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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 28 Aug 2020 10:39:05 -0700
Message-ID: <CALCETrUUKb6oyBmB3CSeVy1xT7mcnV=BD2eipAnKUhma7K3qKw@mail.gmail.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     "H.J. Lu" <hjl.tools@gmail.com>
Cc:     Florian Weimer <fweimer@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@intel.com>,
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On Fri, Aug 28, 2020 at 4:38 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Thu, Aug 27, 2020 at 11:24 PM Florian Weimer <fweimer@redhat.com> wrote:
> >
> > * H. J. Lu:
> >
> > > Can you think of ANY issues of passing more arguments to arch_prctl?
> >
> > On x32, the glibc arch_prctl system call wrapper only passes two
> > arguments to the kernel, and applications have no way of detecting that.
> > musl only passes two arguments on all architectures.  It happens to work
> > anyway with default compiler flags, but that's an accident.
>
> In the current glibc, there is no arch_prctl wrapper for i386.  There are
> arch_prctl wrappers with 2 arguments for x86-64 and x32.  But this isn't an
> issue for glibc since glibc is both the provider and the user of the new
> arch_prctl extension.  Besides,
>
> long syscall(long number, ...);
>
> is always available.

Userspace is probably full of tools and libraries that contain tables
of system calls and their signatures.  Think tracing, audit, container
management, etc.  I don't know how they will react to the addition of
new arguments.