UBSAN: Undefined behaviour in ../include/linux/percpu_counter.h:137:13

[Michael Ellerman]

Mathieu Malaterre <malat at debian.org> writes:

> Hi there,
>
> I have a reproducible UBSAN appearing in dmesg after a while on my G4
> (*). Could anyone suggest a way to diagnose the actual root issue here
> (or is it just a false positive) ?

It looks like a real overflow, I guess the question is why are we seeing it.

The first thing to work out would be what exactly is overflowing.

Is it in here?

	cfqg_stats_update_completion(cfqq->cfqg, rq->start_time_ns,
				     rq->io_start_time_ns, rq->cmd_flags);


If so that would suggest something is taking multiple hours to complete,
which seems unlikely. Is time going backward?

cheers

> (*)
> [41877.514338] ================================================================================
> [41877.514364] UBSAN: Undefined behaviour in
> ../include/linux/percpu_counter.h:137:13
> [41877.514373] signed integer overflow:
> [41877.514378] 9223352809007201260 + 41997676517838 cannot be
> represented in type 'long long int'
> [41877.514389] CPU: 0 PID: 0 Comm: swapper Not tainted 4.17.0+ #54
> [41877.514394] Call Trace:
> [41877.514411] [dffedd30] [c047a5f8] ubsan_epilogue+0x18/0x4c (unreliable)
> [41877.514422] [dffedd40] [c047af98] handle_overflow+0xbc/0xdc
> [41877.514437] [dffeddc0] [c043aaa8] cfq_completed_request+0x560/0x1234
> [41877.514446] [dffede40] [c03f595c] __blk_put_request+0xb0/0x2dc
> [41877.514460] [dffede80] [c05aa41c] scsi_end_request+0x19c/0x344
> [41877.514469] [dffedeb0] [c05abba0] scsi_io_completion+0x4b4/0x854
> [41877.514482] [dffedf10] [c040604c] blk_done_softirq+0xe4/0x1e0
> [41877.514496] [dffedf60] [c07eef84] __do_softirq+0x16c/0x5f0
> [41877.514508] [dffedfd0] [c0065160] irq_exit+0x110/0x1a8
> [41877.514520] [dffedff0] [c001646c] call_do_irq+0x24/0x3c
> [41877.514533] [c0ce5e80] [c0009a2c] do_IRQ+0x98/0x1a0
> [41877.514541] [c0ce5eb0] [c001b93c] ret_from_except+0x0/0x14
> [41877.514549] --- interrupt: 501 at arch_cpu_idle+0x30/0x78
>                    LR = arch_cpu_idle+0x30/0x78
> [41877.514558] [c0ce5f70] [c0ce4000] 0xc0ce4000 (unreliable)
> [41877.514570] [c0ce5f80] [c00a3928] do_idle+0xc4/0x158
> [41877.514577] [c0ce5fb0] [c00a3b74] cpu_startup_entry+0x24/0x28
> [41877.514585] [c0ce5fc0] [c0988820] start_kernel+0x47c/0x490
> [41877.514592] [c0ce5ff0] [00003444] 0x3444
> [41877.514597] ================================================================================
> [41886.390210] ================================================================================
> [41886.390236] UBSAN: Undefined behaviour in
> ../include/linux/percpu_counter.h:137:13
> [41886.390245] signed integer overflow:
> [41886.390250] 9223366156262940402 + 42006563339289 cannot be
> represented in type 'long long int'
> [41886.390260] CPU: 0 PID: 0 Comm: swapper Not tainted 4.17.0+ #54
> [41886.390265] Call Trace:
> [41886.390282] [dffedd30] [c047a5f8] ubsan_epilogue+0x18/0x4c (unreliable)
> [41886.390293] [dffedd40] [c047af98] handle_overflow+0xbc/0xdc
> [41886.390309] [dffeddc0] [c043a8c4] cfq_completed_request+0x37c/0x1234
> [41886.390317] [dffede40] [c03f595c] __blk_put_request+0xb0/0x2dc
> [41886.390331] [dffede80] [c05aa41c] scsi_end_request+0x19c/0x344
> [41886.390340] [dffedeb0] [c05abba0] scsi_io_completion+0x4b4/0x854
> [41886.390353] [dffedf10] [c040604c] blk_done_softirq+0xe4/0x1e0
> [41886.390367] [dffedf60] [c07eef84] __do_softirq+0x16c/0x5f0
> [41886.390379] [dffedfd0] [c0065160] irq_exit+0x110/0x1a8
> [41886.390391] [dffedff0] [c001646c] call_do_irq+0x24/0x3c
> [41886.390404] [c0ce5e80] [c0009a2c] do_IRQ+0x98/0x1a0
> [41886.390411] [c0ce5eb0] [c001b93c] ret_from_except+0x0/0x14
> [41886.390420] --- interrupt: 501 at arch_cpu_idle+0x30/0x78
>                    LR = arch_cpu_idle+0x30/0x78
> [41886.390429] [c0ce5f70] [c0ce4000] 0xc0ce4000 (unreliable)
> [41886.390441] [c0ce5f80] [c00a3928] do_idle+0xc4/0x158
> [41886.390449] [c0ce5fb0] [c00a3b74] cpu_startup_entry+0x24/0x28
> [41886.390457] [c0ce5fc0] [c0988820] start_kernel+0x47c/0x490
> [41886.390463] [c0ce5ff0] [00003444] 0x3444
> [41886.390468] ================================================================================