NAME

SYNOPSIS

DESCRIPTION

sandboxfs is designed to allow running commands with limited access to the file system by using the virtual tree as their new root, and to do so consistently across a variety of platforms.

The sandbox instance mounted at mount_point is initially configured with the mapping specifications provided via repeated instances of --mapping. Once running, the mount point can be reconfigured any number of times: sandboxfs listens for reconfiguration requests on the file specified by --input and reconfigures the mappings as requested, emiting a response on the file specified by --output.

The following flags are recognized:

--allow who

Specifies who should have access to the file system. who must be one of: ‘other’ to indicate that everyone, including root, can access the file system; ‘root’ to indicate that only the current user and root can access the file system; and ‘self’ to indicate that only the current user can access the file system.

The default value is ‘self’ because the standard FUSE configuration does not allow more relaxed permissions, and it would be a pity if you couldn't run sandboxfs successfully without additional configuration. To change this behavior, add ‘user_allow_other’ to /etc/fuse.conf on Linux or set the ‘vfs.generic.osxfuse.tunables.allow_other’ sysctl(8) tunable to 1 on macOS.

Note that, at least on macOS, you will need to consider granting ‘other’ permissions if you intend to run signed binaries through the sandbox. This is because the amfid(8) daemon, which implements the signature validation, runs as a different user and must be able to access the executables.

--cpu_profile path

Enables CPU profiling and stores the pprof log to the given path. This feature is only available if sandboxfs was built with gperftools support (i.e. with the compile-time ‘profiler’ feature). Passing this flag when support is not enabled results in an error.

--input path

Points to the file from which to read new configuration requests, or ‘-’ (the default) for stdin. See the Reconfigurations subsection for details on the contents and behavior of the input file.

--help

Prints global help details and exits. Specifying this flag causes all other valid flags and arguments to be ignored.

--mapping type:mapping:target

Registers a new mapping. This flag can be given an arbitrary number of times as long as the same mapping is not repeated. See the Mapping specifications subsection for details on how a mapping is specified.

--node_cache

Enables the path-based node cache, which causes nodes to be reused across reconfigurations when they map to the same underlying paths. This should offer a performance boost when the same set of files are mapped over and over again across different reconfiguration operations.

This cache used to be enabled by default but it turned out to be problematic. For example, on macOS, OSXFUSE seems to keep track of the path where a node first appeared, which causes trouble if the node disappears and then reappears in a different location (for example, dladdr(3) returns invalid paths). For this reason, the cache is now disabled by default but is still kept around in the code for experimentation. The cache may be removed in a future release altogether if these problems cannot be worked around.

--output path

Points to the file to which to write confirmations of reconfiguration, or ‘-’ (the default) for stdout. See the Reconfigurations subsection for details on the contents and behavior of the output file.

--ttl duration

Specifies how long the kernel is allowed to cache file metadata for. The duration is currently specified as a number of seconds followed by the ‘s’ suffix.

--reconfig_threads count

Sets the number of threads to use to process reconfiguration requests. Defaults to the number of logical CPUs in the system.

--version

Prints version information and exits. Specifying this flag causes all other valid flags and arguments to be ignored except for --help, which has priority over this one.

The first line printed to stdout is machine parseable and follows the following structure:

sandboxfs MAJOR.MINOR[.ADDITIONAL OPTIONAL QUALIFIERS]
    

Programs automating invocations of sandboxfs can use this information to determine the correct command-line syntax to use.

--xattrs

Enables support for extended attributes, which causes all extended attribute operations to propagate to the underlying files.

When disabled (the default), the behavior of extended attribute operations is platform-dependent: they may either fail as not supported or they may act as no-op stubs. The reason is that this feature is disabled by telling the kernel that none of these operations are implemented in sandboxfs, which then causes the kernel to never contact the daemon for them. Keeping extended attributes disabled can result in a performance boost.

EXIT STATUS

Sending a termination signal to sandboxfs will cause the file system to exit in a controlled manner, ensuring that the mount point is released. If the file system is busy, the signal will be queued until all open file descriptors on the file system are released at which point the file system will try to exit cleanly again. Note that, due to limitations in signal handling in Rust (which is the language in which sandboxfs is implemented), the reception of a signal will cause sandboxfs to return 1 instead of terminating with a signal condition.

