Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Yu, Yu-cheng]

On 8/26/2020 10:04 AM, Andy Lutomirski wrote:
> On Wed, Aug 26, 2020 at 9:52 AM Florian Weimer <fweimer@xxxxxxxxxx> wrote:
> > * Dave Martin:
> >
> > > On Tue, Aug 25, 2020 at 04:34:27PM -0700, Yu, Yu-cheng wrote:
> > > > On 8/25/2020 4:20 PM, Dave Hansen wrote:
> > > > > On 8/25/20 2:04 PM, Yu, Yu-cheng wrote:
> > > > > > > > I think this is more arch-specific.  Even if it becomes a new syscall,
> > > > > > > > we still need to pass the same parameters.
> > > > > > >
> > > > > > > Right, but without the copying in and out of memory.
> > > > > > Linux-api is already on the Cc list.  Do we need to add more people to
> > > > > > get some agreements for the syscall?
> > > > > What kind of agreement are you looking for?  I'd suggest just coding it
> > > > > up and posting the patches.  Adding syscalls really is really pretty
> > > > > straightforward and isn't much code at all.
> > > >
> > > > Sure, I will do that.
> > >
> > > Alternatively, would a regular prctl() work here?
> >
> > Is this something appliation code has to call, or just the dynamic
> > loader?
> >
> > prctl in glibc is a variadic function, so if there's a mismatch between
> > the kernel/userspace syscall convention and the userspace calling
> > convention (for variadic functions) for specific types, it can't be made
> > to work in a generic way.
> >
> > The loader can use inline assembly for system calls and does not have
> > this issue, but applications would be implcated by it.
>
> I would expect things like Go and various JITs to call it directly.
>
> If we wanted to be fancy and add a potentially more widely useful
> syscall, how about:
>
> mmap_special(void *addr, size_t length, int prot, int flags, int type);
>
> Where type is something like MMAP_SPECIAL_X86_SHSTK.  Fundamentally,
> this is really just mmap() except that we want to map something a bit
> magical, and we don't want to require opening a device node to do it.

One benefit of MMAP_SPECIAL_* is there are more free bits than MAP_*.
Does ARM have similar needs for memory mapping, Dave?

Yu-cheng