Re: [PATCH v3 2/3] x86/ldt: Make modify_ldt optional

["Jan Beulich" <JBeulich@suse.com>]

>>> On 22.07.15 at 21:23, <luto@kernel.org> wrote:
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1015,6 +1015,7 @@ config VM86
>  config X86_16BIT
>  	bool "Enable support for 16-bit segments" if EXPERT
>  	default y
> +	depends on MODIFY_LDT_SYSCALL
>  	---help---
>  	  This option is required by programs like Wine to run 16-bit
>  	  protected mode legacy code on x86 processors.  Disabling
> @@ -2053,6 +2054,22 @@ config CMDLINE_OVERRIDE
>  	  This is used to work around broken boot loaders.  This should
>  	  be set to 'N' under normal conditions.
>  
> +config MODIFY_LDT_SYSCALL
> +       bool "Enable the LDT (local descriptor table)" if EXPERT
> +       default y
> +       ---help---
> +         Linux can allow user programs to install a per-process x86
> +	 Local Descriptor Table (LDT) using the modify_ldt(2) system
> +	 call.  This is required to run 16-bit or segmented code such as
> +	 DOSEMU or some Wine programs.  It is also used by some very old
> +	 threading libraries.
> +
> +	 Enabling this feature adds a small amount of overhead to
> +	 context switches and increases the low-level kernel attack
> +	 surface.  Disabling it removes the modify_ldt(2) system call.
> +
> +	 Saying 'N' here may make sense for embedded or server kernels.
> +

I think it would be better to place this ahead of the one being
made dependent on it, to avoid the user being prompted for
X86_16BIT despite it possibly becoming unavailable (when this
one gets set to n).

Jan