ENVIRONMENT

RUST_LOG

Sets the maximum level of logging messages sent to stderr. Possible values include ‘error’, ‘warn’, ‘info’ and ‘debug’, though many more syntaxes are supported. See the documentation for Rust's ‘env_logger’ crate for more details.

sandboxfs may recognize other standard Rust variables like RUST_BACKTRACE but the list above attempts to describe the ones a user may find most useful.

EXAMPLES

sandboxfs --mapping=ro:/:/ --mapping=rw:/tmp:/tmp/fresh-tmp /mnt

[
    {"CreateSandbox": {
        "id": "first",
        "mappings": [
            {"path": "/tmp", "underlying_path": "/tmp/abc", "writable": true}
        ]
    }},
    {"DestroySandbox": "first"}
    {"CreateSandbox": {
        "id": "second",
        "mappings": [
            {"path": "/foo/bar", "underlying_path": "/tmp/x/y",
             "writable": false}
        ]
    }}
]

{"id": "first", "error": null}
{"id": "first", "error": null}
{"id": "second", "error": null}

[
    {"CreateSandbox": {
        "id": "first",
        "mappings": [
            {"path": "", "path_prefix": 1,
             "underlying_path": "abc", "underlying_path_prefix": 1,
             "writable": true}
        ],
        "prefixes": {
            "1": "/tmp"
        }
    }},
    {"DestroySandbox": "first"}
    {"CreateSandbox": {
        "id": "second",
        "mappings": [
            {"path": "bar", "path_prefix": 2,
             "underlying_path": "x/y", "underlying_path_prefix": 1,
             "writable": false}
        ],
        "prefixes": {
            "2": "/foo"
        }
    }}
]

[
    {"C": {
        "i": "first",
        "m": [
            {"p": "", "x": 1, "u": "abc", "y": 1, "w": true}
        ],
        "q": {
            "1": "/tmp"
        }
    }},
    {"D": "first"}
    {"C": {
        "i": "second",
        "m": [
            {"p": "bar", "x": 2, "u": "x/y", "y": 1}
        ],
        "q": {
            "2": "/foo"
        }
    }}
]

AUTHORS

BUGS

• Hard links are not supported.
• Mapping the same external file or directory under two different locations within the mount point results in undefined behavior. Writes may not be reflected at both mapped locations at the same time, which can lead to data corruption. This is true even for read-only mappings because each separate view within the mount point may have cached different contents, returning different data than what's truly on disk. Using --node_cache may help mitigate this issue but it doesn't always do.
• The --allow root setting does not work on Linux; use --allow other as the alternative. See https://github.com/bazil/fuse/issues/144 for details.
• It is currently impossible to terminate sandboxfs cleanly while the file system is busy. Signals received while the file system is in use will be queued as described in EXIT STATUS and fatal signals will cause sandboxfs to leak the mount point (possibly irrecoverably without a reboot because of kernel bugs).
• Any explicitly-mapped directories and any scaffold directories (those directories that appear to represent intermediate path components that do not exist anywhere else in the file system) cannot be removed. Attempts to remove them will result in a “permission denied” error. While it could be possible to implement some different behavior, this is what sandboxfs currently exposes. You may or may not consider this to be a bug.
• If --node_cache is enabled, node data is cached in-memory for all files accessed through a sandboxfs instance in order to offer good performance across reconfigurations. However, this cache does not currently implement any expiration policy, which means that memory usage can grow unboundedly if many different files are mapped and accessed through the sandbox.
• If a FIFO is used for -input, sandboxfs will block until a separate process opens the FIFO for writing. This happens even before the file system starts serving, which means the file system is not usable until the FIFO is opened.
• While it is possible to reconfigure the entries of the root directory of a running file system, it is not possible to reconfigure the root mapping itself to point to a different location or to change its writability properties.
• Unmapping entries doesn't fully work. The FUSE library that sandboxfs currently uses does not support sending cache invalidation requests to the kernel, which means unmapped entries will not vanish immediately from the file system. You may be able to mitigate this by setting a low node TTL with the --ttl flag, but this doesn't work on macOS either because OSXFUSE does not honor node TTLs.
• Handling of extended attributes on open-but-deleted-files does not work properly. Those files will appear as if they didn't have any extended attributes any longer, and attempts to modify them will fail.