Re: [LKP] Re: [rcuperf] 4e88ec4a9e: UBSAN:division-overflow_in_arch/x86/include/asm/div64.h

[Rong Chen]

On 8/31/20 11:50 PM, Paul E. McKenney wrote:
> On Mon, Aug 31, 2020 at 08:01:22PM +0800, kernel test robot wrote:
> > Greeting,
> >
> > FYI, we noticed the following commit (built with gcc-9):
> >
> > commit: 4e88ec4a9eb17527e640b063f79e5b875733eb53 ("rcuperf: Change rcuperf to rcuscale")
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> >
> >
> > in testcase: trinity
> > with following parameters:
> >
> > 	runtime: 300s
> >
> > test-description: Trinity is a linux system call fuzz tester.
> > test-url: http://codemonkey.org.uk/projects/trinity/
> >
> >
> > on test machine: qemu-system-i386 -enable-kvm -cpu SandyBridge -smp 2 -m 8G
> >
> > caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
> >
> >
> > +---------------------------------------------------------+------------+------------+
> > |                                                         | 65bd77f554 | 4e88ec4a9e |
> > +---------------------------------------------------------+------------+------------+
> > | boot_successes                                          | 13         | 0          |
> > | boot_failures                                           | 0          | 14         |
> > | UBSAN:division-overflow_in_arch/x86/include/asm/div64.h | 0          | 14         |
> > | error:#[##]                                             | 0          | 14         |
> > | EIP:main_func.cold                                      | 0          | 14         |
> > | Kernel_panic-not_syncing:Fatal_exception                | 0          | 14         |
> > +---------------------------------------------------------+------------+------------+
> >
> >
> > If you fix the issue, kindly add following tag
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Does the patch below fix this for you?

Yes, this patch can fix the issue, and nreaders was adjusted to 1:

[    5.953645] The force parameter has not been set to 1. The Iris 
poweroff handler will not be installed.
[   12.546587] rcu-ref-scale: --- Start of test:  verbose=0 shutdown=1 
holdoff=10 loops=10000 nreaders=-1 nruns=30 readdelay=0
[   12.561495] ------------[ cut here ]------------
[   12.562016] ref_scale_init: nreaders = 0, adjusted to 1
[   12.562601] WARNING: CPU: 0 PID: 1 at kernel/rcu/refscale.c:684 
ref_scale_init+0x653/0x80

Best Regards,
Rong Chen

> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit d301e320e952e2e604d83d9540e52510b0eb3d94
> Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Date:   Thu Aug 27 09:58:19 2020 -0700
>
>      refscale: Bounds-check module parameters
>      
>      The default value for refscale.nreaders is -1, which results in the code
>      setting the value to three-quarters of the number of CPUs.  On single-CPU
>      systems, this results in three-quarters of the value one, which the C
>      language's integer arithmetic rounds to zero.  This in turn results in
>      a divide-by-zero error.
>      
>      This commit therefore adds bounds checking to the refscale module
>      parameters, so that if they are less than one, they are set to the
>      value one.
>      
>      Reported-by: kernel test robot <lkp@xxxxxxxxx>
>      Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
> index 952595c..fb5f20d 100644
> --- a/kernel/rcu/refscale.c
> +++ b/kernel/rcu/refscale.c
> @@ -681,6 +681,12 @@ ref_scale_init(void)
>   	// Reader tasks (default to ~75% of online CPUs).
>   	if (nreaders < 0)
>   		nreaders = (num_online_cpus() >> 1) + (num_online_cpus() >> 2);
> +	if (WARN_ONCE(loops <= 0, "%s: loops = %ld, adjusted to 1\n", __func__, loops))
> +		loops = 1;
> +	if (WARN_ONCE(nreaders <= 0, "%s: nreaders = %d, adjusted to 1\n", __func__, nreaders))
> +		nreaders = 1;
> +	if (WARN_ONCE(nruns <= 0, "%s: nruns = %d, adjusted to 1\n", __func__, nruns))
> +		nruns = 1;
>   	reader_tasks = kcalloc(nreaders, sizeof(reader_tasks[0]),
>   			       GFP_KERNEL);
>   	if (!reader_tasks) {
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