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Diffstat (limited to 'man2/modify_ldt.2')
| -rw-r--r-- | man2/modify_ldt.2 | 196 |
1 files changed, 0 insertions, 196 deletions
diff --git a/man2/modify_ldt.2 b/man2/modify_ldt.2 deleted file mode 100644 index 07f3cddb0..000000000 --- a/man2/modify_ldt.2 +++ /dev/null @@ -1,196 +0,0 @@ -.\" Copyright (c) 1995 Michael Chastain (mec@duracef.shout.net), 22 July 1995. -.\" Copyright (c) 2015 Andrew Lutomirski -.\" -.\" SPDX-License-Identifier: GPL-2.0-or-later -.\" -.TH modify_ldt 2 (date) "Linux man-pages (unreleased)" -.SH NAME -modify_ldt \- get or set a per-process LDT entry -.SH LIBRARY -Standard C library -.RI ( libc ", " \-lc ) -.SH SYNOPSIS -.nf -.BR "#include <asm/ldt.h>" " /* Definition of " "struct user_desc" " */" -.BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */" -.B #include <unistd.h> -.PP -.BI "int syscall(SYS_modify_ldt, int " func ", void " ptr [. bytecount ], -.BI " unsigned long " bytecount ); -.fi -.PP -.IR Note : -glibc provides no wrapper for -.BR modify_ldt (), -necessitating the use of -.BR syscall (2). -.SH DESCRIPTION -.BR modify_ldt () -reads or writes the local descriptor table (LDT) for a process. -The LDT -is an array of segment descriptors that can be referenced by user code. -Linux allows processes to configure a per-process (actually per-mm) LDT. -For more information about the LDT, see the Intel Software Developer's -Manual or the AMD Architecture Programming Manual. -.PP -When -.I func -is 0, -.BR modify_ldt () -reads the LDT into the memory pointed to by -.IR ptr . -The number of bytes read is the smaller of -.I bytecount -and the actual size of the LDT, although the kernel may act as though -the LDT is padded with additional trailing zero bytes. -On success, -.BR modify_ldt () -will return the number of bytes read. -.PP -When -.I func -is 1 or 0x11, -.BR modify_ldt () -modifies the LDT entry indicated by -.IR ptr\->entry_number . -.I ptr -points to a -.I user_desc -structure -and -.I bytecount -must equal the size of this structure. -.PP -The -.I user_desc -structure is defined in \fI<asm/ldt.h>\fP as: -.PP -.in +4n -.EX -struct user_desc { - unsigned int entry_number; - unsigned int base_addr; - unsigned int limit; - unsigned int seg_32bit:1; - unsigned int contents:2; - unsigned int read_exec_only:1; - unsigned int limit_in_pages:1; - unsigned int seg_not_present:1; - unsigned int useable:1; -}; -.EE -.in -.PP -In Linux 2.4 and earlier, this structure was named -.IR modify_ldt_ldt_s . -.PP -The -.I contents -field is the segment type (data, expand-down data, non-conforming code, or -conforming code). -The other fields match their descriptions in the CPU manual, although -.BR modify_ldt () -cannot set the hardware-defined "accessed" bit described in the CPU manual. -.PP -A -.I user_desc -is considered "empty" if -.I read_exec_only -and -.I seg_not_present -are set to 1 and all of the other fields are 0. -An LDT entry can be cleared by setting it to an "empty" -.I user_desc -or, if -.I func -is 1, by setting both -.I base -and -.I limit -to 0. -.PP -A conforming code segment (i.e., one with -.IR contents==3 ) -will be rejected if -.I -func -is 1 or if -.I seg_not_present -is 0. -.PP -When -.I func -is 2, -.BR modify_ldt () -will read zeros. -This appears to be a leftover from Linux 2.4. -.SH RETURN VALUE -On success, -.BR modify_ldt () -returns either the actual number of bytes read (for reading) -or 0 (for writing). -On failure, -.BR modify_ldt () -returns \-1 and sets -.I errno -to indicate the error. -.SH ERRORS -.TP -.B EFAULT -.I ptr -points outside the address space. -.TP -.B EINVAL -.I ptr -is 0, -or -.I func -is 1 and -.I bytecount -is not equal to the size of the structure -.IR user_desc , -or -.I func -is 1 or 0x11 and the new LDT entry has invalid values. -.TP -.B ENOSYS -.I func -is neither 0, 1, 2, nor 0x11. -.SH STANDARDS -Linux. -.SH NOTES -.BR modify_ldt () -should not be used for thread-local storage, as it slows down context -switches and only supports a limited number of threads. -Threading libraries should use -.BR set_thread_area (2) -or -.BR arch_prctl (2) -instead, except on extremely old kernels that do not support those system -calls. -.PP -The normal use for -.BR modify_ldt () -is to run legacy 16-bit or segmented 32-bit code. -Not all kernels allow 16-bit segments to be installed, however. -.PP -Even on 64-bit kernels, -.BR modify_ldt () -cannot be used to create a long mode (i.e., 64-bit) code segment. -The undocumented field "lm" in -.I user_desc -is not useful, and, despite its name, -does not result in a long mode segment. -.SH BUGS -On 64-bit kernels before Linux 3.19, -.\" commit e30ab185c490e9a9381385529e0fd32f0a399495 -setting the "lm" bit in -.I user_desc -prevents the descriptor from being considered empty. -Keep in mind that the -"lm" bit does not exist in the 32-bit headers, but these buggy kernels -will still notice the bit even when set in a 32-bit process. -.SH SEE ALSO -.BR arch_prctl (2), -.BR set_thread_area (2), -.BR vm86 (2) |