Weird writev() behaviour on EFAULT - also successfully modifying the file

[Tuomas Tynkkynen <tuomas@tuxera.com>]

Greetings,

I've noticed a corner case with writev() both modifying the file and
returning -EFAULT at the same time. This happens on filesystems using
generic_perform_write() (i.e. ext4, vfat) on 4.6.3 kernel and below,
down to 3.16. Here's the reproducer:

// 8<---- cut here ------------------------ >8
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/uio.h>

int main(int argc, char** argv) {
    int fd;
    ssize_t ret;
    struct iovec iov[] = {
        { .iov_base = NULL, .iov_len = 0, },
        { .iov_base = NULL, .iov_len = 4096, },
    };

    system("dd if=/dev/zero bs=8k count=1 | tr '\\0' 'A' > foo");

    fd = open("foo", O_RDWR);
    if (fd < 0) {
        perror("open()");
        return 1;
    }

    ret = writev(fd, iov, 2);
    if (ret < 0) {
        perror("writev()");
        return 1;
    }

    return 0;
}
// 8<---- cut here ------------------------ >8

Running that prints "writev(): Bad address" but also some NUL bytes have
appeared at the beginning file, in addition to the 'A's by the dd.

00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00001000  41 41 41 41 41 41 41 41  41 41 41 41 41 41 41 41  |AAAAAAAAAAAAAAAA|
*
00002000

Is that intented behaviour?