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SubjectRe: [RFC Patch 1/2][Bugfix][x86][hw-breakpoint] Clear reserved bits of DR6 in do_debug()
On Sat, Dec 26, 2009 at 11:57:25PM +0530, K.Prasad wrote:
> Clear the reserved bits from the stored copy of debug status register (DR6).
> This will help easy bitwise operations.
>
> Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
> ---
> arch/x86/include/asm/debugreg.h | 3 +++
> arch/x86/kernel/traps.c | 3 +++
> 2 files changed, 6 insertions(+)
>
> Index: linux-2.6-tip/arch/x86/include/asm/debugreg.h
> ===================================================================
> --- linux-2.6-tip.orig/arch/x86/include/asm/debugreg.h
> +++ linux-2.6-tip/arch/x86/include/asm/debugreg.h
> @@ -14,6 +14,9 @@
> which debugging register was responsible for the trap. The other bits
> are either reserved or not of interest to us. */
>
> +/* Define reserved bits in DR6 which are always set to 1 */
> +#define DR6_RESERVED (0xFFFF0FF0)
> +


The 12th bit seems to be also reserved.
Shouldn't it be 0xffff1ff0 ?
What kind of bitwise operations do you think it could help?

All of the operations I can find on dr6 are simple masks
test/set/clear.



> #define DR_TRAP0 (0x1) /* db0 */
> #define DR_TRAP1 (0x2) /* db1 */
> #define DR_TRAP2 (0x4) /* db2 */
> Index: linux-2.6-tip/arch/x86/kernel/traps.c
> ===================================================================
> --- linux-2.6-tip.orig/arch/x86/kernel/traps.c
> +++ linux-2.6-tip/arch/x86/kernel/traps.c
> @@ -534,6 +534,9 @@ dotraplinkage void __kprobes do_debug(st
>
> get_debugreg(dr6, 6);
>
> + /* Filter out all the reserved bits which are preset to 1 */
> + dr6 &= ~DR6_RESERVED;
> +
> /* Catch kmemcheck conditions first of all! */
> if ((dr6 & DR_STEP) && kmemcheck_trap(regs))
> return;
>



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