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    SubjectRe: [PATCH 08/24] Allow a 32bit ABI to use the naming of the 64bit ABI syscalls to avoid confusion of not splitting the registers
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    On Wednesday 01 October 2014 13:42:27 Catalin Marinas wrote:
    > On Thu, Sep 04, 2014 at 11:11:04AM +0100, Arnd Bergmann wrote:
    > > On Wednesday 03 September 2014 14:19:02 Andrew Pinski wrote:
    > > > + * For 32bit abis where 64bit can be passed via one
    > > > + * register, use the same naming as the 64bit ones
    > > > + * as they will only have a 64 bit off_t.
    > > > */
    > > > -#if __BITS_PER_LONG == 64 && !defined(__SYSCALL_COMPAT)
    > > > +#if (__BITS_PER_LONG == 64 && !defined(__SYSCALL_COMPAT)) || \
    > > > + defined(__ARCH_WANT_64BIT_SYSCALLS)
    > >
    > > I'm not sure if __ARCH_WANT_64BIT_SYSCALLS is the best name for
    > > this, since it's really only about off_t. It took me a while
    > > to understand what you are doing here.
    >
    > I'm not sure I fully get it yet. So with this change, we avoid using
    > syscall numbers like __NR_ftruncate64 in favour of __NR_ftruncate. Why?
    > (maybe there's a valid reason, just not getting it).

    glibc depends on the name to decide which calling conventions it
    uses. I assume this is the same on IPL32 ARM.

    The general rule is that on a 32-bit architecture, __NR_ftruncate refers
    to the system call that takes a 32-bit off_t argument, while __NR_ftruncate64
    refers to the syscall that takes a 64-bit loff_t.

    I would assume that the new ABI does not actually allow using 32-bit off_t
    in applications (that would be silly) and defaults to using 64-bit offsets,
    but it still needs to generate the right system calls.

    > Either way, ILP32 would still end up calling sys_ftruncate64() (rather
    > than the native sys_ftruncate()).

    sys_ftruncate64 does not exist in 64-bit kernels, it can either call
    compat_sys_ftruncate64_wrapper or sys_ftruncate. I'd assume it would
    call the latter and pass a single 64-bit register, but that is another
    matter.

    Arnd


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