PKEY_ALLOC(2)

NAME
pkey_alloc, pkey_free - allocate or free a protection key

SYNOPSIS
#include <sys/mman.h>

int pkey_alloc(unsigned long flags, unsigned long access_rights);
int pkey_free(int pkey);

DESCRIPTION
pkey_alloc() allocates a protection key (pkey) and allows it to be passed to pkey_mprotect(2).

The pkey_alloc() flags is reserved for future use and currently must always be specified as 0.

The pkey_alloc() access_rights argument may contain zero or more disable operations:

PKEY_DISABLE_ACCESS
    Disable all data access to memory covered by the returned protection key.

PKEY_DISABLE_WRITE
    Disable write access to memory covered by the returned protection key.

pkey_free() frees a protection key and makes it available for later allocations. After a protection key has been freed, it may no longer be used in any protection-key-related operations.

An application should not call pkey_free() on any protection key which has been assigned to an address range by pkey_mprotect(2) and which is still in use. The behavior in this case is undefined and may result in an error.

RETURN VALUE
On success, pkey_alloc() returns a positive protection key value. On success, pkey_free() returns zero. On error, -1 is returned, and errno is set appropriately.

ERRORS

EINVAL
    pkey, flags, or access_rights is invalid.

ENOSPC
    (pkey_alloc()) All protection keys available for the current process have been allocated. The number of keys available is architecture-specific and implementation-specific and may be reduced by kernel-internal use of certain keys. There are currently 15 keys available to user programs on x86.

    This error will also be returned if the processor or operating system does not support protection keys. Applications should always be prepared to handle this error, since factors outside of the application's control can reduce the number of available pkeys.

VERSIONS
pkey_alloc() and pkey_free() were added to Linux in kernel 4.9; library support was added in glibc 2.27.

CONFORMING TO
The pkey_alloc() and pkey_free() system calls are Linux-specific.

NOTES
pkey_alloc() is always safe to call regardless of whether or not the operating system supports protection keys. It can be used in lieu of any other mechanism for detecting pkey support and will simply fail with the error ENOSPC if the operating system has no pkey support.

The kernel guarantees that the contents of the hardware rights register (PKRU) will be preserved only for allocated protection keys. Any time a key is unallocated (either before the first call returning that key from pkey_alloc() or after it is freed via pkey_free()), the kernel may make arbitrary changes to the parts of the rights register affecting access to that key.

EXAMPLE
See pkeys(7).

SEE ALSO
pkey_mprotect(2), pkeys(7)

COLOPHON
This page is part of release 4.16 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.