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

[Eugene Syromiatnikov <esyr-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org/