Re: [PATCH 2/2] selftests/timens: add a test for vfork+exit

[Alexey Izbyshev]

On 2022-10-13 20:46, Andrei Vagin wrote:
> On Sun, Oct 9, 2022 at 9:10 AM Alexey Izbyshev <izbyshev@xxxxxxxxx> 
> wrote:
> > On 2022-09-21 03:31, Andrei Vagin wrote:
> > > From: Andrei Vagin <avagin@xxxxxxxxx>
>
> <snip>
>
> > > +     if (pid == 0) {
> > > +             char now_str[64];
> > > +             char *cargv[] = {"exec", now_str, NULL};
> > > +             char *cenv[] = {NULL};
> > > +
> > > +             // Check that we are still in the source timens.
> > > +             if (check("child before exec", &now))
> > > +                     return 1;
> >
> > I know this is just a test, but...
> >
> > Creating threads in a vfork()-child is quite dangerous (like most 
> > other
> > things that touch the libc state, which is shared with the parent
> > process). Here it works probably only because pthread_create() 
> > followed
> > by pthread_join() restores everything into more-or-less the original
> > state before returning control to the parent, but this is something 
> > that
> > libcs don't guarantee and that can break at any moment.
> >
> > Also, returning from a vfork()-child is explicitly forbidden by the
> > vfork() contract because the parent would then return to an invalid
> > stack frame that could be arbitrarily clobbered by code executed in 
> > the
> > child after main() returned. Moreover, if I'm not mistaken, on x86 
> > with
> > Intel CET-enabled glibc (assuming the support for CET is ever merged
> > into the kernel) such return would cause the parent to always trap
> > because the shadow stack will become inconsistent with the normal 
> > stack.
> > Instead, _exit() should be used here...
>
> Hi Alexey,
>
> You are right, it isn't a good idea to create threads from the vfork-ed
> process. Now, vfork isn't a special case in the kernel code, so I think
> we can just remove the check() call from here. I have sent an updated
> version of this patch, pls take a look at it.
Hi, Andrei,

While I think you could just skip check_in_thread() in the vfork()-child 
instead of removing check() completely (the rest of the code in check() 
is unlikely to mess up the libc state), I agree that the test is still 
able to catch problems unconditionally affecting all CLONE_VM tasks 
thanks to check_in_thread() in the parent, so I don't see much point in 
holding it up further. Your v2 patch looks good enough to me, thanks!

Alexey