[dm-devel] [PATCH 08/35] libmultipath: create bitfield abstraction

[Benjamin Marzinski]

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 at suse.com wrote:
> > > > From: Martin Wilck <mwilck at suse.com>
> > > > +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
>