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



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