Re: [-mm patch] ptrace: Fix EFL_OFFSET value according to i386 pda changes (was Re: BUG on 2.6.20-rc1 when using gdb)

[Jeremy Fitzhardinge <jeremy@goop.org>]

Frederik Deweerdt wrote:
> Same problems here with 2.6.20-rc1-mm1 (ie with the %gs->%fs patch).
> It seems to me that the problem comes from the EFL_OFFSET no longer
> beeing accurate.
> The following patch fixes the problem for me.
>   

Thanks Frederik; that's exactly the kind of thing I thought it might
be.  I wonder if there's some way we can make this more robust
though...  Does this work for you?  I did a slightly larger cleanup
which should make it less fragile and more comprehensible.

    J

diff -r e775f6e42258 arch/i386/kernel/ptrace.c
--- a/arch/i386/kernel/ptrace.c	Tue Dec 19 10:32:40 2006 -0800
+++ b/arch/i386/kernel/ptrace.c	Wed Dec 20 11:18:56 2006 -0800
@@ -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_
 }
 
 /*
- * 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 
 {
 	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 *ch
 	}
 	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_
 		default:
 			if (regno > ES*4)
 				regno -= 1*4;
-			regno = regno - sizeof(struct pt_regs);
 			retval &= get_stack_long(child, regno);
 	}
 	return retval;