locking/lockdep: Fix stacktrace mess

There is some complication between check_prevs_add() and
check_prev_add() wrt. saving stack traces. The problem is that we want
to be frugal with saving stack traces, since it consumes static
resources.

We'll only know in check_prev_add() if we need the trace, but we can
call into it multiple times. So we want to do on-demand and re-use.

A further complication is that check_prev_add() can drop graph_lock
and mess with our static resources.

In any case, the current state; after commit:

  ce07a9415f26 ("locking/lockdep: Make check_prev_add() able to handle external stack_trace")

is that we'll assume the trace contains valid data once
check_prev_add() returns '2'. However, as noted by Josh, this is
false, check_prev_add() can return '2' before having saved a trace,
this then result in the possibility of using uninitialized data.
Testing, as reported by Wu, shows a NULL deref.

So simplify.

Since the graph_lock() thing is a debug path that hasn't
really been used in a long while, take it out back and avoid the
head-ache.

Further initialize the stack_trace to a known 'empty' state; as long
as nr_entries == 0, nothing should deref entries. We can then use the
'entries == NULL' test for a valid trace / on-demand saving.

Analyzed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: ce07a9415f26 ("locking/lockdep: Make check_prev_add() able to handle external stack_trace")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 44c8d0d17170af808ab0f11c4e556d49044a6c78..e36e652d996fe682157c52768949f0b9c7f1e6d1 100644 (file)
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1873,10 +1873,10 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
               struct held_lock *next, int distance, struct stack_trace *trace,
               int (*save)(struct stack_trace *trace))
 {
+       struct lock_list *uninitialized_var(target_entry);
        struct lock_list *entry;
-       int ret;
        struct lock_list this;
-       struct lock_list *uninitialized_var(target_entry);
+       int ret;
 
        /*
         * Prove that the new <prev> -> <next> dependency would not
@@ -1890,8 +1890,17 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
        this.class = hlock_class(next);
        this.parent = NULL;
        ret = check_noncircular(&this, hlock_class(prev), &target_entry);
-       if (unlikely(!ret))
+       if (unlikely(!ret)) {
+               if (!trace->entries) {
+                       /*
+                        * If @save fails here, the printing might trigger
+                        * a WARN but because of the !nr_entries it should
+                        * not do bad things.
+                        */
+                       save(trace);
+               }
                return print_circular_bug(&this, target_entry, next, prev, trace);
+       }
        else if (unlikely(ret < 0))
                return print_bfs_bug(ret);
 
@@ -1938,7 +1947,7 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
                return print_bfs_bug(ret);
 
 
-       if (save && !save(trace))
+       if (!trace->entries && !save(trace))
                return 0;
 
        /*
@@ -1958,20 +1967,6 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
        if (!ret)
                return 0;
 
-       /*
-        * Debugging printouts:
-        */
-       if (verbose(hlock_class(prev)) || verbose(hlock_class(next))) {
-               graph_unlock();
-               printk("\n new dependency: ");
-               print_lock_name(hlock_class(prev));
-               printk(KERN_CONT " => ");
-               print_lock_name(hlock_class(next));
-               printk(KERN_CONT "\n");
-               dump_stack();
-               if (!graph_lock())
-                       return 0;
-       }
        return 2;
 }
 
@@ -1986,8 +1981,12 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
 {
        int depth = curr->lockdep_depth;
        struct held_lock *hlock;
-       struct stack_trace trace;
-       int (*save)(struct stack_trace *trace) = save_trace;
+       struct stack_trace trace = {
+               .nr_entries = 0,
+               .max_entries = 0,
+               .entries = NULL,
+               .skip = 0,
+       };
 
        /*
         * Debugging checks.
@@ -2018,17 +2017,10 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
                         */
                        if (hlock->read != 2 && hlock->check) {
                                int ret = check_prev_add(curr, hlock, next,
-                                                        distance, &trace, save);
+                                                        distance, &trace, save_trace);
                                if (!ret)
                                        return 0;
 
-                               /*
-                                * Stop saving stack_trace if save_trace() was
-                                * called at least once:
-                                */
-                               if (save && ret == 2)
-                                       save = NULL;
-
                                /*
                                 * Stop after the first non-trylock entry,
                                 * as non-trylock entries have added their