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, 32 insertions, 10 deletions

diff --git a/kernel/hw_breakpoint.c b/kernel/hw_breakpoint.c
index 03a0773ac2b2..366eedf949c0 100644
--- a/kernel/hw_breakpoint.c
+++ b/kernel/hw_breakpoint.c
@@ -320,18 +320,40 @@ EXPORT_SYMBOL_GPL(register_user_hw_breakpoint);
  * @triggered: callback to trigger when we hit the breakpoint
  * @tsk: pointer to 'task_struct' of the process to which the address belongs
  */
-struct perf_event *
-modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *attr)
+int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *attr)
 {
-        /*
-         * FIXME: do it without unregistering
-         * - We don't want to lose our slot
-         * - If the new bp is incorrect, don't lose the older one
-         */
-        unregister_hw_breakpoint(bp);
+        u64 old_addr = bp->attr.bp_addr;
+        int old_type = bp->attr.bp_type;
+        int old_len = bp->attr.bp_len;
+        int err = 0;
+
+        perf_event_disable(bp);
+
+        bp->attr.bp_addr = attr->bp_addr;
+        bp->attr.bp_type = attr->bp_type;
+        bp->attr.bp_len = attr->bp_len;
+
+        if (attr->disabled)
+                goto end;
 
-        return perf_event_create_kernel_counter(attr, -1, bp->ctx->task->pid,
-                                                bp->overflow_handler);
+        err = arch_validate_hwbkpt_settings(bp, bp->ctx->task);
+        if (!err)
+                perf_event_enable(bp);
+
+        if (err) {
+                bp->attr.bp_addr = old_addr;
+                bp->attr.bp_type = old_type;
+                bp->attr.bp_len = old_len;
+                if (!bp->attr.disabled)
+                        perf_event_enable(bp);
+
+                return err;
+        }
+
+end:
+        bp->attr.disabled = attr->disabled;
+
+        return 0;
 }
 EXPORT_SYMBOL_GPL(modify_user_hw_breakpoint);