preadv/pwritev: create compat_readv()

This patch series:

Implement the preadv() and pwritev() syscalls.  *BSD has this syscall for
quite some time.

Test code:

#if 0
set -x
gcc -Wall -O2 -o preadv $0
exit 0
#endif
/*
 * preadv demo / test
 *
 * (c) 2008 Gerd Hoffmann <kraxel@redhat.com>
 *
 * build with "sh $thisfile"
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <inttypes.h>
#include <sys/uio.h>

/* ----------------------------------------------------------------- */
/* syscall windup                                                    */

#include <sys/syscall.h>
#if 0
/* WARNING: Be sure you know what you are doing if you enable this.
 * linux syscall code isn't upstream yet, syscall numbers are subject
 * to change */
# ifndef __NR_preadv
#  ifdef __i386__
#   define __NR_preadv  333
#   define __NR_pwritev 334
#  endif
#  ifdef __x86_64__
#   define __NR_preadv  295
#   define __NR_pwritev 296
#  endif
# endif
#endif
#ifndef __NR_preadv
# error preadv/pwritev syscall numbers are unknown
#endif

static ssize_t preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
{
    uint32_t pos_high = (offset >> 32) & 0xffffffff;
    uint32_t pos_low  =  offset        & 0xffffffff;

    return syscall(__NR_preadv, fd, iov, iovcnt, pos_high, pos_low);
}

static ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
{
    uint32_t pos_high = (offset >> 32) & 0xffffffff;
    uint32_t pos_low  =  offset        & 0xffffffff;

    return syscall(__NR_pwritev, fd, iov, iovcnt, pos_high, pos_low);
}

/* ----------------------------------------------------------------- */
/* demo/test app                                                     */

static char filename[] = "/tmp/preadv-XXXXXX";
static char outbuf[11] = "0123456789";
static char inbuf[11]  = "----------";

static struct iovec ovec[2] = {{
        .iov_base = outbuf + 5,
        .iov_len  = 5,
    },{
        .iov_base = outbuf + 0,
        .iov_len  = 5,
    }};

static struct iovec ivec[3] = {{
        .iov_base = inbuf + 6,
        .iov_len  = 2,
    },{
        .iov_base = inbuf + 4,
        .iov_len  = 2,
    },{
        .iov_base = inbuf + 2,
        .iov_len  = 2,
    }};

void cleanup(void)
{
    unlink(filename);
}

int main(int argc, char **argv)
{
    int fd, rc;

    fd = mkstemp(filename);
    if (-1 == fd) {
        perror("mkstemp");
        exit(1);
    }
    atexit(cleanup);

    /* write to file: "56789-01234" */
    rc = pwritev(fd, ovec, 2, 0);
    if (rc < 0) {
        perror("pwritev");
        exit(1);
    }

    /* read from file: "78-90-12" */
    rc = preadv(fd, ivec, 3, 2);
    if (rc < 0) {
        perror("preadv");
        exit(1);
    }

    printf("result  : %s\n", inbuf);
    printf("expected: %s\n", "--129078--");
    exit(0);
}

This patch:

Factor out some code from compat_sys_readv() which can be shared with the
upcoming compat_sys_preadv().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <linux-api@vger.kernel.org>
Cc: <linux-arch@vger.kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>