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    SubjectRe: [RFC PATCH 4/6] mm: provide generic compat_sys_readahead() implementation
    On Mon, Mar 19, 2018 at 11:23:42PM +0000, Al Viro wrote:
    > static inline long C_S_moron(int, loff_t, size_t);
    > long compat_SyS_moron(long a0, long a1, long a2, long a3, long a4, long a5, long a6)
    > {
    > return C_S_moron((__force int)a0,
    > (__force loff_t)(((u64)a2 << 32)|a1),
    > (__force size_t)a3);
    > }
    > static inline long C_S_moron(int fd, loff_t offset, size_t count)
    > {
    > whatever body you had for it
    > }
    >
    > That - from
    > COMPAT_SYSCALL_DEFINE3(moron, int, fd, loff_t, offset, size_t, count)
    > {
    > whatever body you had for it
    > }
    >
    > We can use similar machinery for SYSCALL_DEFINE itself, so that
    > SyS_moron() would be defined with (long, long, long, long, long, long)
    > as arguments and not (long, long long, long) as we have now.

    That would be great, as it would allow to use a struct pt_regs * based
    syscall calling convention on i386 as well, and not only on x86-64, right?

    > It's not impossible to do. It won't be pretty, but that use of local
    > enums allows to avoid unbearably long expansions.
    >
    > Benefits:
    > * all SyS... wrappers (i.e. the thing that really ought to
    > go into syscall tables) have the same type.
    > * we could have SYSCALL_DEFINE produce a trivial compat
    > wrapper, have explicit COMPAT_SYSCALL_DEFINE discard that thing
    > and populate the compat syscall table *entirely* with compat_SyS_...,
    > letting the linker sort it out. That way we don't need to keep
    > track of what can use native and what needs compat in each compat
    > table on biarch.
    > * s390 compat wrappers would disappear with that approach.
    > * we could even stop generating sys_... aliases - if
    > syscall table is generated by slapping SyS_... or compat_SyS_...
    > on the name given there, we don't need to _have_ those sys_...
    > things at all. All SyS_... would have the same type, so the pile
    > in syscalls.h would not be needed - we could generate the externs
    > at the same time we generate the syscall table.
    >
    > And yes, it's a high-squick approach. I know and I'm not saying
    > it's a good idea. OTOH, to quote the motto of philosophers and
    > shell game operators, "there's something in it"...

    ... and getting rid of all in-kernel calls to sys_*() is needed as
    groundwork for that. So I'll continue to do that "mindless" conversion
    first. On top of that, three things (which are mostly orthogonal to each
    other) can be done:

    1) ptregs system call conversion for x86-64

    Original implementation by Linus exists; needs a bit of tweaking
    but should be doable soon. Need to double-check it does the right
    thing for IA32_EMULATION, though.

    2) re-work initramfs etc. code to not use in-kernel equivalents of
    syscalls, but operate on the VFS level instead.

    3) re-work SYSCALL_DEFINEx() / COMPAT_SYSCALL_DEFINEx() based on
    your suggestions.

    Does that sound sensible?

    Thanks,
    Dominik

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