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SubjectRe: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer dereference at 000001f2
On Tue, Oct 03, 2017 at 10:05:38AM -0500, Josh Poimboeuf wrote:
> I don't know the lockdep code, but one more comment from the peanut
> gallery. This code looks suspect to me:
>
>
> /*
> * Stop saving stack_trace if save_trace() was
> * called at least once:
> */
> if (save && ret == 2)
> save = NULL;
>
>
> From looking at check_prev_add(), a return value of 2 doesn't
> necessarily imply that save_trace() was called. If the
> check_redundant() call returns 0, then check_prev_add() can return 2,
> and the trace will still be uninitialized, but save will be set to NULL
> even though save_trace() hasn't been called. Then a subsequent call to
> check_prev_add() could add an uninitialized stack_trace struct to the
> dependency list.

Right, and print_circular_bug() uses @trace before it ever can be set,
although I suspect the intention is that that only ever gets called from
commit_xhlock() where we pass in an initialized @trace. A comment
would've been good :/

So the whole point of that trace wankery here is to not do save_trace()
when we'll not in fact use the stack trace.

It seems to me we can do that much saner by actually initializing our
@trace buffer and testing if it contains an actual stacktrace.

Also killed that verbose thing, because dropping that graph_lock thing
hurts my brain -- although I think it should work.

Does this make the corruption thing go away?

---
kernel/locking/lockdep.c | 48 ++++++++++++++++++++----------------------------
1 file changed, 20 insertions(+), 28 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 44c8d0d17170..e36e652d996f 100644
--- 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
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