Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 2 Jun 2023 11:32:35 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Baolu Lu <baolu.lu@linux.intel.com>
Cc:     Yanfei Xu <yanfei.xu@intel.com>, dwmw2@infradead.org,
        joro@8bytes.org, will@kernel.org, robin.murphy@arm.com,
        iommu@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] iommu/vt-d: Use BUG_ON to check NULL value of 'table'
Message-ID: <ZHn9g6QoMUo3ZPog@ziepe.ca>
References: <20230530092503.152926-1-yanfei.xu@intel.com>
 <20230530092503.152926-3-yanfei.xu@intel.com>
 <3e75f9de-4c67-93aa-9fb4-28da9849dd48@linux.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <3e75f9de-4c67-93aa-9fb4-28da9849dd48@linux.intel.com>
Precedence: bulk

On Wed, May 31, 2023 at 11:26:55AM +0800, 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@intel.com>
> > ---
> >   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;
> 
> ?

If it is not something you think needs active debugging then just let
it crash on the NULL pointer deref. You get a nice backtrace from that
already.

The additional value of the WARN is it gives you a line number, a
prettier print and it might recover more cleanly (or not, it might
just crash somewhere else).

Jason