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

[Frederik Deweerdt <deweerdt@free.fr>]

On Wed, Dec 20, 2006 at 03:21:53AM -0800, Jeremy Fitzhardinge wrote:
> "walt" <w41ter@gmail.com> reported a similar problem which he bisected
> down to the PDA changeset which touches ptrace
> (66e10a44d724f1464b5e8b5a3eae1e2cbbc2cca6).  I haven't managed to repo
> the problem, but I guess there's something nasty going on in ptrace -
> maybe its screwing up eflags on the stack or something.  Need to
> double-check all the conversions from kernel<->usermode registers.  Hm,
> wonder if its fixed with the %gs->%fs conversion patch applied?
> 
Hi Jeremy,

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.

Regards,
Frederik

Signed-off-by: Frederik Deweerdt <frederik.deweerdt@gmail.com>

diff --git a/arch/i386/kernel/ptrace.c b/arch/i386/kernel/ptrace.c
index 7f7d830..00d8a5a 100644
--- a/arch/i386/kernel/ptrace.c
+++ b/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 ((EFL-1)*4-sizeof(struct pt_regs))
 
 static inline struct pt_regs *get_child_regs(struct task_struct *task)
 {