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    SubjectRe: [PATCH v3 0/3] preadv & pwritev syscalls.
    On Mon, Dec 15, 2008 at 09:57:24PM +0100, Gerd Hoffmann wrote:

    > > It fixes the alignment issue but still won't work; on MIPS 32-bit userspace
    > > will pass the 64-bit argument in two registers but the 64-bit kernel code
    > > will assume it to be passed in a single registers. It'd be ugly but passing
    > > a pointer to a 64-bit argument would solve the issue; something like this:
    > >
    > > sys_preadv(unsigned long fd, const struct iovec __user *vec,
    > > unsigned long vlen, loff_t __user *pos);
    > > compat_sys_preadv(unsigned long fd, const struct compat_iovec __user *vec,
    > > unsigned long vlen, loff_t __user *pos);
    > Suggestion from the s390 front was to explicitly pass high and low part
    > of pos as two arguments. A bit ugly too, but should work fine as well
    > and it avoids the user pointer dereference. What do you think about this?

    That's what the wrapper which you deleted, was doing ;-) So yes, I like
    it. It just raises one new problem, endianess - are arguments being passed
    as low/high or high/low? On MIPS we've been solving the issue with the
    merge_64() macro which is defined depending on the byte order:

    #ifdef __MIPSEB__
    #define merge_64(r1, r2) ((((r1) & 0xffffffffUL) << 32) + ((r2) & 0xffffffffUL))
    #ifdef __MIPSEL__
    #define merge_64(r1, r2) ((((r2) & 0xffffffffUL) << 32) + ((r1) & 0xffffffffUL))

    The actual syscall wrapper could use it like:

    asmlinkage int compat_sys_pwritev(unsigned long fd,
    const struct compat_iovec __user *vec,
    unsigned a3, unsigned a4, unsigned long vlen)
    loff_t offset = merge_64(a3, a4);

    If merge_64() was a standard macro in <asm/compat.h> on all architectures,
    compat_sys_pwrite() could invoke it directly and the need for the compat
    wrapper around the compat wrapper would go away.

    > > I'm surprised this works for x86; does x86-64 code really expect 64-bit
    > > arguments as 2 32-bit arguments?
    > Args are passed on the stack, not in registers.

    Same as 32-bit MIPS for argument #4 and up which will be passed on the
    stack - but the requirement for passing a long long is for it to be
    passed as an aligned pair of arguments and the original patch got
    that wrong.


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      Last update: 2008-12-16 17:09    [W:0.022 / U:16.036 seconds]
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