Re: [Patch 06/12] Use the new wrapper routines to access debug registers in process/thread code

["K.Prasad" <prasad@linux.vnet.ibm.com>]

On Thu, May 28, 2009 at 04:42:38PM +1000, David Gibson wrote:
> On Mon, May 11, 2009 at 05:23:44PM +0530, K.Prasad wrote:
> > From: Alan Stern <stern@rowland.harvard.edu>
> > 
> > This patch enables the use of abstract debug registers in
> > process-handling routines.
> 
> [snip]
> >  
> > +	p->thread.io_bitmap_ptr = NULL;
> 
> Why is manipulating the io_bitmap_ptr relevant to debug register
> handling?

I *re-read* the patch but was unable to find how this change had sneaked
in. It shouldn't be there although it is harmless.

Hi Frederic,
	I am attaching a new version of this patch 06/12 that:

- removes the line that assigns NULL to "p->thread.io_bitmap_ptr"

- Updates the comment in __switch_to() function which was stale (was
  relevant when 'last_debugged_task' was used to detect lazy debug
  register switching).

Kindly integrate this version in lieu of the older version sent here:
http://lkml.org/lkml/2009/5/21/149.

Hi Alan,
	I'm retaining the
"Reviewed-by: Alan Stern <stern@rowland.harvard.edu>" tag on the patch
even after the above changes and guess it is acceptable to you. Please
let me know if you have any concerns about that.

Thanks,
K.Prasad


Use the new wrapper routines to access debug registers in process/thread code

This patch enables the use of abstract debug registers in
process-handling routines.

Original-patch-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
---
 arch/x86/kernel/process.c    |   23 ++++++-----------------
 arch/x86/kernel/process_32.c |   27 +++++++++++++++++++++++++++
 arch/x86/kernel/process_64.c |   30 ++++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 17 deletions(-)

Index: linux-2.6-tip.hbkpt/arch/x86/kernel/process.c
===================================================================
--- linux-2.6-tip.hbkpt.orig/arch/x86/kernel/process.c
+++ linux-2.6-tip.hbkpt/arch/x86/kernel/process.c
@@ -17,6 +17,8 @@
 #include <asm/uaccess.h>
 #include <asm/i387.h>
 #include <asm/ds.h>
+#include <asm/debugreg.h>
+#include <asm/hw_breakpoint.h>
 
 unsigned long idle_halt;
 EXPORT_SYMBOL(idle_halt);
@@ -48,6 +50,8 @@ void free_thread_xstate(struct task_stru
 		kmem_cache_free(task_xstate_cachep, tsk->thread.xstate);
 		tsk->thread.xstate = NULL;
 	}
+	if (unlikely(test_tsk_thread_flag(tsk, TIF_DEBUG)))
+		flush_thread_hw_breakpoint(tsk);
 
 	WARN(tsk->thread.ds_ctx, "leaking DS context\n");
 }
@@ -106,14 +110,9 @@ void flush_thread(void)
 	}
 #endif
 
-	clear_tsk_thread_flag(tsk, TIF_DEBUG);
+	if (unlikely(test_tsk_thread_flag(tsk, TIF_DEBUG)))
+		flush_thread_hw_breakpoint(tsk);
 
-	tsk->thread.debugreg0 = 0;
-	tsk->thread.debugreg1 = 0;
-	tsk->thread.debugreg2 = 0;
-	tsk->thread.debugreg3 = 0;
-	tsk->thread.debugreg6 = 0;
-	tsk->thread.debugreg7 = 0;
 	memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));
 	/*
 	 * Forget coprocessor state..
@@ -195,16 +194,6 @@ void __switch_to_xtra(struct task_struct
 	else if (next->debugctlmsr != prev->debugctlmsr)
 		update_debugctlmsr(next->debugctlmsr);
 
-	if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
-		set_debugreg(next->debugreg0, 0);
-		set_debugreg(next->debugreg1, 1);
-		set_debugreg(next->debugreg2, 2);
-		set_debugreg(next->debugreg3, 3);
-		/* no 4 and 5 */
-		set_debugreg(next->debugreg6, 6);
-		set_debugreg(next->debugreg7, 7);
-	}
-
 	if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
 	    test_tsk_thread_flag(next_p, TIF_NOTSC)) {
 		/* prev and next are different */
Index: linux-2.6-tip.hbkpt/arch/x86/kernel/process_32.c
===================================================================
--- linux-2.6-tip.hbkpt.orig/arch/x86/kernel/process_32.c
+++ linux-2.6-tip.hbkpt/arch/x86/kernel/process_32.c
@@ -58,6 +58,8 @@
 #include <asm/idle.h>
 #include <asm/syscalls.h>
 #include <asm/ds.h>
+#include <asm/debugreg.h>
+#include <asm/hw_breakpoint.h>
 
 asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");
 
