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    SubjectRe: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack
    On Thu, Aug 27, 2020 at 12:38 PM H.J. Lu <hjl.tools@gmail.com> wrote:
    >
    > On Thu, Aug 27, 2020 at 11:56 AM Andy Lutomirski <luto@amacapital.net> wrote:
    > >
    > >
    > >
    > > > On Aug 27, 2020, at 11:13 AM, Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
    > > >
    > > > On 8/27/2020 6:36 AM, Florian Weimer wrote:
    > > >> * H. J. Lu:
    > > >>>> On Thu, Aug 27, 2020 at 6:19 AM Florian Weimer <fweimer@redhat.com> wrote:
    > > >>>>>
    > > >>>>> * Dave Martin:
    > > >>>>>
    > > >>>>>> You're right that this has implications: for i386, libc probably pulls
    > > >>>>>> more arguments off the stack than are really there in some situations.
    > > >>>>>> This isn't a new problem though. There are already generic prctls with
    > > >>>>>> fewer than 4 args that are used on x86.
    > > >>>>>
    > > >>>>> As originally posted, glibc prctl would have to know that it has to pull
    > > >>>>> an u64 argument off the argument list for ARCH_X86_CET_DISABLE. But
    > > >>>>> then the u64 argument is a problem for arch_prctl as well.
    > > >>>>>
    > > >>>
    > > >>> Argument of ARCH_X86_CET_DISABLE is int and passed in register.
    > > >> The commit message and the C source say otherwise, I think (not sure
    > > >> about the C source, not a kernel hacker).
    > > >
    > > > H.J. Lu suggested that we fix x86 arch_prctl() to take four arguments, and then keep MMAP_SHSTK as an arch_prctl(). Because now the map flags and size are all in registers, this also solves problems being pointed out earlier. Without a wrapper, the shadow stack mmap call (from user space) will be:
    > > >
    > > > syscall(_NR_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, size, MAP_32BIT).
    > >
    > > I admit I don’t see a show stopping technical reason we can’t add arguments to an existing syscall, but I’m pretty sure it’s unprecedented, and it doesn’t seem like a good idea.
    >
    > prctl prototype is:
    >
    > extern int prctl (int __option, ...)
    >
    > and implemented in kernel as:
    >
    > int prctl(int option, unsigned long arg2, unsigned long arg3,
    > unsigned long arg4, unsigned long arg5);
    >
    > Not all prctl operations take all 5 arguments. It also applies
    > to arch_prctl. It is quite normal for different operations of
    > arch_prctl to take different numbers of arguments.

    If by "quite normal" you mean "does not happen", then I agree.

    In any event, I will not have anything to do with a patch that changes
    an existing syscall signature unless Linus personally acks it. So if
    you want to email him and linux-abi, be my guest.

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