Re: [PATCH v2] The value may overflow

[Mathieu Desnoyers]

On 9/5/23 05:31, David Laight wrote:
> From: Mathieu Desnoyers
> > Sent: 04 September 2023 11:24
> >
> > On 9/4/23 05:42, Denis Arefev wrote:
> > > The value of an arithmetic expression 1 << (cpu - sdp->mynode->grplo)
> > > is subject to overflow due to a failure to cast operands to a larger
> > > data type before performing arithmetic
> >
> > The patch title should identify more precisely its context, e.g.:
> >
> > "srcu: Fix srcu_struct node grpmask overflow on 64-bit systems"
> >
> > Also, as I stated in my reply to the previous version, the patch commit
> > message should describe the impact of the bug it fixes.
>
> And is the analysis complete?
> Is 1UL right for 32bit archs??
> Is 64 bits even enough??

I understand from include/linux/rcu_node_tree.h and kernel/rcu/Kconfig 
RCU_FANOUT and RCU_FANOUT_LEAF ranges that a 32-bit integer is 
sufficient to hold the mask on 32-bit architectures, and a 64-bit 
integer is enough on 64-bit architectures given the current implementation.

At least this appears to be the intent. I did not do a thorough analysis 
of the various parameter limits.

Thanks,

Mathieu

> 	David
>
> > Thanks,
> >
> > Mathieu
> >
> >
> > > Found by Linux Verification Center (linuxtesting.org) with SVACE.
> > >
> > > Signed-off-by: Denis Arefev <arefev@xxxxxxxxx>
> > > ---
> > > v2: Added fixes to the srcu_schedule_cbs_snp function as suggested by
> > > Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > >    kernel/rcu/srcutree.c | 4 ++--
> > >    1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > index 20d7a238d675..6c18e6005ae1 100644
> > > --- a/kernel/rcu/srcutree.c
> > > +++ b/kernel/rcu/srcutree.c
> > > @@ -223,7 +223,7 @@ static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
> > >    				snp->grplo = cpu;
> > >    			snp->grphi = cpu;
> > >    		}
> > > -		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
> > > +		sdp->grpmask = 1UL << (cpu - sdp->mynode->grplo);
> > >    	}
> > >    	smp_store_release(&ssp->srcu_sup->srcu_size_state, SRCU_SIZE_WAIT_BARRIER);
> > >    	return true;
> > > @@ -833,7 +833,7 @@ static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp
> > >    	int cpu;
> > >
> > >    	for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
> > > -		if (!(mask & (1 << (cpu - snp->grplo))))
> > > +		if (!(mask & (1UL << (cpu - snp->grplo))))
> > >    			continue;
> > >    		srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, cpu), delay);
> > >    	}
> >
> > --
> > Mathieu Desnoyers
> > EfficiOS Inc.
> > https://www.efficios.com
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com