@@ -263,6 +265,11 @@ int copy_thread(unsigned long clone_flag
 	task_user_gs(p) = get_user_gs(regs);
 
 	tsk = current;
+	err = -ENOMEM;
+	if (unlikely(test_tsk_thread_flag(tsk, TIF_DEBUG)))
+		if (copy_thread_hw_breakpoint(tsk, p, clone_flags))
+			goto out;
+
 	if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
 		p->thread.io_bitmap_ptr = kmemdup(tsk->thread.io_bitmap_ptr,
 						IO_BITMAP_BYTES, GFP_KERNEL);
@@ -282,10 +289,13 @@ int copy_thread(unsigned long clone_flag
 		err = do_set_thread_area(p, -1,
 			(struct user_desc __user *)childregs->si, 0);
 
+out:
 	if (err && p->thread.io_bitmap_ptr) {
 		kfree(p->thread.io_bitmap_ptr);
 		p->thread.io_bitmap_max = 0;
 	}
+	if (err)
+		flush_thread_hw_breakpoint(p);
 
 	clear_tsk_thread_flag(p, TIF_DS_AREA_MSR);
 	p->thread.ds_ctx = NULL;
@@ -424,6 +434,23 @@ __switch_to(struct task_struct *prev_p, 
 		lazy_load_gs(next->gs);
 
 	percpu_write(current_task, next_p);
+	/*
+	 * There's a problem with moving the arch_install_thread_hw_breakpoint()
+	 * call before current is updated.  Suppose a kernel breakpoint is
+	 * triggered in between the two, the hw-breakpoint handler will see that
+	 * the 'current' task does not have TIF_DEBUG flag set and will think it
+	 * is leftover from an old task (lazy switching) and will erase it. Then
+	 * until the next context switch, no user-breakpoints will be installed.
+	 *
+	 * The real problem is that it's impossible to update both current and
+	 * physical debug registers at the same instant, so there will always be
+	 * a window in which they disagree and a breakpoint might get triggered.
+	 * Since we use lazy switching, we are forced to assume that a
+	 * disagreement means that current is correct and the exception is due
+	 * to lazy debug register switching.
+	 */
+	if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
+		arch_install_thread_hw_breakpoint(next_p);
 
 	return prev_p;
 }
Index: linux-2.6-tip.hbkpt/arch/x86/kernel/process_64.c
===================================================================
--- linux-2.6-tip.hbkpt.orig/arch/x86/kernel/process_64.c
+++ linux-2.6-tip.hbkpt/arch/x86/kernel/process_64.c
@@ -52,6 +52,8 @@
 #include <asm/idle.h>
 #include <asm/syscalls.h>
 #include <asm/ds.h>
+#include <asm/debugreg.h>
+#include <asm/hw_breakpoint.h>
 
 asmlinkage extern void ret_from_fork(void);
 
@@ -245,6 +247,8 @@ void release_thread(struct task_struct *
 			BUG();
 		}
 	}
+	if (unlikely(dead_task->thread.debugreg7))
+		flush_thread_hw_breakpoint(dead_task);
 }
 
 static inline void set_32bit_tls(struct task_struct *t, int tls, u32 addr)
@@ -306,6 +310,11 @@ int copy_thread(unsigned long clone_flag
 	savesegment(es, p->thread.es);
 	savesegment(ds, p->thread.ds);
 
+	err = -ENOMEM;
+	if (unlikely(test_tsk_thread_flag(me, TIF_DEBUG)))
+		if (copy_thread_hw_breakpoint(me, p, clone_flags))
+			goto out;
+
 	if (unlikely(test_tsk_thread_flag(me, TIF_IO_BITMAP))) {
 		p->thread.io_bitmap_ptr = kmalloc(IO_BITMAP_BYTES, GFP_KERNEL);
 		if (!p->thread.io_bitmap_ptr) {
@@ -344,6 +353,9 @@ out:
 		kfree(p->thread.io_bitmap_ptr);
 		p->thread.io_bitmap_max = 0;
 	}
+	if (err)
+		flush_thread_hw_breakpoint(p);
+
 	return err;
 }
 
@@ -489,6 +501,24 @@ __switch_to(struct task_struct *prev_p, 
 	 */
 	if (tsk_used_math(next_p) && next_p->fpu_counter > 5)
 		math_state_restore();
+	/*
+	 * There's a problem with moving the arch_install_thread_hw_breakpoint()
+	 * call before current is updated.  Suppose a kernel breakpoint is
+	 * triggered in between the two, the hw-breakpoint handler will see that
+	 * the 'current' task does not have TIF_DEBUG flag set and will think it
+	 * is leftover from an old task (lazy switching) and will erase it. Then
+	 * until the next context switch, no user-breakpoints will be installed.
+	 *
+	 * The real problem is that it's impossible to update both current and
+	 * physical debug registers at the same instant, so there will always be
+	 * a window in which they disagree and a breakpoint might get triggered.
+	 * Since we use lazy switching, we are forced to assume that a
+	 * disagreement means that current is correct and the exception is due
+	 * to lazy debug register switching.
+	 */
+	if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
+		arch_install_thread_hw_breakpoint(next_p);
+
 	return prev_p;
 }