ioctl BLKGETSIZE /dev/md0 produces Permission denied error for non root

From: Pascal Bourguignon (spam_at_mouse-potato.com)
Date: 10/30/05 

Date: Sun, 30 Oct 2005 00:58:16 +0200

I've got a problem with ioctl BLKGETSIZE on a RAID device
(I want to pass ubd0=/dev/md0 to user-mode-linux (with a linux-2.6.8 kernel)
on a linux-2.4.20).

The problem is that ioctl BLKGETSIZE /dev/md0 fails with a Permission
denied error. But I have access rigths on /dev/md0 and its underlying
partitions:

[pjb@janus-1 user-mode-linux]$ cat blkgetsize.c
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mount.h>

    const char* msg(const char* operation,const char* arg){
        static char msg[80];
        if(strlen(operation)+strlen(arg)<sizeof(msg)-2){
            sprintf(msg,"%s %s",operation,arg);
        }else{
            sprintf(msg,"%s",operation);
        }
        return(msg);}

    int main(int argc,char** argv){
        int i;
        for(i=1;i<argc;i++){
            const char* fname=argv[i];
            int fd=open(fname,O_RDONLY,0);
            if(fd<0){
                perror(msg("open",fname));}
            else{
                int block;
                int res=ioctl(fd,BLKGETSIZE,&block);
                if(res<0){
                    perror(msg("ioctl BLKGETSIZE",fname));}
                else{
                    printf("ioctl BLKGETSIZE %s: block=%ld\n",fname,block);}
                close(fd);}}
        return(0);}

[pjb@janus-1 user-mode-linux]$ grep md0 /proc/mdstat
md0 : active raid1 hdc2[1] hda2[0]
[pjb@janus-1 user-mode-linux]$ ls -l /dev/md0 /dev/hd[ac]2
brw-rw---- 1 pjb disk 3, 2 2003-03-14 14:07 /dev/hda2
brw-rw---- 1 pjb disk 22, 2 2003-03-14 14:07 /dev/hdc2
brw-rw---- 1 pjb disk 9, 0 2003-03-14 14:07 /dev/md0
[pjb@janus-1 user-mode-linux]$ ./blkgetsize /dev/md0 /dev/hd[ac]2
ioctl BLKGETSIZE /dev/md0: Permission denied
ioctl BLKGETSIZE /dev/hda2: block=16803990
ioctl BLKGETSIZE /dev/hdc2: block=16803990
[pjb@janus-1 user-mode-linux]$ sudo ./blkgetsize /dev/md0 /dev/hd[ac]2
Password:xxxxxxxx
ioctl BLKGETSIZE /dev/md0: block=16803840
ioctl BLKGETSIZE /dev/hda2: block=16803990
ioctl BLKGETSIZE /dev/hdc2: block=16803990
[pjb@janus-1 user-mode-linux]$ uname -a
Linux janus-1 2.4.20-4GB #4 Sat May 1 08:55:53 CEST 2004 i686 unknown unknown GNU/Linux

What can I do to get it work?
I get the same error when I "hown pjb /dev/hd[ac]" too. 

-- 
"Klingon function calls do not have "parameters" -- they have
"arguments" and they ALWAYS WIN THEM."