >> --- linux-2.6.26.orig/arch/s390/kernel/signal.c >> +++ linux-2.6.26/arch/s390/kernel/signal.c >> @@ -409,6 +409,11 @@ handle_signal(unsigned long sig, struct >> spin_unlock_irq(¤t->sighand->siglock); >> } >> >> + if (current->instrumentation) { >> + clear_thread_flag(TIF_SYSCALL_TRACE); >> + current->instrumentation &= ~PTS_SELF; >> + } >> + >> return ret; >> } >> > > I still think this patch shouldn't change handle_signal(). > > Once again. The signal handler for SIGSYS can first do > sys_ptrace(PTRACE_SELF_OFF) (which is filtered out), and then use any > other syscall, so this change is not needed, afaics. > Yes it can but what if the application forget to do it? It is a security so that the application do not bounce for ever. > The overhead of the additional PTRACE_SELF_OFF syscall is very small, > especially compared to signal delivery. I don't think this functionality > will be widely used, but this change adds the unconditional overhead > to handle_signal(). > > Btw, the check above looks wrong, shouldn't it be > > if (current->instrumentation & PTS_SELF) > > ? > Yes you are right, in fact I do not need two flags, I will remove the PTS_INSTRUMENTED flag. > And. According to the prior discussion, this requires to hook every > signal handler in user space, otherwise we can miss syscall. But every > hook should start with PTRACE_SELF_ON, so I can't see any gain. > > >> +#define PTS_INSTRUMENTED 0x00000001 >> +#define PTS_SELF 0x00000002 >> > > I don't really understand why do we need 2 flags, see also below, > Yes, I remove PTS_INSTRUMENTED, a bad idea. > >> --- linux-2.6.26.orig/kernel/ptrace.c >> +++ linux-2.6.26/kernel/ptrace.c >> @@ -543,6 +543,38 @@ asmlinkage long sys_ptrace(long request, >> * This lock_kernel fixes a subtle race with suid exec >> */ >> lock_kernel(); >> + if (request == PTRACE_SELF_ON) { >> + task_lock(current); >> + if (current->ptrace) { >> + task_unlock(current); >> + ret = -EPERM; >> + goto out; >> + } >> + set_thread_flag(TIF_SYSCALL_TRACE); >> + current->instrumentation |= PTS_INSTRUMENTED|PTS_SELF; >> + task_unlock(current); >> + ret = 0; >> + goto out; >> > > The code looks strange. How about > > if (request == PTRACE_SELF_ON) { > ret = -EPERM; > task_lock(current); > if (!current->ptrace) { > set_thread_flag(TIF_SYSCALL_TRACE); > current->instrumentation |= PTS_INSTRUMENTED|PTS_SELF; > ret = 0; > } > task_unlock(current); > goto out; > } > > ? > > I don't understand how task_lock() can help. This code runs under > lock_kernel(), and without this lock the code is racy anyway. >
I use task_lock to protect the current->ptrace bit-field which can be accessed by another thread, like the one you pointed out previously. I agree it is not necessary with lock_kernel(). I will put the code before the lock_kernel() to be more efficient. > >> + } >> + if (request == PTRACE_SELF_OFF) { >> + task_lock(current); >> + if (current->ptrace) { >> + task_unlock(current); >> + ret = -EPERM; >> + goto out; >> + } >> + clear_thread_flag(TIF_SYSCALL_TRACE); >> + current->instrumentation &= ~PTS_SELF; >> > > So. PTRACE_SELF_OFF doesn't clear PTS_INSTRUMENTED? How can the task > reset ->instrumentation ? > You are right again, I will remove the PTS_INSTRUMENTED flag. > >> + if (current->instrumentation) { >> + ret = -EPERM; >> + goto out; >> + } >> > > So, PTRACE_SELF_XXX disables the "normal" ptrace. Not sure this is good. > I think that having two tracing system one over the other may be quite difficult to handle.
Pierre > Oleg. > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
-- ============= Pierre Morel RTOS and Embedded Linux
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