[PATCH] percpu_counter: `percpu_counter_read_positive' should not return negative number

[Mathieu Malaterre]

Since function `percpu_counter_add' may result in a signed integer overflow
the result stored in `fbc->count' could be negative. Make sure that
function `percpu_counter_read_positive' does not return a negative number
in this case. This will match behavior when CONFIG_SMP=y.

Detected wth CONFIG_UBSAN=y

[76404.888450] ================================================================================
[76404.888477] UBSAN: Undefined behaviour in ../include/linux/percpu_counter.h:136:13
[76404.888485] signed integer overflow:
[76404.888490] 9223308017647617321 + 76624449492175 cannot be represented in type 'long long int'
[76404.888501] CPU: 0 PID: 0 Comm: swapper Not tainted 4.17.0+ #50
[76404.888506] Call Trace:
[76404.888523] [dffedd30] [c0478b90] ubsan_epilogue+0x18/0x4c (unreliable)
[76404.888533] [dffedd40] [c0479530] handle_overflow+0xbc/0xdc
[76404.888548] [dffeddc0] [c0439044] cfq_completed_request+0x560/0x1234
[76404.888557] [dffede40] [c03f3fc4] __blk_put_request+0xb0/0x2dc
[76404.888570] [dffede80] [c05a81d0] scsi_end_request+0x19c/0x344
[76404.888579] [dffedeb0] [c05a9954] scsi_io_completion+0x4b4/0x854
[76404.888592] [dffedf10] [c04046b4] blk_done_softirq+0xe4/0x1e0
[76404.888605] [dffedf60] [c07ec1d4] __do_softirq+0x16c/0x5f0
[76404.888617] [dffedfd0] [c0065160] irq_exit+0x110/0x1a8
[76404.888629] [dffedff0] [c001646c] call_do_irq+0x24/0x3c
[76404.888641] [c0cdbe80] [c0009a2c] do_IRQ+0x98/0x1a0
[76404.888649] [c0cdbeb0] [c001b93c] ret_from_except+0x0/0x14
[76404.888659] --- interrupt: 501 at arch_cpu_idle+0x30/0x78
                   LR = arch_cpu_idle+0x30/0x78
[76404.888667] [c0cdbf70] [c0cda000] 0xc0cda000 (unreliable)
[76404.888679] [c0cdbf80] [c00a3844] do_idle+0xc4/0x158
[76404.888687] [c0cdbfb0] [c00a3a90] cpu_startup_entry+0x24/0x28
[76404.888696] [c0cdbfc0] [c097f820] start_kernel+0x47c/0x490
[76404.888703] [c0cdbff0] [00003444] 0x3444
[76404.888708] ================================================================================
[76409.458652] ================================================================================
[76409.458679] UBSAN: Undefined behaviour in ../include/linux/percpu_counter.h:136:13
[76409.458687] signed integer overflow:
[76409.458692] 9223369047059056210 + 76629034867964 cannot be represented in type 'long long int'
[76409.458703] CPU: 0 PID: 0 Comm: swapper Not tainted 4.17.0+ #50
[76409.458708] Call Trace:
[76409.458725] [dffedd30] [c0478b90] ubsan_epilogue+0x18/0x4c (unreliable)
[76409.458736] [dffedd40] [c0479530] handle_overflow+0xbc/0xdc
[76409.458751] [dffeddc0] [c0438e60] cfq_completed_request+0x37c/0x1234
[76409.458759] [dffede40] [c03f3fc4] __blk_put_request+0xb0/0x2dc
[76409.458773] [dffede80] [c05a81d0] scsi_end_request+0x19c/0x344
[76409.458781] [dffedeb0] [c05a9954] scsi_io_completion+0x4b4/0x854
[76409.458794] [dffedf10] [c04046b4] blk_done_softirq+0xe4/0x1e0
[76409.458807] [dffedf60] [c07ec1d4] __do_softirq+0x16c/0x5f0
[76409.458820] [dffedfd0] [c0065160] irq_exit+0x110/0x1a8
[76409.458832] [dffedff0] [c001646c] call_do_irq+0x24/0x3c
[76409.458844] [c0cdbe80] [c0009a2c] do_IRQ+0x98/0x1a0
[76409.458852] [c0cdbeb0] [c001b93c] ret_from_except+0x0/0x14
[76409.458861] --- interrupt: 501 at arch_cpu_idle+0x30/0x78
                   LR = arch_cpu_idle+0x30/0x78
[76409.458870] [c0cdbf70] [c0cda000] 0xc0cda000 (unreliable)
[76409.458882] [c0cdbf80] [c00a3844] do_idle+0xc4/0x158
[76409.458889] [c0cdbfb0] [c00a3a8c] cpu_startup_entry+0x20/0x28
[76409.458899] [c0cdbfc0] [c097f820] start_kernel+0x47c/0x490
[76409.458905] [c0cdbff0] [00003444] 0x3444
[76409.458910] ================================================================================

Signed-off-by: Mathieu Malaterre <malat@xxxxxxxxxx>
---
 include/linux/percpu_counter.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 4f052496cdfd..fd14f629123f 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -133,6 +133,7 @@ static inline void
 percpu_counter_add(struct percpu_counter *fbc, s64 amount)
 {
 	preempt_disable();
+	/* possible signed integer overflow */
 	fbc->count += amount;
 	preempt_enable();
 }
@@ -154,7 +155,10 @@ static inline s64 percpu_counter_read(struct percpu_counter *fbc)
  */
 static inline s64 percpu_counter_read_positive(struct percpu_counter *fbc)
 {
-	return fbc->count;
+	if (fbc->count >= 0)
+		return fbc->count;
+
+	return 0;
 }
 
 static inline s64 percpu_counter_sum_positive(struct percpu_counter *fbc)
-- 
2.11.0