On Wed, Nov 28, 2018 at 01:35:46PM +0100, Oleg Nesterov wrote:
> On 11/28, Dmitry V. Levin wrote:
> >
> > > Just like ptrace_request(PTRACE_LISTEN)
> > > does but you can do this lockless (no need to lock_task_sighand()).
> >
> > Why this can be done lockless?  All other places in that file do
> > the locking,
> 
> PTRACE_LISTEN too doesn't need lock_task_sighand() to access ->last_siginfo,
> this code predates ptrace_freeze_traced() which ensures that the tracee can't
> go away and clear ->last_siginfo.
> 
> However, unlike ptrace_get_syscall(), PTRACE_LISTEN needs spin_lock_irq(siglock),
> it modifies ->jobctl and calls signal_wake_up().

What about PTRACE_GETSIGINFO?  Can it also be done lockless because
ptrace_check_attach() has already called ptrace_freeze_traced()?

> > > Of course, debugger can do PTRACE_SETSIGINFO and confuse itself but probably we
> > > do not care?
> >
> > The only potential issue I could think of is whether PTRACE_SETSIGINFO
> > could be used this way to cause an information leak by making
> > PTRACE_GET_SYSCALL_INFO access some unrelated data.
> 
> Well, afaics ptrace_get_syscall() does nothing "special", debugger can use other
> PTRACE_ requests to get the same info?

I agree.


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ldv