[PATCH v6 4/6] trace_uprobe/sdt: Prevent multiple reference counter for same uprobe

[Ravi Bangoria <ravi.bangoria@linux.ibm.com>]

We assume to have only one reference counter for one uprobe.
Don't allow user to add multiple trace_uprobe entries having
same inode+offset but different reference counter.

Ex,
  # echo "p:sdt_tick/loop2 /home/ravi/tick:0x6e4(0x10036)" > uprobe_events
  # echo "p:sdt_tick/loop2_1 /home/ravi/tick:0x6e4(0xfffff)" >> uprobe_events
  bash: echo: write error: Invalid argument

  # dmesg
  trace_kprobe: Reference counter offset mismatch.

There are two exceptions though:
1) When user is trying to replace the old entry with the new
   one, we allow this if the new entry does not conflict with
   any other existing entry.
2) Any one of the reference counter offset is 0 while comparing
   new entry with old one.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
---
 kernel/trace/trace_uprobe.c | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
index bf2be098eb08..9fafb183c49d 100644
--- a/kernel/trace/trace_uprobe.c
+++ b/kernel/trace/trace_uprobe.c
@@ -324,6 +324,38 @@ static int unregister_trace_uprobe(struct trace_uprobe *tu)
 	return 0;
 }
 
+/*
+ * Uprobe with multiple reference counter is not allowed. i.e.
+ * If inode and offset matches, reference counter offset *must*
+ * match as well. Though, there are two exceptions:
+ * 1) We are replacing old trace_uprobe with new one(same
+ *    group/event) then we don't care as far as the new entry
+ *    does not conflict with any other existing entry.
+ * 2) Any one of the reference counter offset is 0 while comparing
+ *    new entry with old one.
+ */
+static struct trace_uprobe *find_old_trace_uprobe(struct trace_uprobe *new)
+{
+	struct trace_uprobe *tmp, *old = NULL;
+	struct inode *new_inode = d_real_inode(new->path.dentry);
+
+	old = find_probe_event(trace_event_name(&new->tp.call),
+				new->tp.call.class->system);
+
+	list_for_each_entry(tmp, &uprobe_list, list) {
+		if ((old ? old != tmp : true) &&
+		    new_inode == d_real_inode(tmp->path.dentry) &&
+		    new->offset == tmp->offset &&
+		    new->ref_ctr_offset != tmp->ref_ctr_offset &&
+		    new->ref_ctr_offset != 0 &&
+		    tmp->ref_ctr_offset != 0) {
+			pr_warn("Reference counter offset mismatch.");
+			return ERR_PTR(-EINVAL);
+		}
+	}
+	return old;
+}
+
 /* Register a trace_uprobe and probe_event */
 static int register_trace_uprobe(struct trace_uprobe *tu)
 {
@@ -333,8 +365,12 @@ static int register_trace_uprobe(struct trace_uprobe *tu)
 	mutex_lock(&uprobe_lock);
 
 	/* register as an event */
-	old_tu = find_probe_event(trace_event_name(&tu->tp.call),
-			tu->tp.call.class->system);
+	old_tu = find_old_trace_uprobe(tu);
+	if (IS_ERR(old_tu)) {
+		ret = PTR_ERR(old_tu);
+		goto end;
+	}
+
 	if (old_tu) {
 		/* delete old event */
 		ret = unregister_trace_uprobe(old_tu);
-- 
2.14.4