Re: [PATCH v10 26/26] x86/cet/shstk: Add arch_prctl functions for shadow stack

Eugene Syromiatnikov
2020-05-22T17:29:58Z

On Fri, May 22, 2020 at 10:17:43AM -0700, Yu-cheng Yu wrote:
> On Thu, 2020-05-21 at 15:42 -0700, Kees Cook wrote:
> > On Wed, Apr 29, 2020 at 03:07:32PM -0700, Yu-cheng Yu wrote:
> [...]
> > > +
> > > +int prctl_cet(int option, u64 arg2)
> > > +{
> > > + struct cet_status *cet;
> > > +
> > > + if (!IS_ENABLED(CONFIG_X86_INTEL_CET))
> > > + return -EINVAL;
> >
> > Using -EINVAL here means userspace can't tell the difference between an
> > old kernel and a kernel not built with CONFIG_X86_INTEL_CET. Perhaps
> > -ENOTSUPP?
>
> Looked into this. The kernel and GLIBC are not in sync. So maybe we still use
> EINVAL here?
>
> Yu-cheng
>
>
>
> In kernel:
> ----------
>
> #define EOPNOTSUPP 95
> #define ENOTSUPP 524
>
> In GLIBC:
> ---------
>
> printf("ENOTSUP=%d\n", ENOTSUP);
> printf("EOPNOTSUPP=%d\n", EOPNOTSUPP);
> printf("%s=524\n", strerror(524));
>
> ENOTSUP=95
> EOPNOTSUPP=95
> Unknown error 524=524

EOPNOTSUPP/ENOTSUP/ENOTSUPP is actually a mess, it's summarized recently
by Michael Kerrisk[1]. From the kernel's point of view, I think it
would be reasonable to return EOPNOTSUPP, and expect that the userspace
would use ENOTSUP to match against it.

[1] https://lore.kernel.org/linux-man/cb4c685b-6c5d-9c16-aade-0c95e57de4b9@gmail.com/