Re: [patch] x86_64: fix is_at_popf() for compat tasks

In-Reply-To: <200607311054.38585.ak@xxxxxxx>

On Mon, 31 Jul 2006 10:54:38 +0200, Andi Kleen wrote:

When testing for the REX instruction prefix, first check
for a 32-bit task because in compat mode the REX prefix is an
increment instruction.

is_compat_task doesn't actually say that a task is in compat mode
(it refers to the Linux compat layer, not x86-64 compat mode)

A better test would be regs->cs == __USER32_CS, but in theory
there could be other code segments in LDT. I guess that can
be ignored though.

How about checking for regs->cs != __USER_CS instead? In 64-bit mode
a program shouldn't have any other value there while in 32-bit mode
it could be using LDT segments.



From: Chuck Ebbert <76306.1226@xxxxxxxxxxxxxx>

When testing for the REX instruction prefix, first check
for 32-bit mode because in compat mode the REX prefix is an
increment instruction.

Signed-off-by: Chuck Ebbert <76306.1226@xxxxxxxxxxxxxx>

--- 2.6.18-rc2-64.orig/arch/x86_64/kernel/ptrace.c
+++ 2.6.18-rc2-64/arch/x86_64/kernel/ptrace.c
@@ -141,8 +141,11 @@ static int is_at_popf(struct task_struct
case 0xf0: case 0xf2: case 0xf3:
continue;

- /* REX prefixes */
case 0x40 ... 0x4f:
+ if (regs->cs != __USER_CS)
+ /* 32-bit mode: register increment */
+ return 0;
+ /* 64-bit mode: REX prefix */
continue;

/* CHECKME: f0, f2, f3 */
--
Chuck
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