Re: [PATCH 2/2] iommu/vt-d: Use BUG_ON to check NULL value of 'table'

[Baolu Lu]

On 5/31/23 3:09 PM, Yanfei Xu wrote:
> On 5/31/2023 11:26 AM, Baolu Lu wrote:
> > On 5/30/23 5:25 PM, Yanfei Xu wrote:
> > > Checking NULL value of 'table' variable deserves a BUG_ON as the
> > > following code will trigger a crash by dereferencing the NULL
> > > 'table' pointer. Crash in advance with BUG_ON to avoid WARN_ON
> > > plus NULL pointer dereferencing can simplify the crash log.
> > >
> > > Signed-off-by: Yanfei Xu<yanfei.xu@xxxxxxxxx>
> > > ---
> > >   drivers/iommu/intel/iommu.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> > > index e98f1b122b49..8aa3bfdb7f95 100644
> > > --- a/drivers/iommu/intel/iommu.c
> > > +++ b/drivers/iommu/intel/iommu.c
> > > @@ -1944,7 +1944,7 @@ static int domain_context_mapping_one(struct 
> > > dmar_domain *domain,
> > >       if (sm_supported(iommu)) {
> > >           unsigned long pds;
> > >   -        WARN_ON(!table);
> > > +        BUG_ON(!table);
> >
> > BUG_ON() is not recommended. Perhaps,
> >
> >         if (!table)
> >             -ENODEV;
> Agree:) It is always better to handle the error than crash kernel.
>
> How about:
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 8096273b034c..7f077e3a4128 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -1944,7 +1944,10 @@ static int domain_context_mapping_one(struct 
> dmar_domain *domain,
>          if (sm_supported(iommu)) {
>                  unsigned long pds;
>
> -               WARN_ON(!table);
> +               if (WARN_ON(!table)) {
> +                       ret = -ENODEV;
> +                       goto out_unlock;
> +               }

I'd recommend to remove this line directly. This pointer will be
accessed in the following code, if empty "table" really happens, the
kernel will report a NULL pointer reference warning at the first place.

In the same function, I also saw "WARN_ON(did == 0);". It's unnecessary
as domain_id_iommu() will never return 0. Perhaps we can clean it up as
well.

Best regards,
baolu