From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Marzinski Subject: Re: [PATCH 08/35] libmultipath: create bitfield abstraction Date: Tue, 4 Aug 2020 11:26:35 -0500 Message-ID: <20200804162635.GJ19233@octiron.msp.redhat.com> References: <20200709101620.6786-1-mwilck@suse.com> <20200709101620.6786-9-mwilck@suse.com> <20200716211708.GM11089@octiron.msp.redhat.com> <2ae4b38b8f07eb1ac9be31099b5be091fa6e9617.camel@suse.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com Content-Disposition: inline To: Martin Wilck Cc: dm-devel@redhat.com List-Id: dm-devel.ids On Tue, Aug 04, 2020 at 05:18:18PM +0200, Martin Wilck wrote: > On Tue, 2020-08-04 at 17:04 +0200, Martin Wilck wrote: > > On Thu, 2020-07-16 at 16:17 -0500, Benjamin Marzinski wrote: > > > On Thu, Jul 09, 2020 at 12:15:53PM +0200, mwilck@suse.com wrote: > > > > From: Martin Wilck > > > > +struct bitfield *alloc_bitfield(unsigned int maxbit) > > > > +{ > > > > + unsigned int n; > > > > + struct bitfield *bf; > > > > + > > > > + n = maxbit > 0 ? (maxbit - 1) / bits_per_slot + 1 : 0; > > > > > > What's the point in accepting 0? That's an empty bitmap. > > > > > Thanks for spotting these, I will fix them. > > Thinking about it once more, I believe that accepting 0 as the bitfield > length is actually the right thing. A bitfield of length 0 makes not > much less sense than one of length 1. The code makes sure that the bit > operations on the 0-length bitfield behave correctly (see > test_bitmask_len_0()). Thus callers can use bitfields without bothering > for extra NULL checks. That was the intention. Like we support 0-length > vectors. But the calloc call itself can return NULL, so deferencing bf (as in bf->len = maxbit), can crash. I'm also still fuzzy on why we want to support zero length bitfields. Since they can't be grown like vectors can, it seem like requesting a zero length bitfield will always be a sign of a coding error. We would get a more useful error by having the failure happen closer to the error in the code. Or is there actually a use for a zero length bitfield that can't be grown? -Ben > Regards, > Martin >