[PATCH] lockdep: avoid signed overflow

[Mark Rutland]

The counters in struct lockdep_stats are all (signed) ints. For some
counters (e.g. hardirqs_on_events, hardirqs_off_events), it's easy to
trigger an overflow in a short period of time, rendering the information
exposed under /proc/lockdep_stats erroneous, and causing UBSAN to
scream.

Since all counters can never validly be negative, we can make them
unsigned, which is sufficient to placate UBSAN. However, this merely
doubles the time until an overflow silently occurs, rendering the
information under /proc/lockdep_stats erroneous.

To avoid the undefined behaviour and to avoid erroneous results, this
patch upgrades all of the counters to unsigned long, which cannot
overflow in a reasonable time frame.

We don't need to change anything else. The counters are only manipulated
via this_cpu inc/dec operations, which are unaffected by the type.
They're only read read by the code generating the /proc output, which
sums the values into unsigned long long values.

Original UBSAN splats:

[137987.143729] ================================================================================
[137987.152266] UBSAN: Undefined behaviour in kernel/locking/lockdep.c:2696:2
[137987.159140] signed integer overflow:
[137987.162801] 2147483647 + 1 cannot be represented in type 'int'
[137987.168723] CPU: 0 PID: 24149 Comm: qemu-system-aar Not tainted 4.11.0-rc1-00002-g23187dc #12
[137987.177329] Hardware name: AMD Seattle (Rev.B0) Development Board (Overdrive) (DT)
[137987.184980] Call trace:
[137987.187523] [<ffff200008094638>] dump_backtrace+0x0/0x538
[137987.193010] [<ffff200008094b90>] show_stack+0x20/0x30
[137987.198150] [<ffff200008bd4c00>] dump_stack+0x120/0x188
[137987.203463] [<ffff200008c7c588>] ubsan_epilogue+0x18/0x98
[137987.208948] [<ffff200008c7da24>] handle_overflow+0x1f0/0x234
[137987.214695] [<ffff200008c7da9c>] __ubsan_handle_add_overflow+0x34/0x44
[137987.221311] [<ffff200008265510>] trace_hardirqs_on_caller+0x728/0x768
[137987.227838] [<ffff200008265560>] trace_hardirqs_on+0x10/0x18
[137987.233585] [<ffff200009fc1b7c>] yield_to+0x3c4/0x700
[137987.238725] [<ffff2000080db340>] kvm_vcpu_yield_to+0x138/0x2f8
[137987.244646] [<ffff2000080db948>] kvm_vcpu_on_spin+0x448/0x718
[137987.250480] [<ffff2000080fed20>] kvm_handle_wfx+0x80/0x228
[137987.256053] [<ffff2000080ff150>] handle_exit+0x258/0x5b0
[137987.261453] [<ffff2000080f11e4>] kvm_arch_vcpu_ioctl_run+0x7ac/0x1170
[137987.267980] [<ffff2000080d85ac>] kvm_vcpu_ioctl+0x6bc/0xe30
[137987.273641] [<ffff2000085fb774>] do_vfs_ioctl+0x194/0x14a0
[137987.279213] [<ffff2000085fcb28>] SyS_ioctl+0xa8/0xb8
[137987.284265] [<ffff200008084770>] el0_svc_naked+0x24/0x28
[137987.289659] ================================================================================
[137987.298322] ================================================================================
[137987.306912] UBSAN: Undefined behaviour in kernel/locking/lockdep.c:2775:3
[137987.313791] signed integer overflow:
[137987.317457] 2147483647 + 1 cannot be represented in type 'int'
[137987.323384] CPU: 0 PID: 24149 Comm: qemu-system-aar Not tainted 4.11.0-rc1-00002-g23187dc #12
[137987.331997] Hardware name: AMD Seattle (Rev.B0) Development Board (Overdrive) (DT)
[137987.339654] Call trace:
[137987.342197] [<ffff200008094638>] dump_backtrace+0x0/0x538
[137987.347691] [<ffff200008094b90>] show_stack+0x20/0x30
[137987.352836] [<ffff200008bd4c00>] dump_stack+0x120/0x188
[137987.358155] [<ffff200008c7c588>] ubsan_epilogue+0x18/0x98
[137987.363648] [<ffff200008c7da24>] handle_overflow+0x1f0/0x234
[137987.369401] [<ffff200008c7da9c>] __ubsan_handle_add_overflow+0x34/0x44
[137987.376023] [<ffff20000825c8a0>] trace_hardirqs_off_caller+0x2b8/0x420
[137987.382645] [<ffff20000825ca18>] trace_hardirqs_off+0x10/0x18
[137987.388484] [<ffff200008083fb0>] el1_irq+0x70/0x130
[137987.393457] [<ffff2000080db340>] kvm_vcpu_yield_to+0x138/0x2f8
[137987.399384] [<ffff2000080db948>] kvm_vcpu_on_spin+0x448/0x718
[137987.405225] [<ffff2000080fed20>] kvm_handle_wfx+0x80/0x228
[137987.410804] [<ffff2000080ff150>] handle_exit+0x258/0x5b0
[137987.416211] [<ffff2000080f11e4>] kvm_arch_vcpu_ioctl_run+0x7ac/0x1170
[137987.422746] [<ffff2000080d85ac>] kvm_vcpu_ioctl+0x6bc/0xe30
[137987.428412] [<ffff2000085fb774>] do_vfs_ioctl+0x194/0x14a0
[137987.433991] [<ffff2000085fcb28>] SyS_ioctl+0xa8/0xb8
[137987.439049] [<ffff200008084770>] el0_svc_naked+0x24/0x28
[137987.444451] ================================================================================

Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 kernel/locking/lockdep_internals.h | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
index c2b8849..9fd970e 100644
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -132,23 +132,23 @@ extern void get_usage_chars(struct lock_class *class,
  * and we want to avoid too much cache bouncing.
  */
 struct lockdep_stats {
-	int	chain_lookup_hits;
-	int	chain_lookup_misses;
-	int	hardirqs_on_events;
-	int	hardirqs_off_events;
-	int	redundant_hardirqs_on;
-	int	redundant_hardirqs_off;
-	int	softirqs_on_events;
-	int	softirqs_off_events;
-	int	redundant_softirqs_on;
-	int	redundant_softirqs_off;
-	int	nr_unused_locks;
-	int	nr_cyclic_checks;
-	int	nr_cyclic_check_recursions;
-	int	nr_find_usage_forwards_checks;
-	int	nr_find_usage_forwards_recursions;
-	int	nr_find_usage_backwards_checks;
-	int	nr_find_usage_backwards_recursions;
+	unsigned long	chain_lookup_hits;
+	unsigned long	chain_lookup_misses;
+	unsigned long	hardirqs_on_events;
+	unsigned long	hardirqs_off_events;
+	unsigned long	redundant_hardirqs_on;
+	unsigned long	redundant_hardirqs_off;
+	unsigned long	softirqs_on_events;
+	unsigned long	softirqs_off_events;
+	unsigned long	redundant_softirqs_on;
+	unsigned long	redundant_softirqs_off;
+	unsigned long	nr_unused_locks;
+	unsigned long	nr_cyclic_checks;
+	unsigned long	nr_cyclic_check_recursions;
+	unsigned long	nr_find_usage_forwards_checks;
+	unsigned long	nr_find_usage_forwards_recursions;
+	unsigned long	nr_find_usage_backwards_checks;
+	unsigned long	nr_find_usage_backwards_recursions;
 };
 
 DECLARE_PER_CPU(struct lockdep_stats, lockdep_stats);
-- 
1.9.1