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modify_ldt.2: Minor tweaks to Andy Lutomirski's patch 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2015-01-30 13:06:08 +01:00
parent 14620a2531
commit 7acf749e4f
1 changed files with 20 additions and 17 deletions
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37
man2/modify_ldt.2
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@ -29,14 +29,15 @@ modify_ldt \- get or set a per-process LDT entry
.nf
.B #include <sys/types.h>
.sp
.BI "int modify_ldt(int " "func" ", void *" "ptr" ", unsigned long " "bytecount" );
.BI "int modify_ldt(int " func ", void *" ptr ", unsigned long " bytecount );
.fi
.PP
.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
.BR modify_ldt ()
reads or writes the local descriptor table (LDT) for a process. The 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
@ -51,7 +52,8 @@ reads the LDT into the memory pointed to by
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,
the LDT is padded with additional trailing zero bytes.
On success,
.BR modify_ldt ()
will return the number of bytes read.
.PP
@ -60,7 +62,7 @@ When
is 1 or 0x11,
.BR modify_ldt ()
modifies the LDT entry indicated by
.I ptr\->entry_number.
.IR ptr\->entry_number .
.I ptr
points to a
.I user_desc
@ -95,8 +97,8 @@ In Linux 2.4 and earlier, this structure was named
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
conforming code).
The other fields match their descriptions in the CPU manual, although
.BR modify_ldt ()
cannot set the accessed bit.
.PP
@ -128,7 +130,8 @@ When
.I func
is 2,
.BR modify_ldt ()
will read zeros. This appears to be a leftover from Linux 2.4.
will read zeros.
This appears to be a leftover from Linux 2.4.
.SH RETURN VALUE
On success,
.BR modify_ldt ()
@ -156,7 +159,7 @@ 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.
is 1 or 0x11 and the new LDT entry has invalid values.
.TP
.B ENOSYS
.I func
@ -164,18 +167,14 @@ is neither 0, 1, 2, nor 0x11.
.SH CONFORMING TO
This call is Linux-specific and should not be used in programs intended
to be portable.
.SH BUGS
On 64-bit kernels before Linux 3.19, one of the padding bits in
.I user_desc,
if set, would prevent the descriptor from being considered empty.
.SH NOTES
Glibc does not provide a wrapper for this system call; call it using
.BR syscall (2).
.PP
.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
switches and only supports a limited number of threads.
Threading libraries should use
.BR set_thread_area (2)
or
.BR arch_prctl (2)
@ -184,9 +183,13 @@ calls.
.PP
The normal use for
.BR modify_ldt (2)
is to run legacy 16-bit or segmented 32-bit code. Not all kernels allow
16-bit segments to be installed, however.
is to run legacy 16-bit or segmented 32-bit code.
Not all kernels allow 16-bit segments to be installed, however.
.SH BUGS
On 64-bit kernels before Linux 3.19, one of the padding bits in
.IR user_desc ,
if set, would prevent the descriptor from being considered empty.
.SH SEE ALSO
.BR set_thread_area (2),
.BR arch_prctl (2),
.BR set_thread_area (2),
.BR vm86 (2)