On Thu, Jul 21, 2011 at 10:20 AM, piyush moghe <pmkernel@gmail.com> wrote:
I have a question regarding mounting filesystem without giving filesystem type as argument to mount command:
Now my question is that if I don't provide FS type name to mount command how the FS gets mounted. Is there some other path for this code or if before calling compat_sys_mount i.e in user mode FS type is determined somehow?
Regards, Piyush
Hello Piyush, The mount command, resolves in userspace. Most mount binaries are compiled against libblkid library that is used to identify the content of block devices. root@aopcjm:~# ldd /bin/mount | grep blkid libblkid.so.1 => /lib/libblkid.so.1 (0xb76e3000) Mounting without specifying the FS type for my pendrive, the mount syscall is invoked with FS type vfat root@aopcjm:~# strace -emount mount /dev/sdb1 /mnt/pendrive/ mount("/dev/sdb1", "/mnt/pendrive/", "vfat", MS_MGC_VAL, NULL) = 0 Is very well described in the mount man page: If no -t option is given, or if the auto type is specified, mount will try to guess the desired type. If mount was compiled with the blkid library, the guessing is done by this library. Otherwise, mount guesses itself by probing the superblock; if that does not turn up anything that looks familiar, mount will try to read the file /etc/filesystems, or, if that does not exist, /proc/filesystems. All of the filesystem types listed there will be tried, except for those that are labeled "nodev" (e.g., devpts, proc, nfs, and nfs4). If /etc/filesystems ends in a line with a single * only, mount will read /proc/filesystems afterwards.
From the blkid man page:
The blkid program is the command-line interface to working with libuuid(3) library. It can determine the type of content (e.g. filesystem, swap) a block device holds, and also attributes (tokens, NAME=value pairs) from the content metadata (e.g. LABEL or UUID fields). Hope this helps, -- Javier MartÃnez Canillas (+34) 682 39 81 69 PhD Student in High Performance Computing Computer Architecture and Operating System Department (CAOS) Universitat Autònoma de Barcelona Barcelona, Spain