linux-kernel - Re: [PATCH manpages] modify_ldt.2: Clarify the lm bit's behavior

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Date:	Thu, 05 Feb 2015 08:51:38 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Andy Lutomirski <luto@...capital.net>, linux-kernel@...r.kernel.org
CC:	mtk.manpages@...il.com
Subject: Re: [PATCH manpages] modify_ldt.2: Clarify the lm bit's behavior

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 <luto@...capital.net>
> ---
>  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/
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