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.TH PKEY_ALLOC 2 2018-02-02 "Linux" "Linux Programmer's Manual"

.SH NAME

pkey_alloc, pkey_free \- allocate or free a protection key

.SH SYNOPSIS

.nf

.B #include <sys/mman.h>

.PP

.BI "int pkey_alloc(unsigned long " flags ", unsigned long " access_rights ");"

.BI "int pkey_free(int " pkey ");"

.fi

.SH DESCRIPTION

.BR pkey_alloc ()

allocates a protection key (pkey) and allows it to be passed to

.BR pkey_mprotect (2).

.PP

The

.BR pkey_alloc ()

.I flags

is reserved for future use and currently must always be specified as 0.

.PP

The

.BR pkey_alloc ()

.I access_rights

.BR

argument may contain zero or more disable operations:

.TP

.B PKEY_DISABLE_ACCESS

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

.TP

.B PKEY_DISABLE_WRITE

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

.PP

.BR 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.

.PP

An application should not call

.BR pkey_free ()

on any protection key which has been assigned to an address

range by

.BR pkey_mprotect (2)

and which is still in use.

The behavior in this case is undefined and may result in an error.

.SH RETURN VALUE

On success,

.BR pkey_alloc ()

returns a positive protection key value.

On success,

.BR pkey_free ()

returns zero.

On error, \-1 is returned, and

.I errno

is set appropriately.

.SH ERRORS

.TP

.B EINVAL

.IR pkey ,

.IR flags ,

or

.I access_rights

is invalid.

.TP

.B ENOSPC

.RB ( 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.

.IP

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.

.SH VERSIONS

.BR pkey_alloc ()

and

.BR pkey_free ()

were added to Linux in kernel 4.9;

library support was added in glibc 2.27.

.SH CONFORMING TO

The

.BR pkey_alloc ()

and

.BR pkey_free ()

system calls are Linux-specific.

.SH NOTES

.BR 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

.B ENOSPC

if the operating system has no pkey support.

.PP

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

.BR pkey_alloc ()

or after it is freed via

.BR pkey_free ()),

the kernel may make arbitrary changes to the parts of the

rights register affecting access to that key.

.SH EXAMPLE

See

.BR pkeys (7).

.SH SEE ALSO

.BR pkey_mprotect (2),

.BR pkeys (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.