hw-breakpoints: 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, according to the new hardware breakpoint
Api.

[ Impact: adapt thread breakpoints handling code to the new breakpoint Api ]

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>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

1 files changed, 31 insertions, 0 deletions

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 5a1a1de292ec..f7b276d4b3fb 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -55,6 +55,8 @@
 #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);
 
@@ -248,6 +250,8 @@ void release_thread(struct task_struct *dead_task)
 			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)
@@ -303,12 +307,18 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 
 	p->thread.fs = me->thread.fs;
 	p->thread.gs = me->thread.gs;
+	p->thread.io_bitmap_ptr = NULL;
 
 	savesegment(gs, p->thread.gsindex);
 	savesegment(fs, p->thread.fsindex);
 	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) {
@@ -347,6 +357,9 @@ out:
 		kfree(p->thread.io_bitmap_ptr);
 		p->thread.io_bitmap_max = 0;
 	}
+	if (err)
+		flush_thread_hw_breakpoint(p);
+
 	return err;
 }
 
@@ -492,6 +505,24 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_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;
 }