Re: [PATCHv2 1/3] uprobe: Add uretprobe syscall to speed up return probe

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 5 Apr 2024 13:02:30 +0200
Oleg Nesterov <oleg@redhat.com> wrote:

> On 04/05, Jiri Olsa wrote:
> >
> > On Fri, Apr 05, 2024 at 10:22:03AM +0900, Masami Hiramatsu wrote:
> > >
> > > I think this expects setjmp/longjmp as below
> > >
> > > foo() { <- retprobe1
> > > 	setjmp()
> > > 	bar() { <- retprobe2
> > > 		longjmp()
> > > 	}
> > > } <- return to trampoline
> > >
> > > In this case, we need to skip retprobe2's instance.
> 
> Yes,
> 
> > > My concern is, if we can not find appropriate return instance, what happen?
> > > e.g.
> > >
> > > foo() { <-- retprobe1
> > >    bar() { # sp is decremented
> > >        sys_uretprobe() <-- ??
> > >     }
> > > }
> > >
> > > It seems sys_uretprobe() will handle retprobe1 at that point instead of
> > > SIGILL.
> >
> > yes, and I think it's fine, you get the consumer called in wrong place,
> > but it's your fault and kernel won't crash
> 
> Agreed.
> 
> With or without this patch userpace can also do
> 
> 	foo() { <-- retprobe1
> 		bar() {
> 			jump to xol_area
> 		}
> 	}
> 
> handle_trampoline() will handle retprobe1.

This is OK because the execution path has been changed to trampoline, but
the above will continue running bar() after sys_uretprobe().

> 
> > this can be fixed by checking the syscall is called from the trampoline
> > and prevent handle_trampoline call if it's not
> 
> Yes, but I still do not think this makes a lot of sense. But I won't argue.
> 
> And what should sys_uretprobe() do if it is not called from the trampoline?
> I'd prefer force_sig(SIGILL) to punish the abuser ;) OK, OK, EINVAL.
> 
> I agree very much with Andrii,
> 
>        sigreturn()  exists only to allow the implementation of signal handlers.  It should never be
>        called directly.  Details of the arguments (if any) passed to sigreturn() vary depending  on
>        the architecture.
> 
> this is how sys_uretprobe() should be treated/documented.

OK.

> 
> sigreturn() can be "improved" too. Say, it could validate sigcontext->ip
> and return -EINVAL if this addr is not valid. But why?

Because sigreturn() never returns, but sys_uretprobe() will return.
If it changes the regs->ip and directly returns to the original return address,
I think it is natural to send SIGILL.


Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>