x86: Make support for 16-bit segments a config option
Embedded systems and the like are likely never going to experience
16-bit software, so make it an option to reject them entirely and not
carry any of the overhead.
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 25d2c6f..f1da8b1 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -914,6 +914,14 @@
XFree86 to initialize some video cards via BIOS. Disabling this
option saves about 6k.
+config X86_16BIT
+ bool "Enable 16-bit segments" if EXPERT
+ default y
+ ---help---
+ This option enables the execution of 16-bit protected mode
+ software. This is primarily used to run Win16 software
+ under Wine and 16-bit extended DOS programs under DOSEMU.
+
config TOSHIBA
tristate "Toshiba Laptop support"
depends on X86_32
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 2780b8f..30dd6e5 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -527,6 +527,7 @@
restore_all:
TRACE_IRQS_IRET
restore_all_notrace:
+#ifdef CONFIG_X86_16BIT
movl PT_EFLAGS(%esp), %eax # mix EFLAGS, SS and CS
# Warning: PT_OLDSS(%esp) contains the wrong/random values if we
# are returning to the kernel.
@@ -537,6 +538,7 @@
cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
CFI_REMEMBER_STATE
je ldt_ss # returning to user-space with LDT SS
+#endif
restore_nocheck:
RESTORE_REGS 4 # skip orig_eax/error_code
irq_return:
@@ -549,6 +551,7 @@
.previous
_ASM_EXTABLE(irq_return,iret_exc)
+#ifdef CONFIG_X86_16BIT
CFI_RESTORE_STATE
ldt_ss:
#ifdef CONFIG_PARAVIRT
@@ -592,6 +595,7 @@
lss (%esp), %esp /* switch to espfix segment */
CFI_ADJUST_CFA_OFFSET -8
jmp restore_nocheck
+#endif /* CONFIG_X86_16BIT */
CFI_ENDPROC
ENDPROC(system_call)
@@ -692,6 +696,7 @@
.popsection
.macro FIXUP_ESPFIX_STACK
+#ifdef CONFIG_X86_16BIT
/*
* Switch back for ESPFIX stack to the normal zerobased stack
*
@@ -708,8 +713,10 @@
pushl_cfi %eax
lss (%esp), %esp /* switch to the normal stack segment */
CFI_ADJUST_CFA_OFFSET -8
+#endif
.endm
.macro UNWIND_ESPFIX_STACK
+#ifdef CONFIG_X86_16BIT
movl %ss, %eax
/* see if on espfix stack */
cmpw $__ESPFIX_SS, %ax
@@ -720,6 +727,7 @@
/* switch to normal stack */
FIXUP_ESPFIX_STACK
27:
+#endif
.endm
/*
@@ -1350,11 +1358,13 @@
ENTRY(nmi)
RING0_INT_FRAME
ASM_CLAC
+#ifdef CONFIG_X86_16BIT
pushl_cfi %eax
movl %ss, %eax
cmpw $__ESPFIX_SS, %ax
popl_cfi %eax
je nmi_espfix_stack
+#endif
cmpl $ia32_sysenter_target,(%esp)
je nmi_stack_fixup
pushl_cfi %eax
@@ -1394,6 +1404,7 @@
FIX_STACK 24, nmi_stack_correct, 1
jmp nmi_stack_correct
+#ifdef CONFIG_X86_16BIT
nmi_espfix_stack:
/* We have a RING0_INT_FRAME here.
*
@@ -1415,6 +1426,8 @@
lss 12+4(%esp), %esp # back to espfix stack
CFI_ADJUST_CFA_OFFSET -24
jmp irq_return
+#endif /* CONFIG_X86_16BIT */
+
CFI_ENDPROC
END(nmi)
diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index af1d14a..cc34ae8 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -229,11 +229,7 @@
}
}
- /*
- * On x86-64 we do not support 16-bit segments due to
- * IRET leaking the high bits of the kernel stack address.
- */
-#ifdef CONFIG_X86_64
+#ifndef CONFIG_X86_16BIT
if (!ldt_info.seg_32bit) {
error = -EINVAL;
goto out_unlock;
