The lm bit should never have existed in the first place. Sigh.
Signed-off-by: Andy Lutomirski <[email protected]>
---
man2/modify_ldt.2 | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
diff --git a/man2/modify_ldt.2 b/man2/modify_ldt.2
index 1c2cf9cdef93..d6e0514973a6 100644
--- a/man2/modify_ldt.2
+++ b/man2/modify_ldt.2
@@ -187,12 +187,21 @@ 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.
+.PP
+Even on 64-bit kernels,
+.BR modify_ldt (2)
+cannot be used to create a long mode (i.e. 64-bit) code segment. The
+undocumented field "lm" in
+.IR 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" padding bit in
+setting the "lm" bit in
.IR user_desc
-prevents the descriptor from being considered empty.
+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),
--
2.1.0
On 02/04/2015 10:53 PM, Andy Lutomirski wrote:
> The lm bit should never have existed in the first place. Sigh.
Thanks, Andy. Applied in the branch.
Cheers,
Michael
> Signed-off-by: Andy Lutomirski <[email protected]>
> ---
> man2/modify_ldt.2 | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/man2/modify_ldt.2 b/man2/modify_ldt.2
> index 1c2cf9cdef93..d6e0514973a6 100644
> --- a/man2/modify_ldt.2
> +++ b/man2/modify_ldt.2
> @@ -187,12 +187,21 @@ 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.
> +.PP
> +Even on 64-bit kernels,
> +.BR modify_ldt (2)
> +cannot be used to create a long mode (i.e. 64-bit) code segment. The
> +undocumented field "lm" in
> +.IR 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" padding bit in
> +setting the "lm" bit in
> .IR user_desc
> -prevents the descriptor from being considered empty.
> +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),
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/