hw-breakpoints: Modify breakpoints without unregistering them

Currently, when ptrace needs to modify a breakpoint, like disabling
it, changing its address, type or len, it calls
modify_user_hw_breakpoint(). This latter will perform the heavy and
racy task of unregistering the old breakpoint and registering a new
one.

This is racy as someone else might steal the reserved breakpoint
slot under us, which is undesired as the breakpoint is only
supposed to be modified, sometimes in the middle of a debugging
workflow. We don't want our slot to be stolen in the middle.

So instead of unregistering/registering the breakpoint, just
disable it while we modify its breakpoint fields and re-enable it
after if necessary.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prasad <prasad@linux.vnet.ibm.com>
LKML-Reference: <1260347148-5519-1-git-send-regression-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 26 insertions, 31 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index b361d28061d0..7079ddaf0731 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -595,7 +595,7 @@ static unsigned long ptrace_get_dr7(struct perf_event *bp[])
 	return dr7;
 }
 
-static struct perf_event *
+static int
 ptrace_modify_breakpoint(struct perf_event *bp, int len, int type,
 			 struct task_struct *tsk, int disabled)
 {
@@ -609,11 +609,11 @@ ptrace_modify_breakpoint(struct perf_event *bp, int len, int type,
 	 * written the address register first
 	 */
 	if (!bp)
-		return ERR_PTR(-EINVAL);
+		return -EINVAL;
 
 	err = arch_bp_generic_fields(len, type, &gen_len, &gen_type);
 	if (err)
-		return ERR_PTR(err);
+		return err;
 
 	attr = bp->attr;
 	attr.bp_len = gen_len;
@@ -658,28 +658,17 @@ restore:
 				if (!second_pass)
 					continue;
 
-				thread->ptrace_bps[i] = NULL;
-				bp = ptrace_modify_breakpoint(bp, len, type,
+				rc = ptrace_modify_breakpoint(bp, len, type,
 							      tsk, 1);
-				if (IS_ERR(bp)) {
-					rc = PTR_ERR(bp);
-					thread->ptrace_bps[i] = NULL;
+				if (rc)
 					break;
-				}
-				thread->ptrace_bps[i] = bp;
 			}
 			continue;
 		}
 
-		bp = ptrace_modify_breakpoint(bp, len, type, tsk, 0);
-
-		/* Incorrect bp, or we have a bug in bp API */
-		if (IS_ERR(bp)) {
-			rc = PTR_ERR(bp);
-			thread->ptrace_bps[i] = NULL;
+		rc = ptrace_modify_breakpoint(bp, len, type, tsk, 0);
+		if (rc)
 			break;
-		}
-		thread->ptrace_bps[i] = bp;
 	}
 	/*
 	 * Make a second pass to free the remaining unused breakpoints
@@ -737,26 +726,32 @@ static int ptrace_set_breakpoint_addr(struct task_struct *tsk, int nr,
 		attr.disabled = 1;
 
 		bp = register_user_hw_breakpoint(&attr, ptrace_triggered, tsk);
+
+		/*
+		 * CHECKME: the previous code returned -EIO if the addr wasn't
+		 * a valid task virtual addr. The new one will return -EINVAL in
+		 *  this case.
+		 * -EINVAL may be what we want for in-kernel breakpoints users,
+		 * but -EIO looks better for ptrace, since we refuse a register
+		 * writing for the user. And anyway this is the previous
+		 * behaviour.
+		 */
+		if (IS_ERR(bp))
+			return PTR_ERR(bp);
+
+		t->ptrace_bps[nr] = bp;
 	} else {
+		int err;
+
 		bp = t->ptrace_bps[nr];
-		t->ptrace_bps[nr] = NULL;
 
 		attr = bp->attr;
 		attr.bp_addr = addr;
-		bp = modify_user_hw_breakpoint(bp, &attr);
+		err = modify_user_hw_breakpoint(bp, &attr);
+		if (err)
+			return err;
 	}
-	/*
-	 * CHECKME: the previous code returned -EIO if the addr wasn't a
-	 * valid task virtual addr. The new one will return -EINVAL in this
-	 * case.
-	 * -EINVAL may be what we want for in-kernel breakpoints users, but
-	 * -EIO looks better for ptrace, since we refuse a register writing
-	 * for the user. And anyway this is the previous behaviour.
-	 */
-	if (IS_ERR(bp))
-		return PTR_ERR(bp);
 
-	t->ptrace_bps[nr] = bp;
 
 	return 0;
 }