1ARCH_PRCTL(2) Linux Programmer's Manual ARCH_PRCTL(2)
2
6 arch_prctl - set architecture-specific thread state
7
9 #include <asm/prctl.h>
10 #include <sys/prctl.h>
11 int arch_prctl(int code, unsigned long addr);
13 int arch_prctl(int code, unsigned long *addr);
14
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.
20 Subfunctions for both x86 and x86-64 are:
22 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).
30 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).
34 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.
39 Subfunctions for x86-64 only are:
41 ARCH_SET_FS
43 Set the 64-bit base for the FS register to addr.
44 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.
48 ARCH_SET_GS
50 Set the 64-bit base for the GS register to addr.
51 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.
55
57 On success, arch_prctl() returns 0; on error, -1 is returned, and errno
58 is set to indicate the error.
59
61 EFAULT addr points to an unmapped address or is outside the process ad‐
62 dress space.
63 EINVAL code is not a valid subcommand.
65 EPERM addr is outside the process address space.
67 ENODEV ARCH_SET_CPUID was requested, but the underlying hardware does
69 not support CPUID faulting.
70
72 arch_prctl() is a Linux/x86-64 extension and should not be used in pro‐
73 grams intended to be portable.
74
76 arch_prctl() is supported only on Linux/x86-64 for 64-bit programs cur‐
77 rently.
78 The 64-bit base changes when a new 32-bit segment selector is loaded.
80 ARCH_SET_GS is disabled in some kernels.
82 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.
89 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.
93 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.
97 FS may be already used by the threading library. Programs that use
99 ARCH_SET_FS directly are very likely to crash.
100
102 mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)
103 AMD X86-64 Programmer's manual
105
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/.
111Linux 2020-04-11 ARCH_PRCTL(2)