Re: UBSAN: Undefined behaviour in kernel/time/tick-sched.c:600:9

From: Dmitry Vyukov
Date: Fri Mar 10 2017 - 16:17:22 EST


On Fri, Mar 10, 2017 at 10:14 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>
>> Cc'ing KASAN folks
>>
>>> (Retry with fixed address ox 86 list)
>>>
>>> This HP Netserver LT6000R, quad P3 Xeon. Trying
>>> 4.11.0-rc1-00088-gec3b93a, I got the following UBSAN warning that was
>>> not there in 4.10:
>>>
>>> [ 14.820437] ================================================================================
>>> [ 14.820534] UBSAN: Undefined behaviour in kernel/time/tick-sched.c:600:9
>>> [ 14.820594] signed integer overflow:
>>> [ 14.820645] 11567464533 + 3985952 cannot be represented in type 'long long int'
>>
>> This does not make any sense:
>>
>> 11567464533 + 3985952 = 0x2b1798055 + 0x3cd220 = 0x2b1b65275
>>
>> which definitely fits into a 'long long int' aka 64 bit
>
>
> Is there a reproducer? Or can you print the values right in
> tick-sched.c with pr_err?
>
> The code in ubsan.c looks more complex than I would expect from that
> code. There is probably some potential for truncation:
>
> static s_max get_signed_val(struct type_descriptor *type, unsigned long val)
> {
> if (is_inline_int(type)) {
> unsigned extra_bits = sizeof(s_max)*8 - type_bit_width(type);
> return ((s_max)val) << extra_bits >> extra_bits;
> }
> if (type_bit_width(type) == 64)
> return *(s64 *)val;
> return *(s_max *)val;
> }


Also what compiler is this?


>>> [ 14.820722] CPU: 3 PID: 135 Comm: startpar Not tainted 4.11.0-rc1-00088-gec3b93a #61
>>> [ 14.820793] Hardware name: Hewlett Packard HP NetServer/HP System Board, BIOS 4.06.46 PW 06/25/2003
>>> [ 14.820863] Call Trace:
>>> [ 14.820950] ? dump_stack+0x45/0x6e
>>> [ 14.821022] ? ubsan_epilogue+0xb/0x40
>>> [ 14.821081] ? handle_overflow+0xb6/0xe0
>>> [ 14.821146] ? smp_trace_apic_timer_interrupt+0x85/0x100
>>> [ 14.821208] ? __ubsan_handle_add_overflow+0xf/0x20
>>> [ 14.821281] ? get_cpu_idle_time_us+0x204/0x2b0
>>> [ 14.821350] ? get_idle_time+0x57/0xb0
>>> [ 14.821405] ? show_stat+0x217/0x10c0
>>> [ 14.821463] ? seq_open+0x33/0xb0
>>> [ 14.821519] ? __d_alloc+0x149/0x2c0
>>> [ 14.821574] ? get_iowait_time+0xb0/0xb0
>>> [ 14.821628] ? single_open+0x4d/0xb0
>>> [ 14.821685] ? single_open_size+0x37/0xc0
>>> [ 14.821748] ? path_openat+0x7d5/0x1bd0
>>> [ 14.821804] ? seq_read+0xca/0x6f0
>>> [ 14.821872] ? _copy_to_user+0x5e/0x90
>>> [ 14.821937] ? cp_new_stat64+0x108/0x130
>>> [ 14.821991] ? __seq_open_private+0x80/0x80
>>> [ 14.822062] ? proc_reg_read+0x7c/0x120
>>> [ 14.822117] ? proc_reg_write+0x120/0x120
>>> [ 14.822172] ? __vfs_read+0x3e/0x240
>>> [ 14.822228] ? vfs_read+0x92/0x1b0
>>> [ 14.822282] ? SyS_read+0x45/0xa0
>>> [ 14.822338] ? do_fast_syscall_32+0xb9/0x220
>>> [ 14.822395] ? entry_SYSENTER_32+0x47/0x71
>>> [ 14.822447]
>>> ================================================================================