ladybird/Base/usr/share/man/man2/mount.md at 04b51e39d6c3bd118e68e87801a6f9392309a24c

Liav A ed67a877a3 Kernel+SystemServer+Base: Introduce the RAMFS filesystem

This filesystem is based on the code of the long-lived TmpFS. It differs
from that filesystem in one keypoint - its root inode doesn't have a
sticky bit on it.

Therefore, we mount it on /dev, to ensure only root can modify files on
that directory. In addition to that, /tmp is mounted directly in the
SystemServer main (start) code, so it's no longer specified in the fstab
file. We ensure that /tmp has a sticky bit and has the value 0777 for
root directory permissions, which is certainly a special case when using
RAM-backed (and in general other) filesystems.

Because of these 2 changes, it's no longer needed to maintain the TmpFS
filesystem, hence it's removed (renamed to RAMFS), because the RAMFS
represents the purpose of this filesystem in a much better way - it
relies on being backed by RAM "storage", and therefore it's easy to
conclude it's temporary and volatile, so its content is gone on either
system shutdown or unmounting of the filesystem.

2023-02-04 15:32:45 -07:00

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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_BIND: Perform a bind-mount (see below).
MS_RDONLY: Mount the filesystem read-only.
MS_REMOUNT: Remount an already mounted filesystem (see below).
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

If MS_REMOUNT is specified in flags, source_fd and fs_type are ignored, and a remount is performed instead. target must point to an existing mount point. The mount flags for that mount point are reset to flags (except the MS_REMOUNT flag itself, which is stripped from the value).

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.

Similarly, to change the mount flags used by the root directory, a process can remount the root filesystem using MS_REMOUNT. However, it only have a noticeable effect if the kernel was to launch more userspace processes directly, the way it does launch the initial userspace process.

Errors

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.

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