kcsan: Show location access was reordered to

Also show the location the access was reordered to. An example report:

| ==================================================================
| BUG: KCSAN: data-race in test_kernel_wrong_memorder / test_kernel_wrong_memorder
|
| read-write to 0xffffffffc01e61a8 of 8 bytes by task 2311 on cpu 5:
|  test_kernel_wrong_memorder+0x57/0x90
|  access_thread+0x99/0xe0
|  kthread+0x2ba/0x2f0
|  ret_from_fork+0x22/0x30
|
| read-write (reordered) to 0xffffffffc01e61a8 of 8 bytes by task 2310 on cpu 7:
|  test_kernel_wrong_memorder+0x57/0x90
|  access_thread+0x99/0xe0
|  kthread+0x2ba/0x2f0
|  ret_from_fork+0x22/0x30
|   |
|   +-> reordered to: test_kernel_wrong_memorder+0x80/0x90
|
| Reported by Kernel Concurrency Sanitizer on:
| CPU: 7 PID: 2310 Comm: access_thread Not tainted 5.14.0-rc1+ #18
| Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
| ==================================================================
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/kcsan/report.c

@@ -308,10 +308,12 @@ static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries
 
 /*
  * Skips to the first entry that matches the function of @ip, and then replaces
- * that entry with @ip, returning the entries to skip.
+ * that entry with @ip, returning the entries to skip with @replaced containing
+ * the replaced entry.
  */
 static int
-replace_stack_entry(unsigned long stack_entries[], int num_entries, unsigned long ip)
+replace_stack_entry(unsigned long stack_entries[], int num_entries, unsigned long ip,
+		    unsigned long *replaced)
 {
 	unsigned long symbolsize, offset;
 	unsigned long target_func;
@@ -330,6 +332,7 @@ replace_stack_entry(unsigned long stack_entries[], int num_entries, unsigned lon
 		func -= offset;
 
 		if (func == target_func) {
+			*replaced = stack_entries[skip];
 			stack_entries[skip] = ip;
 			return skip;
 		}
@@ -342,9 +345,10 @@ replace_stack_entry(unsigned long stack_entries[], int num_entries, unsigned lon
 }
 
 static int
-sanitize_stack_entries(unsigned long stack_entries[], int num_entries, unsigned long ip)
+sanitize_stack_entries(unsigned long stack_entries[], int num_entries, unsigned long ip,
+		       unsigned long *replaced)
 {
-	return ip ? replace_stack_entry(stack_entries, num_entries, ip) :
+	return ip ? replace_stack_entry(stack_entries, num_entries, ip, replaced) :
 			  get_stack_skipnr(stack_entries, num_entries);
 }
 
@@ -360,6 +364,14 @@ static int sym_strcmp(void *addr1, void *addr2)
 	return strncmp(buf1, buf2, sizeof(buf1));
 }
 
+static void
+print_stack_trace(unsigned long stack_entries[], int num_entries, unsigned long reordered_to)
+{
+	stack_trace_print(stack_entries, num_entries, 0);
+	if (reordered_to)
+		pr_err("  |\n  +-> reordered to: %pS\n", (void *)reordered_to);
+}
+
 static void print_verbose_info(struct task_struct *task)
 {
 	if (!task)
@@ -378,10 +390,12 @@ static void print_report(enum kcsan_value_change value_change,
 			 struct other_info *other_info,
 			 u64 old, u64 new, u64 mask)
 {
+	unsigned long reordered_to = 0;
 	unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
 	int num_stack_entries = stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1);
-	int skipnr = sanitize_stack_entries(stack_entries, num_stack_entries, ai->ip);
+	int skipnr = sanitize_stack_entries(stack_entries, num_stack_entries, ai->ip, &reordered_to);
 	unsigned long this_frame = stack_entries[skipnr];
+	unsigned long other_reordered_to = 0;
 	unsigned long other_frame = 0;
 	int other_skipnr = 0; /* silence uninit warnings */
 
@@ -394,7 +408,7 @@ static void print_report(enum kcsan_value_change value_change,
 	if (other_info) {
 		other_skipnr = sanitize_stack_entries(other_info->stack_entries,
 						      other_info->num_stack_entries,
-						      other_info->ai.ip);
+						      other_info->ai.ip, &other_reordered_to);
 		other_frame = other_info->stack_entries[other_skipnr];
 
 		/* @value_change is only known for the other thread */
@@ -434,10 +448,9 @@ static void print_report(enum kcsan_value_change value_change,
 		       other_info->ai.cpu_id);
 
 		/* Print the other thread's stack trace. */
-		stack_trace_print(other_info->stack_entries + other_skipnr,
+		print_stack_trace(other_info->stack_entries + other_skipnr,
 				  other_info->num_stack_entries - other_skipnr,
-				  0);
-
+				  other_reordered_to);
 		if (IS_ENABLED(CONFIG_KCSAN_VERBOSE))
 			print_verbose_info(other_info->task);
 
@@ -451,9 +464,7 @@ static void print_report(enum kcsan_value_change value_change,
 		       get_thread_desc(ai->task_pid), ai->cpu_id);
 	}
 	/* Print stack trace of this thread. */
-	stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr,
-			  0);
-
+	print_stack_trace(stack_entries + skipnr, num_stack_entries - skipnr, reordered_to);
 	if (IS_ENABLED(CONFIG_KCSAN_VERBOSE))
 		print_verbose_info(current);