bsd/man/man2/i386_get_ldt.2

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  35 .Dd February 14, 2020
  36 .Dt I386_GET_LDT 2
  37 .Os
  38 .Sh NAME
  39 .Nm i386_get_ldt ,
  40 .Nm i386_set_ldt
  41 .Nd manage i386 per-process Local Descriptor Table entries
  42 .Sh LIBRARY
  43 .Lb libc
  44 .Sh SYNOPSIS
  45 .In architecture/i386/table.h
  46 .In i386/user_ldt.h
  47 .Ft int
  48 .Fn i386_get_ldt "int start_sel" "union ldt_entry *descs" "int num_sels"
  49 .Ft int
  50 .Fn i386_set_ldt "int start_sel" "union ldt_entry *descs" "int num_sels"
  51 .Sh DESCRIPTION
  52 The
  53 .Fn i386_get_ldt
  54 system call
  55 will return the list of i386 descriptors that the process has in its
  56 LDT.
  57 The
  58 .Fn i386_set_ldt
  59 system call
  60 will set a list of i386 descriptors for the current process in its
  61 LDT.
  62 Both routines accept a starting selector number
  63 .Fa start_sel ,
  64 an array of memory that
  65 will contain the descriptors to be set or returned
  66 .Fa descs ,
  67 and the number of entries to set or return
  68 .Fa num_sels .
  69 .Pp
  70 The argument
  71 .Fa descs
  72 can be either code_desc_t, data_desc_t or call_gate_t and are defined in
  73 .In architecture/i386/desc.h .
  74 These structures are defined by the architecture
  75 as disjoint bit-fields, so care must be taken in constructing them.
  76 .Pp
  77 If
  78 .Fa start_sel
  79 is
  80 .Em LDT_AUTO_ALLOC ,
  81 .Fa num_sels
  82 is 1 and the descriptor pointed to by
  83 .Fa descs
  84 is legal, then
  85 .Fn i386_set_ldt
  86 will allocate a descriptor and return its
  87 selector number.
  88 .Pp
  89 If
  90 .Fa num_descs
  91 is 1,
  92 .Fa start_sels
  93 is valid, and
  94 .Fa descs
  95 is NULL, then
  96 .Fn i386_set_ldt
  97 will free that descriptor
  98 (making it available to be reallocated again later).
  99 .Pp
 100 If
 101 .Fa num_descs
 102 is 0,
 103 .Fa start_sels
 104 is 0 and
 105 .Fa descs
 106 is NULL then, as a special case,
 107 .Fn i386_set_ldt
 108 will free all descriptors.
 109 .Sh NOTES
 110 .Pp
 111 .Fn i386_set_ldt
 112 and
 113 .Fn i386_get_ldt
 114 may be used by 64-bit processes to create 32-bit (compatibility mode) code segments
 115 (in addition to the set of other segments already specified), that, together with
 116 additional infrastructure not provided by macOS, enables 32-bit code execution.
 117 Some platforms may reject segments with non-zero base addresses by returning -1 and
 118 setting errno to EINVAL.
 119 .Sh RETURN VALUES
 120 Upon successful completion,
 121 .Fn i386_get_ldt
 122 returns the number of descriptors currently in the LDT.
 123 The
 124 .Fn i386_set_ldt
 125 system call
 126 returns the first selector set.
 127 In the case when a descriptor is allocated by the kernel, its number will
 128 be returned.
 129 Otherwise, a value of -1 is returned and the global
 130 variable
 131 .Va errno
 132 is set to indicate the error.
 133 .Sh ERRORS
 134 The
 135 .Fn i386_get_ldt
 136 and
 137 .Fn i386_set_ldt
 138 system calls
 139 will fail if:
 140 .Bl -tag -width Er
 141 .It Bq Er EINVAL
 142 An inappropriate value was used for
 143 .Fa start_sel
 144 or
 145 .Fa num_sels ,
 146 or the platform does not support non-zero base addresses in custom descriptors and the descriptor base address passed to
 147 .Fn i386_set_ldt
 148 is non-zero.
 149 .It Bq Er EACCES
 150 The caller attempted to use a descriptor that would
 151 circumvent protection or cause a failure.
 152 .El
 153 .Sh SEE ALSO
 154 i386 Microprocessor Programmer's Reference Manual, Intel
 155 .Sh WARNING
 156 You can really hose your process using this.