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Filesystem

Second Extended Filesystem

Table of contents

  1. Ext2
  2. tmpfs
  3. Mount
  4. chroot

history:

  • MINIX filesystem
  • Extended filesystem, when Linux matured
  • Ext2, 1994
  • Ext3, widely used
#include <sys/statvfs.h>
int statvfs(const char *pathname, struct statvfs *statvfsbuf);
int fstatvfs(int fd, struct statvfs *statvfsbuf);

Ext2

features:

  • may choose the block size (1024 ~ 4096 bytes)
  • may choose the number of inodes for a partition of a given size
  • disk block are partitioned into groups. Each group includes data blocks and inodes stored in adjacent tracks.
  • preallocates disk data blocks for regular files
  • support fast symbolic links
  • consistency check by e2fsck

Ext2 does not support journalling.

tmpfs

The tmpfs file system differs from other memory-based file systems in that it is a virtual memory file system. This means that tmpfs uses not only RAM, but also the swap space, if RAM is exhausted.

mount -t tmpfs source target
# source can be any name

Mount

/proc/[pid]/mounts

All the filesystems currently mounted in the process’s mount namespace.

/etc/fstab

Descriptive information about the filesystems the system can mount. Read-only.

Format: https://man7.org/linux/man-pages/man5/fstab.5.html

sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
<file system> <mount point>  <type>  <options>  <dump>  <pass>

/etc/mtab

system-specific options given to mount

umount

Specifies the mount point of the file system to be unmounted. It is not possible to unmount a file system that is busy; that is, if there are open files on the file system, or a process’s current working directory is somewhere in the file system.

mount

  • Mounting a File System at Multiple Mount Points: changes made via one mount point are visible through the other(s)
  • Stacking Multiple Mounts on the Same Mount Point: Each new mount hides the directory subtree previously visible at that mount point.
  • Bind Mounts (--bind) is somewhat like a hard link, but
    • It is possible to make a bind mount for a directory.
    • A bind mount can cross file-system mount points (and even chroot jails).
  • Recursive version bind mounts (--rbind)

chroot

How to break the jail:

  • Calling chroot() doesn’t change the process’s current working directory. Thus, a call to chroot() is typically preceded or followed by a call to chdir() (e.g., chdir(“/”) after the chroot() call). If this is not done, then a process can use relative pathnames to access files and directories outside the jail.
  • If a process holds an open file descriptor for a directory outside the jail, then the combination of fchdir() plus chroot() can be used to break out of the jail.
    • Using sendmsg() and recvmsg(), we can pass ancillary data containing a file descriptor from one process to another process on the same host via a UNIX domain socket.