Ubuntu Manpage: F_GET_SEALS, F_ADD_SEALS

NAME

       F_GET_SEALS, F_ADD_SEALS - get/add file seals

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <fcntl.h>

       int fcntl(int fd, F_ADD_SEALS, int arg);
       int fcntl(int fd, F_GET_SEALS);

DESCRIPTION

File seals limit the set of allowed operations on a given file. For each seal that is set on a file, a
specific set of operations will fail with EPERM on this file from now on. The file is said to be sealed.
The default set of seals depends on the type of the underlying file and filesystem. For an overview of
file sealing, a discussion of its purpose, and some code examples, see memfd_create(2).

Currently, file seals can be applied only to a file descriptor returned by memfd_create(2) (if the
MFD_ALLOW_SEALING was employed). On other filesystems, all fcntl() operations that operate on seals will
return EINVAL.

Seals are a property of an inode. Thus, all open file descriptors referring to the same inode share the
same set of seals. Furthermore, seals can never be removed, only added.

F_ADD_SEALS
Add the seals given in the bit-mask argument arg to the set of seals of the inode referred to by
the file descriptor fd. Seals cannot be removed again. Once this call succeeds, the seals are
enforced by the kernel immediately. If the current set of seals includes F_SEAL_SEAL (see below),
then this call will be rejected with EPERM. Adding a seal that is already set is a no-op, in case
F_SEAL_SEAL is not set already. In order to place a seal, the file descriptor fd must be
writable.

F_GET_SEALS
Return (as the function result) the current set of seals of the inode referred to by fd. If no
seals are set, 0 is returned. If the file does not support sealing, -1 is returned and errno is
set to EINVAL.

The following seals are available:

F_SEAL_SEAL
If this seal is set, any further call to fcntl() with F_ADD_SEALS fails with the error EPERM.
Therefore, this seal prevents any modifications to the set of seals itself. If the initial set of
seals of a file includes F_SEAL_SEAL, then this effectively causes the set of seals to be constant
and locked.

F_SEAL_SHRINK
If this seal is set, the file in question cannot be reduced in size. This affects open(2) with
the O_TRUNC flag as well as truncate(2) and ftruncate(2). Those calls fail with EPERM if you try
to shrink the file in question. Increasing the file size is still possible.

F_SEAL_GROW
If this seal is set, the size of the file in question cannot be increased. This affects write(2)
beyond the end of the file, truncate(2), ftruncate(2), and fallocate(2). These calls fail with
EPERM if you use them to increase the file size. If you keep the size or shrink it, those calls
still work as expected.

F_SEAL_WRITE
If this seal is set, you cannot modify the contents of the file. Note that shrinking or growing
the size of the file is still possible and allowed. Thus, this seal is normally used in
combination with one of the other seals. This seal affects write(2) and fallocate(2) (only in
combination with the FALLOC_FL_PUNCH_HOLE flag). Those calls fail with EPERM if this seal is set.
Furthermore, trying to create new shared, writable memory-mappings via mmap(2) will also fail with
EPERM.

Using the F_ADD_SEALS operation to set the F_SEAL_WRITE seal fails with EBUSY if any writable,
shared mapping exists. Such mappings must be unmapped before you can add this seal. Furthermore,
if there are any asynchronous I/O operations (io_submit(2)) pending on the file, all outstanding
writes will be discarded.

F_SEAL_FUTURE_WRITE (since Linux 5.1)
The effect of this seal is similar to F_SEAL_WRITE, but the contents of the file can still be
modified via shared writable mappings that were created prior to the seal being set. Any attempt
to create a new writable mapping on the file via mmap(2) will fail with EPERM. Likewise, an
attempt to write to the file via write(2) will fail with EPERM.

Using this seal, one process can create a memory buffer that it can continue to modify while
sharing that buffer on a "read-only" basis with other processes.

RETURN VALUE

       F_GET_SEALS
              A bit mask identifying the seals that have been set for the inode referred to by fd.

       F_ADD_SEALS
              Zero.

       On error, -1 is returned, and errno is set to indicate the error.

ERRORS

       See fcntl(2).

       EBUSY  op  is  F_ADD_SEALS, arg includes F_SEAL_WRITE, and there exists a writable, shared mapping on the
              file referred to by fd.

       EINVAL op is F_ADD_SEALS and arg includes an unrecognized sealing bit.

       EINVAL The filesystem containing the inode referred to by fd does not support sealing.

       EPERM  op was F_ADD_SEALS, but fd was not open for writing or the  current  set  of  seals  on  the  file
              already includes F_SEAL_SEAL.

STANDARDS

       Linux.

HISTORY

       Linux 3.17.