RFC: Disable clone3 for glibc 2.34

[Christian Brauner]

On Tue, Jul 27, 2021 at 11:11:17AM +0200, Florian Weimer via Libc-alpha wrote:
> * Florian Weimer via Libc-alpha:
> 
> > Reportedly, the docker package in Ubuntu as used by Github Actions and
> > others does not provide a way to enable the clone3 system call.  It
> > always fails with EPERM.
> >
> > Should we apply a patch like this for the release?
> >
> > diff --git a/sysdeps/unix/sysv/linux/clone-internal.c b/sysdeps/unix/sysv/linux/clone-internal.c
> > index 1e7a8f6b35..4046c81180 100644
> > --- a/sysdeps/unix/sysv/linux/clone-internal.c
> > +++ b/sysdeps/unix/sysv/linux/clone-internal.c
> > @@ -48,17 +48,6 @@ __clone_internal (struct clone_args *cl_args,
> >  		  int (*func) (void *arg), void *arg)
> >  {
> >    int ret;
> > -#ifdef HAVE_CLONE3_WAPPER
> > -  /* Try clone3 first.  */
> > -  int saved_errno = errno;
> > -  ret = __clone3 (cl_args, sizeof (*cl_args), func, arg);
> > -  if (ret != -1 || errno != ENOSYS)
> > -    return ret;
> > -
> > -  /* NB: Restore errno since errno may be checked against non-zero
> > -     return value.  */
> > -  __set_errno (saved_errno);
> > -#endif
> >  
> >    /* Map clone3 arguments to clone arguments.  NB: No need to check
> >       invalid clone3 specific bits in flags nor exit_signal since this
> >
> > My concern with this is that we don't know yet where the CET kernel API
> > will land exactly and if CET will require clone3.  So clone3 might have
> > to come back once we turn on CET, which is hopefully soon.
> 
> Ubuntu 20.04 LTS may have already been fixed, I cannot reproduce the
> issue with its docker.io/containerd/runc packages.
> 
> I could trivially fix a previously failing Github Action with:
> 
> diff --git a/.github/workflows/fedora.yml b/.github/workflows/fedora.yml
> index d2381ec..7b10286 100644
> --- a/.github/workflows/fedora.yml
> +++ b/.github/workflows/fedora.yml
> @@ -22,6 +22,7 @@ jobs:
>      runs-on: ubuntu-latest
>      container:
>        image: fedora:${{matrix.release}}
> +      options: --security-opt seccomp=unconfined
>  
>      steps:
>        - name: Checkout repository
> 
> So I think we need to figure out what people are actually complaining
> about.

This relates to the discussion what errno value should be used in a
seccomp filter to indicate that a syscall is blocked.

So there are two problems I see with seccomp and clone3():
1. the profile doesn't include clone3() at all and therefore the syscall
   is blocked and the default action is EPERM
2. the profile does include clone3() and decided to block it but the
   runtime has decided to make seccomp return EPERM and not ENOSYS when
   clone3() is attempted

The correct fix in both scenarios is to add clone3() to the seccomp
profile and either allow it or return ENOSYS.

Note that this ENOSYS/EPERM problem is a general problem. Not just glibc
doesn't know when to fallback gracefully other tools don't know either.
Application container usually just get lucky because their applications
don't need to issue the syscalls that are blocked. On a generic system
container with systemd inside this is always an issue and not using
ENOSYS is guaranteed to fail across the board.

Christian