1ARCH_PRCTL(2)              Linux Programmer's Manual             ARCH_PRCTL(2)
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NAME

6       arch_prctl - set architecture-specific thread state
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SYNOPSIS

9       #include <asm/prctl.h>
10       #include <sys/prctl.h>
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12       int arch_prctl(int code, unsigned long addr);
13       int arch_prctl(int code, unsigned long *addr);
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DESCRIPTION

16       arch_prctl()  sets architecture-specific process or thread state.  code
17       selects a subfunction and passes argument addr to it;  addr  is  inter‐
18       preted  as  either  an unsigned long for the "set" operations, or as an
19       unsigned long *, for the "get" operations.
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21       Subfunctions for both x86 and x86-64 are:
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23       ARCH_SET_CPUID (since Linux 4.12)
24              Enable (addr != 0) or disable (addr == 0) the cpuid  instruction
25              for  the calling thread.  The instruction is enabled by default.
26              If disabled, any execution of a cpuid instruction  will  instead
27              generate  a SIGSEGV signal.  This feature can be used to emulate
28              cpuid results that differ  from  what  the  underlying  hardware
29              would have produced (e.g., in a paravirtualization setting).
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31              The  ARCH_SET_CPUID  setting  is  preserved  across  fork(2) and
32              clone(2) but reset to the default (i.e., cpuid enabled)  on  ex‐
33              ecve(2).
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35       ARCH_GET_CPUID (since Linux 4.12)
36              Return  the setting of the flag manipulated by ARCH_SET_CPUID as
37              the result of the system call (1 for enabled, 0  for  disabled).
38              addr is ignored.
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40       Subfunctions for x86-64 only are:
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42       ARCH_SET_FS
43              Set the 64-bit base for the FS register to addr.
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45       ARCH_GET_FS
46              Return  the 64-bit base value for the FS register of the calling
47              thread in the unsigned long pointed to by addr.
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49       ARCH_SET_GS
50              Set the 64-bit base for the GS register to addr.
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52       ARCH_GET_GS
53              Return the 64-bit base value for the GS register of the  calling
54              thread in the unsigned long pointed to by addr.
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RETURN VALUE

57       On success, arch_prctl() returns 0; on error, -1 is returned, and errno
58       is set to indicate the error.
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ERRORS

61       EFAULT addr points to an unmapped address or is outside the process ad‐
62              dress space.
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64       EINVAL code is not a valid subcommand.
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66       EPERM  addr is outside the process address space.
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68       ENODEV ARCH_SET_CPUID  was  requested, but the underlying hardware does
69              not support CPUID faulting.
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CONFORMING TO

72       arch_prctl() is a Linux/x86-64 extension and should not be used in pro‐
73       grams intended to be portable.
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NOTES

76       arch_prctl() is supported only on Linux/x86-64 for 64-bit programs cur‐
77       rently.
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79       The 64-bit base changes when a new 32-bit segment selector is loaded.
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81       ARCH_SET_GS is disabled in some kernels.
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83       Context switches for 64-bit segment bases are rather expensive.  As  an
84       optimization,  if  a  32-bit TLS base address is used, arch_prctl() may
85       use a real TLS entry as if set_thread_area(2) had been called,  instead
86       of  manipulating  the  segment  base  register directly.  Memory in the
87       first 2 GB of address space can be allocated by using mmap(2) with  the
88       MAP_32BIT flag.
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90       Because  of  the  aforementioned  optimization,  using arch_prctl() and
91       set_thread_area(2) in the same thread is dangerous, as they  may  over‐
92       write each other's TLS entries.
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94       As  of  version 2.7, glibc provides no prototype for arch_prctl().  You
95       have to declare it yourself for now.  This may be fixed in future glibc
96       versions.
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98       FS  may  be  already  used by the threading library.  Programs that use
99       ARCH_SET_FS directly are very likely to crash.
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SEE ALSO

102       mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)
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104       AMD X86-64 Programmer's manual
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COLOPHON

107       This page is part of release 5.10 of the Linux  man-pages  project.   A
108       description  of  the project, information about reporting bugs, and the
109       latest    version    of    this    page,    can     be     found     at
110       https://www.kernel.org/doc/man-pages/.
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114Linux                             2020-04-11                     ARCH_PRCTL(2)
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