Re: ptrace denies access to EFLAGS_RF

[Chuck Ebbert <76306.1226@compuserve.com>]

In-Reply-To: <20060105105130.GC3712@frankl.hpl.hp.com>

On Thu, 5 Jan 2006 at 02:51:30 -0800, Stephane Eranian wrote:

> I am trying to the user HW debug registers on i386
> and I am running into a problem with ptrace() not allowing access
> to EFLAGS_RF for POKEUSER (see FLAG_MASK).
> 
> I am not sure I understand the motivation for denying access
> to this flag which can be used to resume after a code
> breakpoint has been reached. It avoids the need to remove the
> breakpoint, single step, and reinstall. The equivalent
> functionality exists on IA-64 and is allowed by ptrace().

I see no reason for denying this.  This patch should fix it:


i386: PTRACE_POKEUSR: allow changing RF bit in EFLAGS register.

Setting RF (resume flag) allows a debugger to resume execution
after a code breakpoint without tripping the breakpoint again.
It is reset by the CPU after execution of one instruction.

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>

--- 2.6.15a.orig/arch/i386/kernel/ptrace.c
+++ 2.6.15a/arch/i386/kernel/ptrace.c
@@ -32,9 +32,12 @@
  * in exit.c or in signal.c.
  */
 
-/* determines which flags the user has access to. */
-/* 1 = access 0 = no access */
-#define FLAG_MASK 0x00044dd5
+/*
+ * Determines which flags the user has access to [1 = access, 0 = no access].
+ * Prohibits changing ID(21), VIP(20), VIF(19), VM(17), IOPL(12-13), IF(9).
+ * Also masks reserved bits (31-22, 15, 5, 3, 1).
+ */
+#define FLAG_MASK 0x00054dd5
 
 /* set's the trap flag. */
 #define TRAP_FLAG 0x100
-- 
Chuck
Currently reading: _Thud!_ by Terry Pratchett