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ladybird/Base/usr/share/man/man2/mount.md
Liav A 0bbd9040ef Kernel+Userland: Split bind-mounting and re-mounting from mount syscall 
These 2 are an actual separate types of syscalls, so let's stop using
special flags for bind mounting or re-mounting and instead let userspace
calling directly for this kind of actions.
2023-05-17 23:39:15 -06:00

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Name

mount - mount a filesystem

Synopsis

#include <unistd.h>

int mount(int source_fd, const char* target, const char* fs_type, int flags);

Description

mount() mounts a filesystem stored at source_fd by overlaying its contents over target.

fs_type must be one of the following supported filesystems:

  • Ext2FS (or ext2): The ext2 filesystem.
  • ProcFS (or proc): The process pseudo-filesystem (normally mounted at /proc).
  • DevPtsFS (or devpts): The pseudoterminal pseudo-filesystem (normally mounted at /dev/pts).
  • RAMFS (or ram): A non-persistent filesystem that stores all its data in RAM. An instance of this filesystem is normally mounted at /tmp.
  • Plan9FS (or 9p): A remote filesystem served over the 9P protocol.

For Ext2FS, source_fd must refer to an open file descriptor to a file containing the filesystem image. This may be a device file or any other seekable file. For Plan9FS, source_fd must refer to a socket or a device connected to a 9P server. All the other filesystems ignore the source_fd - you can even pass an invalid file descriptor such as -1.

The following flags are supported:

  • MS_NODEV: Disallow opening any devices from this file system.
  • MS_NOEXEC: Disallow executing any executables from this file system.
  • MS_NOSUID: Ignore set-user-id bits on executables from this file system.
  • MS_RDONLY: Mount the filesystem read-only.
  • MS_WXALLOWED: Allow W^X protection circumvention for executables on this file system.
  • MS_AXALLOWED: Allow anonymous executable mappings for executables on this file system.
  • MS_NOREGULAR: Disallow opening any regular files from this file system.

These flags can be used as a security measure to limit the possible abuses of the newly mounted file system.

Bind mounts

If MS_BIND is specified in flags, fs_type is ignored and a bind mount is performed instead. In this case, the file or directory specified by source_fd is overlaid over target — the target appears to be replaced by a copy of the source. This can be used as an alternative to symlinks or hardlinks.

Each bind mount has its own set of flags, independent of the others or the original file system. It is possible to bind-mount a file or directory over itself, which may be useful for changing mount flags for a part of a filesystem.

Remounting

Note that remounting a file system will only affect future operations with the file system, not any already opened files. For example, if you open a directory on a filesystem that's mounted with MS_NODEV, then remount the filesystem to allow opening devices, attempts to open a devices relative to the directory file descriptor (such as by using openat()) will still fail.

In particular, current working directory and root directory of any already running processes behave the same way, and don't automatically "pick up" changes in mount flags of the underlying file system. To "refresh" the working directory to use the new mount flags after remounting a filesystem, a process can call chdir() with the path to the same directory.

Errors

  • EINVAL: The flags value contains deprecated flags such as MS_REMOUNT or MS_BIND.
  • EFAULT: The fs_type or target are invalid strings.
  • EPERM: The current process does not have superuser privileges.
  • ENODEV: The fs_type is unrecognized, or the file descriptor to source is not found, or the source doesn't contain a valid filesystem image. Also, this error occurs if fs_type is valid and required to be seekable, but the file descriptor from source_fd is not seekable.
  • EBADF: If the source_fd is not valid, and either fs_type specifies a file-backed filesystem (and not a pseudo filesystem), or MS_BIND is specified in flags.
  • ENOTBLK: If the source_fd is not a block device, but one is required (i.e. when fs_type is Ext2FS)

All of the usual path resolution errors may also occur.

See also