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

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

On Mon, 8 Apr 2024 18:02:13 +0200
Jiri Olsa <olsajiri@gmail.com> wrote:

> On Fri, Apr 05, 2024 at 01:02:30PM +0200, Oleg Nesterov 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 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.
> 
> so the similar behaviour with int3 ends up with immediate SIGTRAP
> and not invoking pending uretprobe consumers, like:
> 
>   - setup uretprobe for foo
>   - foo() {
>       executes int 3 -> sends SIGTRAP
>     }
> 
> because the int3 handler checks if it got executed from the uretprobe's
> trampoline.. if not it treats that int3 as regular trap

Yeah, that is consistent behavior. Sounds good to me.

> 
> while for uretprobe syscall we have at the moment following behaviour:
> 
>   - setup uretprobe for foo
>   - foo() {
>      uretprobe_syscall -> executes foo's uretprobe consumers
>     }
>   - at some point we get to the 'ret' instruction that jump into uretprobe
>     trampoline and the uretprobe_syscall won't find pending uretprobe and
>     will send SIGILL
> 
> 
> so I think we should mimic int3 behaviour and:
> 
>   - setup uretprobe for foo
>   - foo() {
>      uretprobe_syscall -> check if we got executed from uretprobe's
>      trampoline and send SIGILL if that's not the case

OK, this looks good to me.

> 
> I think it's better to have the offending process killed right away,
> rather than having more undefined behaviour, waiting for final 'ret'
> instruction that jumps to uretprobe trampoline and causes SIGILL
> 
> > 
> > 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.
> 
> yes, will include man page patch in new version

And please follow Documentation/process/adding-syscalls.rst in new version,
then we can avoid repeating the same discussion :-)

Thank you!

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


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