Virtualization - Apr 2007 - [PATCH 6/14] i386 / Add some segment convenience functions

Add some convenient segment macros to the kernel.  This makes the
rather obfuscated 'seg & 4' go away.

Patch-keys: i386 segment cleanup
Patch-base: 2.6.13-rc5-mm1
Signed-off-by: Zachary Amsden <zach@vmware.com>
Index: linux-2.6.13/include/asm-i386/segment.h
==================================================================---
linux-2.6.13.orig/include/asm-i386/segment.h	2005-08-09 19:36:36.000000000 -0700
+++ linux-2.6.13/include/asm-i386/segment.h	2005-08-09 19:43:47.000000000 -0700
@@ -98,4 +98,14 @@
  */
 #define IDT_ENTRIES 256
 
+/*
+ * This bit is set to indicate segment selectors are in the LDT
+ */
+#define LDT_SEGMENT 4
+
+#ifndef __ASSEMBLY__
+#define segment_index(seg) ((seg) >> 3)
+#define segment_from_ldt(seg) ((seg) & LDT_SEGMENT)
+#endif
+
 #endif
Index: linux-2.6.13/arch/i386/kernel/ptrace.c
==================================================================---
linux-2.6.13.orig/arch/i386/kernel/ptrace.c	2005-08-09 19:36:36.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/ptrace.c	2005-08-10 20:40:51.000000000 -0700
@@ -146,8 +146,6 @@
 	return retval;
 }
 
-#define LDT_SEGMENT 4
-
 static unsigned long convert_eip_to_linear(struct task_struct *child, struct
pt_regs *regs)
 {
 	unsigned long addr, seg;
@@ -165,7 +163,7 @@
 	 * TLS segments are used for data, and the PNPBIOS
 	 * and APM bios ones we just ignore here.
 	 */
-	if (seg & LDT_SEGMENT) {
+	if (segment_from_ldt(seg)) {
 		u32 *desc;
 		unsigned long base;
 
Index: linux-2.6.13/arch/i386/kernel/kprobes.c
==================================================================---
linux-2.6.13.orig/arch/i386/kernel/kprobes.c	2005-08-09 19:36:36.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/kprobes.c	2005-08-10 20:42:20.000000000 -0700
@@ -163,8 +163,8 @@
 	/* Check if the application is using LDT entry for its code segment and
 	 * calculate the address by reading the base address from the LDT entry.
 	 */
-	if ((regs->xcs & 4) && (current->mm)) {
-		lp = (unsigned long *) ((unsigned long)((regs->xcs >> 3) * 8)
+	if (segment_from_ldt(regs->xcs) && (current->mm)) {
+		lp = (unsigned long *) ((unsigned long)(segment_index(regs->xcs) * 8)
 					+ (char *) current->mm->context.ldt);
 		addr = (kprobe_opcode_t *) (get_desc_base(lp) + regs->eip -
 						sizeof(kprobe_opcode_t));
Index: linux-2.6.13/arch/i386/kernel/entry.S
==================================================================---
linux-2.6.13.orig/arch/i386/kernel/entry.S	2005-08-09 19:36:36.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/entry.S	2005-08-09 19:43:47.000000000 -0700
@@ -268,8 +268,8 @@
 	# See comments in process.c:copy_thread() for details.
 	movb OLDSS(%esp), %ah
 	movb CS(%esp), %al
-	andl $(VM_MASK | (4 << 8) | 3), %eax
-	cmpl $((4 << 8) | 3), %eax
+	andl $(VM_MASK | (LDT_SEGMENT << 8) | 3), %eax
+	cmpl $((LDT_SEGMENT << 8) | 3), %eax
 	je ldt_ss			# returning to user-space with LDT SS
 restore_nocheck:
 	RESTORE_REGS
Index: linux-2.6.13/arch/i386/mm/fault.c
==================================================================---
linux-2.6.13.orig/arch/i386/mm/fault.c	2005-08-09 19:43:38.000000000 -0700
+++ linux-2.6.13/arch/i386/mm/fault.c	2005-08-10 20:42:20.000000000 -0700
@@ -101,7 +101,7 @@
 	/* Get the GDT/LDT descriptor base. 
 	   When you look for races in this code remember that
 	   LDT and other horrors are only used in user space. */
-	if (seg & (1<<2)) {
+	if (segment_from_ldt(seg)) {
 		/* Must lock the LDT while reading it. */
 		down(&current->mm->context.sem);
 		desc = current->mm->context.ldt;
@@ -115,7 +115,7 @@
 	/* Decode the code segment base from the descriptor */
 	base = get_desc_base((unsigned long *)desc);
 
-	if (seg & (1<<2)) { 
+	if (segment_from_ldt(seg)) { 
 		up(&current->mm->context.sem);
 	} else
 		put_cpu();

On Wed, Aug 10, 2005 at 09:54:31PM -0700, zach@vmware.com
wrote:
> Add some convenient segment macros to the kernel.  This makes the
> rather obfuscated 'seg & 4' go away.
segment_from_ldt is a weird name for this. I wouldn't guess it to 
be a test for something. How about is_ldt_segment() instead? 

-Andi