[PATCH] ptrace: Fix EFL_OFFSET value according to i386 pda changes

The PDA patches introduced a bug in ptrace: it reads eflags from the wrong
place on the target's stack, but writes it back to the correct place.  The
result is a corrupted eflags, which is most visible when it turns interrupts
off unexpectedly.

This patch fixes this by making the ptrace code a little less fragile.  It
changes [gs]et_stack_long to take a straightforward byte offset into struct
pt_regs, rather than requiring all callers to do a sizeof(struct pt_regs)
offset adjustment.  This means that the eflag's offset (EFL_OFFSET) on the
target stack can be simply computed with offsetof().

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Frederik Deweerdt <deweerdt@free.fr>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/ptrace.c

@@ -45,7 +45,7 @@
 /*
  * Offset of eflags on child stack..
  */
-#define EFL_OFFSET ((EFL-2)*4-sizeof(struct pt_regs))
+#define EFL_OFFSET offsetof(struct pt_regs, eflags)
 
 static inline struct pt_regs *get_child_regs(struct task_struct *task)
 {
@@ -54,24 +54,24 @@ static inline struct pt_regs *get_child_regs(struct task_struct *task)
 }
 
 /*
- * this routine will get a word off of the processes privileged stack. 
- * the offset is how far from the base addr as stored in the TSS.  
- * this routine assumes that all the privileged stacks are in our
+ * This routine will get a word off of the processes privileged stack.
+ * the offset is bytes into the pt_regs structure on the stack.
+ * This routine assumes that all the privileged stacks are in our
  * data space.
  */   
 static inline int get_stack_long(struct task_struct *task, int offset)
 {
 	unsigned char *stack;
 
-	stack = (unsigned char *)task->thread.esp0;
+	stack = (unsigned char *)task->thread.esp0 - sizeof(struct pt_regs);
 	stack += offset;
 	return (*((int *)stack));
 }
 
 /*
- * this routine will put a word on the processes privileged stack. 
- * the offset is how far from the base addr as stored in the TSS.  
- * this routine assumes that all the privileged stacks are in our
+ * This routine will put a word on the processes privileged stack.
+ * the offset is bytes into the pt_regs structure on the stack.
+ * This routine assumes that all the privileged stacks are in our
  * data space.
  */
 static inline int put_stack_long(struct task_struct *task, int offset,
@@ -79,7 +79,7 @@ static inline int put_stack_long(struct task_struct *task, int offset,
 {
 	unsigned char * stack;
 
-	stack = (unsigned char *) task->thread.esp0;
+	stack = (unsigned char *)task->thread.esp0 - sizeof(struct pt_regs);
 	stack += offset;
 	*(unsigned long *) stack = data;
 	return 0;
@@ -114,7 +114,7 @@ static int putreg(struct task_struct *child,
 	}
 	if (regno > ES*4)
 		regno -= 1*4;
-	put_stack_long(child, regno - sizeof(struct pt_regs), value);
+	put_stack_long(child, regno, value);
 	return 0;
 }
 
@@ -137,7 +137,6 @@ static unsigned long getreg(struct task_struct *child,
 		default:
 			if (regno > ES*4)
 				regno -= 1*4;
-			regno = regno - sizeof(struct pt_regs);
 			retval &= get_stack_long(child, regno);
 	}
 	return retval;