Re: lockdep null-ptr-deref

Peter Zijlstra
2020-09-30T09:49:56Z

On Wed, Sep 30, 2020 at 11:16:11AM +0200, Peter Zijlstra wrote:
> On Wed, Sep 30, 2020 at 07:08:23AM +0800, Boqun Feng wrote:
> > I think there are two problems here:
> >
> > 1) the "(null)" means we don't have the "usage_str" for a usage bit,
> > which I think is the LOCK_USED_READ bit introduced by Peter at
> > 23870f122768 ('locking/lockdep: Fix "USED" <- "IN-NMI" inversions').
> >
> > 2) the next null-ptr-deref, and I think this is also caused by
> > LOCK_USED_READ bit, because in the loop inside
> > print_lock_class_header(), we iterate from 0 to LOCK_USAGE_STATES (which
> > is 4*2 + 3), however the class->usage_traces[] only has
> > XXX_LOCK_USAGE_STATES (which is 4*2 + 1) elements, so if we have
> > LOCK_USED_READ bit set in ->usage_mask, we will try to access an element
> > out of the ->usage_traces[] array.
> >
> > Probably the following helps? And another possible fix is to enlarge the
> > ->usage_trace[] array and record the call trace of LOCK_READ_USED.
>
> Urgh.. yeah, I wanted to avoid saving that trace; it's pretty useless :/
> The existing USED trace is already mostly pointless, the consistent
> thing would be to remove both but that might be too radical.
>
> But you're right in that I made a right mess of it. Not sure what's
> best here.
>
> Let me have a play.

How's something like this? It's bigger than I'd like, but I feel the
result is more consistent/readable.

---
diff --git a/include/linux/lockdep_types.h b/include/linux/lockdep_types.h
index bb35b449f533..a55b1d314ae8 100644
--- a/include/linux/lockdep_types.h
+++ b/include/linux/lockdep_types.h
@@ -35,8 +35,12 @@ enum lockdep_wait_type {
/*
* We'd rather not expose kernel/lockdep_states.h this wide, but we do need
* the total number of states... :-(
+ *
+ * XXX_LOCK_USAGE_STATES is the number of lines in lockdep_states.h, for each
+ * of those we generates 4 states, Additionally we (for now) report on USED.
*/
-#define XXX_LOCK_USAGE_STATES (1+2*4)
+#define XXX_LOCK_USAGE_STATES 2
+#define LOCK_TRACE_STATES (XXX_LOCK_USAGE_STATES*4 + 1)

/*
* NR_LOCKDEP_CACHING_CLASSES ... Number of classes
@@ -106,7 +110,7 @@ struct lock_class {
* IRQ/softirq usage tracking bits:
*/
unsigned long usage_mask;
- const struct lock_trace *usage_traces[XXX_LOCK_USAGE_STATES];
+ const struct lock_trace *usage_traces[LOCK_TRACE_STATES];

/*
* Generation counter, when doing certain classes of graph walking,
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 454355c033d2..4f98ac8b4575 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -600,6 +600,8 @@ static const char *usage_str[] =
#include "lockdep_states.h"
#undef LOCKDEP_STATE
[LOCK_USED] = "INITIAL USE",
+ [LOCK_USED_READ] = "INITIAL READ USE",
+ /* abused as string storage for verify_lock_unused() */
[LOCK_USAGE_STATES] = "IN-NMI",
};
#endif
@@ -2231,7 +2233,7 @@ static void print_lock_class_header(struct lock_class *class, int depth)
#endif
printk(KERN_CONT " {\n");

- for (bit = 0; bit < LOCK_USAGE_STATES; bit++) {
+ for (bit = 0; bit < LOCK_TRACE_STATES; bit++) {
if (class->usage_mask & (1 << bit)) {
int len = depth;

@@ -4354,27 +4356,24 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
old_mask = hlock_class(this)->usage_mask;
hlock_class(this)->usage_mask |= new_mask;

- /*
- * Save one usage_traces[] entry and map both LOCK_USED and
- * LOCK_USED_READ onto the same entry.
- */
- if (new_bit == LOCK_USED || new_bit == LOCK_USED_READ) {
- if (old_mask & (LOCKF_USED | LOCKF_USED_READ))
- goto unlock;
- new_bit = LOCK_USED;
+ if (new_bit < LOCK_TRACE_STATES) {
+ if (!(hlock_class(this)->usage_traces[new_bit] = save_trace()))
+ return 0;
}

- if (!(hlock_class(this)->usage_traces[new_bit] = save_trace()))
- return 0;
-
switch (new_bit) {
+ case 0 ... LOCK_USED-1:
+ ret = mark_lock_irq(curr, this, new_bit);
+ if (!ret)
+ return 0;
+ break;
+
case LOCK_USED:
debug_atomic_dec(nr_unused_locks);
break;
+
default:
- ret = mark_lock_irq(curr, this, new_bit);
- if (!ret)
- return 0;
+ break;
}

unlock:
diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
index b0be1560ed17..67dc46e46552 100644
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -20,9 +20,12 @@ enum lock_usage_bit {
#undef LOCKDEP_STATE
LOCK_USED,
LOCK_USED_READ,
- LOCK_USAGE_STATES
+ LOCK_USAGE_STATES,
};

+/* states after LOCK_USED are not traced and printed */
+static_assert(LOCK_TRACE_STATES == LOCK_USED_READ);
+
#define LOCK_USAGE_READ_MASK 1
#define LOCK_USAGE_DIR_MASK 2
#define LOCK_USAGE_STATE_MASK (~(LOCK_USAGE_READ_MASK | LOCK_USAGE_DIR_MASK))
@@ -121,7 +124,7 @@ static const unsigned long LOCKF_USED_IN_IRQ_READ =
extern struct list_head all_lock_classes;
extern struct lock_chain lock_chains[];

-#define LOCK_USAGE_CHARS (1+LOCK_USAGE_STATES/2)
+#define LOCK_USAGE_CHARS (2*XXX_LOCK_USAGE_STATES + 1)

extern void get_usage_chars(struct lock_class *class,
char usage[LOCK_USAGE_CHARS]);