man2/pkey_alloc.2

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  25 .TH PKEY_ALLOC 2 2018-02-02 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 pkey_alloc, pkey_free \- allocate or free a protection key
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <sys/mman.h>
  31 .PP
  32 .BI "int pkey_alloc(unsigned int " flags ", unsigned int " access_rights ");"
  33 .BI "int pkey_free(int " pkey ");"
  34 .fi
  35 .SH DESCRIPTION
  36 .BR pkey_alloc ()
  37 allocates a protection key (pkey) and allows it to be passed to
  38 .BR pkey_mprotect (2).
  39 .PP
  40 The
  41 .BR pkey_alloc ()
  42 .I flags
  43 is reserved for future use and currently must always be specified as 0.
  44 .PP
  45 The
  46 .BR pkey_alloc ()
  47 .I access_rights
  48 argument may contain zero or more disable operations:
  49 .TP
  50 .B PKEY_DISABLE_ACCESS
  51 Disable all data access to memory covered by the returned protection key.
  52 .TP
  53 .B PKEY_DISABLE_WRITE
  54 Disable write access to memory covered by the returned protection key.
  55 .PP
  56 .BR pkey_free ()
  57 frees a protection key and makes it available for later
  58 allocations.
  59 After a protection key has been freed, it may no longer be used
  60 in any protection-key-related operations.
  61 .PP
  62 An application should not call
  63 .BR pkey_free ()
  64 on any protection key which has been assigned to an address
  65 range by
  66 .BR pkey_mprotect (2)
  67 and which is still in use.
  68 The behavior in this case is undefined and may result in an error.
  69 .SH RETURN VALUE
  70 On success,
  71 .BR pkey_alloc ()
  72 returns a positive protection key value.
  73 On success,
  74 .BR pkey_free ()
  75 returns zero.
  76 On error, \-1 is returned, and
  77 .I errno
  78 is set appropriately.
  79 .SH ERRORS
  80 .TP
  81 .B EINVAL
  82 .IR pkey ,
  83 .IR flags ,
  84 or
  85 .I access_rights
  86 is invalid.
  87 .TP
  88 .B ENOSPC
  89 .RB ( pkey_alloc ())
  90 All protection keys available for the current process have
  91 been allocated.
  92 The number of keys available is architecture-specific and
  93 implementation-specific and may be reduced by kernel-internal use
  94 of certain keys.
  95 There are currently 15 keys available to user programs on x86.
  96 .IP
  97 This error will also be returned if the processor or operating system
  98 does not support protection keys.
  99 Applications should always be prepared to handle this error, since
 100 factors outside of the application's control can reduce the number
 101 of available pkeys.
 102 .SH VERSIONS
 103 .BR pkey_alloc ()
 104 and
 105 .BR pkey_free ()
 106 were added to Linux in kernel 4.9;
 107 library support was added in glibc 2.27.
 108 .SH CONFORMING TO
 109 The
 110 .BR pkey_alloc ()
 111 and
 112 .BR pkey_free ()
 113 system calls are Linux-specific.
 114 .SH NOTES
 115 .BR pkey_alloc ()
 116 is always safe to call regardless of whether or not the operating system
 117 supports protection keys.
 118 It can be used in lieu of any other mechanism for detecting pkey support
 119 and will simply fail with the error
 120 .B ENOSPC
 121 if the operating system has no pkey support.
 122 .PP
 123 The kernel guarantees that the contents of the hardware rights
 124 register (PKRU) will be preserved only for allocated protection
 125 keys.
 126 Any time a key is unallocated (either before the first call
 127 returning that key from
 128 .BR pkey_alloc ()
 129 or after it is freed via
 130 .BR pkey_free ()),
 131 the kernel may make arbitrary changes to the parts of the
 132 rights register affecting access to that key.
 133 .SH EXAMPLE
 134 See
 135 .BR pkeys (7).
 136 .SH SEE ALSO
 137 .BR pkey_mprotect (2),
 138 .BR pkeys (7)