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i386_get_ldt, i386_set_ldt - manage i386 per-process Local Descriptor
Standard C Library (libc, -lc)
i386_get_ldt(int start_sel, union descriptor *descs, int num_sels);
i386_set_ldt(int start_sel, union descriptor *descs, int num_sels);
The i386_get_ldt() system call returns a list of the i386 descriptors in
the current process’ LDT. The i386_set_ldt() system call sets a list of
i386 descriptors in the current process’ LDT. For both routines,
start_sel specifies the index of the selector in the LDT at which to
begin and descs points to an array of num_sels descriptors to be set or
Each entry in the descs array can be either a segment_descriptor or
gate_descriptor and are defined in These structures are defined by the
architecture as disjoint bit-fields, so care must be taken in
If start_sel is LDT_AUTO_ALLOC, num_sels is 1 and the descriptor pointed
to by descs is legal, then i386_set_ldt() will allocate a descriptor and
return its selector number.
If num_descs is 1, start_sels is valid, and descs is NULL, then
i386_set_ldt() will free that descriptor (making it available to be
reallocated again later).
If num_descs is 0, start_sels is 0 and descs is NULL then, as a special
case, i386_set_ldt() will free all descriptors.
Upon successful completion, i386_get_ldt() returns the number of
descriptors currently in the LDT. The i386_set_ldt() system call returns
the first selector set on success. If the kernel allocated a descriptor
in the LDT, the allocated index is returned. Otherwise, a value of -1 is
returned and the global variable errno is set to indicate the error.
The i386_get_ldt() and i386_set_ldt() system calls will fail if:
[EINVAL] An inappropriate value was used for start_sel or
[EACCES] The caller attempted to use a descriptor that would
circumvent protection or cause a failure.
i386 Microprocessor Programmer’s Reference Manual, Intel
You can really hose your process using this.