File System Manipulation
In this chapter, we discuss general file system manipulation, with a focus on access files and directories to which an other, potentially untrusted user has write access.
Temporary files are covered in their own chapter, Temporary Files.
Sometimes, it is necessary to operate on files and directories owned by other (potentially untrusted) users. For example, a system administrator could remove the home directory of a user, or a package manager could update a file in a directory which is owned by an application-specific user. This differs from accessing the file system as a specific user; see Accessing the File System as a Different User.
Accessing files across trust boundaries faces several challenges, particularly if an entire directory tree is being traversed:
Another user might add file names to a writable directory at any time. This can interfere with file creation and the order of names returned by
Merely opening and closing a file can have side effects. For instance, an automounter can be triggered, or a tape device rewound. Opening a file on a local file system can block indefinitely, due to mandatory file locking, unless the
O_NONBLOCKflag is specified.
Hard links and symbolic links can redirect the effect of file system operations in unexpected ways. The
AT_SYMLINK_NOFOLLOWvariants of system calls only affected final path name component.
The structure of a directory tree can change. For example, the parent directory of what used to be a subdirectory within the directory tree being processed could suddenly point outside that directory tree.
Files should always be created with the
so that creating the file will fail if it already exists. This
guards against the unexpected appearance of file names, either
due to creation of a new file, or hard-linking of an existing
file. In multi-threaded programs, rather than manipulating the
umask, create the files with mode
possible, and adjust it afterwards with
To avoid issues related to symbolic links and directory tree
restructuring, the “
at” variants of system
calls have to be used (that is, functions like
unlinkat, together with
AT_SYMLINK_NOFOLLOW). Path names passed to
these functions must have just a single component (that is,
without a slash). When descending, the descriptors of parent
directories must be kept open. The missing
opendirat function can be emulated with
openat (with an
O_DIRECTORY flag, to avoid opening special
files with side effects), followed by
If the “
at” functions are not available, it
is possible to emulate them by changing the current directory.
(Obviously, this only works if the process is not multi-threaded.)
fchdir has to be used to change the current
directory, and the descriptors of the parent directories have to
be kept open, just as with the “
chdir("…") is unsafe because it
might ascend outside the intended directory tree.
at” function emulation is currently
required when manipulating extended attributes. In this case,
lsetxattr function can be used, with a
relative path name consisting of a single component. This also
applies to SELinux contexts and the
Currently, it is not possible to avoid opening special files
and changes to files with hard links if the
directory containing them is owned by an untrusted user.
(Device nodes can be hard-linked, just as regular files.)
affect files whose link count is greater than one. But opening
the files, checking that the link count is one with
fstat, and using
the file descriptor may have unwanted side effects, due to item
2 above. When creating directories, it is therefore important
to change the ownership and permissions only after it has been
fully created. Until that point, file names are stable, and no
files with unexpected hard links can be introduced.
Similarly, when just reading a directory owned by an untrusted user, it is currently impossible to reliably avoid opening special files.
There is no workaround against the instability of the file list
modification of the directory can result in a list of files
being returned which never actually existed on disk.
Hard links and symbolic links can be safely deleted using
unlinkat without further checks because
deletion only affects the name within the directory tree being
This section deals with access to the file system as a specific user. This is different from accessing files and directories owned by a different, potentially untrusted user; see Accessing the File System as a Different User.
One approach is to spawn a child process which runs under the
target user and group IDs (both effective and real IDs). Note
that this child process can block indefinitely, even when
processing regular files only. For example, a special FUSE file
system could cause the process to hang in uninterruptible sleep
stat system call.
An existing process could change its user and group ID using
(These functions are preferred over
setegid because they do not allow the
impersonated user to send signals to the process.) These
functions are not thread safe. In multi-threaded processes,
these operations need to be performed in a single-threaded child
process. Unexpected blocking may occur as well.
It is not recommended to try to reimplement the kernel permission checks in user space because the required checks are complex. It is also very difficult to avoid race conditions during path name resolution.
For historical reasons, there are preprocessor constants such as
However, on most systems, the length of canonical path names
(absolute path names with all symbolic links resolved, as
canonicalize_file_name) can exceed
PATH_MAX bytes, and individual file name
components can be longer than
is also true of the
_PC_NAME_MAX values returned by
pathconf, and the
f_namemax member of
statvfs. Therefore, these constants should not be
used. This is also reason why the
readdir_r should never be used (instead,
You should not write code in a way that assumes that there is an upper limit on the number of subdirectories of a directory, the number of regular files in a directory, or the link count of an inode.
Not all file systems support all features. This makes it very difficult to write general-purpose tools for copying files. For example, a copy operation intending to preserve file permissions will generally fail when copying to a FAT file system.
Some file systems are case-insensitive. Most should be case-preserving, though.
Name length limits vary greatly, from eight to thousands of bytes. Path length limits differ as well. Most systems impose an upper bound on path names passed to the kernel, but using relative path names, it is possible to create and access files whose absolute path name is essentially of unbounded length.
Some file systems do not store names as fairly unrestricted byte sequences, as it has been traditionally the case on GNU systems. This means that some byte sequences (outside the POSIX safe character set) are not valid names. Conversely, names of existing files may not be representable as byte sequences, and the files are thus inaccessible on GNU systems. Some file systems perform Unicode canonicalization on file names. These file systems preserve case, but reading the name of a just-created file using
readdirmight still result in a different byte sequence.
Permissions and owners are not universally supported (and SUID/SGID bits may not be available). For example, FAT file systems assign ownership based on a mount option, and generally mark all files as executable. Any attempt to change permissions would result in an error.
Non-regular files (device nodes, FIFOs) are not generally available.
Only on some file systems, files can have holes, that is, not all of their contents is backed by disk storage.
ioctlsupport (even fairly generic functionality such as
FIEMAPfor discovering physical file layout and holes) is file-system-specific.
Not all file systems support extended attributes, ACLs and SELinux metadata. Size and naming restriction on extended attributes vary.
Hard links may not be supported at all (FAT) or only within the same directory (AFS). Symbolic links may not be available, either. Reflinks (hard links with copy-on-write semantics) are still very rare. Recent systems restrict creation of hard links to users which own the target file or have read/write access to it, but older systems do not.
Renaming (or moving) files using
renamecan fail (even when
statindicates that the source and target directories are located on the same file system). This system call should work if the old and new paths are located in the same directory, though.
Locking semantics vary among file systems. This affects advisory and mandatory locks. For example, some network file systems do not allow deleting files which are opened by any process.
Resolution of time stamps varies from two seconds to nanoseconds. Not all time stamps are available on all file systems. File creation time (birth time) is not exposed over the
fstatinterface, even if stored by the file system.
fstatvfs functions allow programs to
examine the number of available blocks and inodes, through the
struct statvfs. Some file systems return
fictional values in the
f_favail fields, so the only reliable way to
discover if the file system still has space for a file is to try
to create it. The
f_bfree field should be
reasonably accurate, though.
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