linux-kernel - Re: [PATCH] IB/mlx4: Avoid implicit enumerated type conversion

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Message-ID: <20180927010803.GA7640@flashbox>
Date:   Wed, 26 Sep 2018 18:08:03 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>, dledford@...hat.com,
        linux-rdma@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] IB/mlx4: Avoid implicit enumerated type conversion

On Mon, Sep 24, 2018 at 08:37:22PM -0600, Jason Gunthorpe wrote:
> On Mon, Sep 24, 2018 at 03:29:38PM -0700, Nick Desaulniers wrote:
> > On Mon, Sep 24, 2018 at 3:27 PM Nathan Chancellor
> > <natechancellor@...il.com> wrote:
> > >
> > > On Mon, Sep 24, 2018 at 03:24:36PM -0700, Nick Desaulniers wrote:
> > > > On Mon, Sep 24, 2018 at 12:57 PM Nathan Chancellor
> > > > <natechancellor@...il.com> wrote:
> > > > >
> > > > > Clang warns when one enumerated type is implicitly converted to another.
> > > > >
> > > > > drivers/infiniband/hw/mlx4/mad.c:1811:41: warning: implicit conversion
> > > > > from enumeration type 'enum mlx4_ib_qp_flags' to different enumeration
> > > > > type 'enum ib_qp_create_flags' [-Wenum-conversion]
> > > > >                 qp_init_attr.init_attr.create_flags = MLX4_IB_SRIOV_TUNNEL_QP;
> > > > >                                                     ~ ^~~~~~~~~~~~~~~~~~~~~~~
> > > > >
> > > > > drivers/infiniband/hw/mlx4/mad.c:1819:41: warning: implicit conversion
> > > > > from enumeration type 'enum mlx4_ib_qp_flags' to different enumeration
> > > > > type 'enum ib_qp_create_flags' [-Wenum-conversion]
> > > > >                 qp_init_attr.init_attr.create_flags = MLX4_IB_SRIOV_SQP;
> > > > >                                                     ~ ^~~~~~~~~~~~~~~~~
> > > > >
> > > > > The type mlx4_ib_qp_flags explicitly provides supplemental values to the
> > > > > type ib_qp_create_flags. Make that clear to Clang by changing the
> > > > > create_flags type to u32.
> > > > >
> > > > > Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > > > > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > > > >  include/rdma/ib_verbs.h | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/include/rdma/ib_verbs.h b/include/rdma/ib_verbs.h
> > > > > index e463d3007a35..f6f4d9e3c8ed 100644
> > > > > +++ b/include/rdma/ib_verbs.h
> > > > > @@ -1149,7 +1149,7 @@ struct ib_qp_init_attr {
> > > > >         struct ib_qp_cap        cap;
> > > > >         enum ib_sig_type        sq_sig_type;
> > > > >         enum ib_qp_type         qp_type;
> > > > > -       enum ib_qp_create_flags create_flags;
> > > > > +       u32                     create_flags;
> > > >
> > > > I think it might be better to just have explicit casts at the
> > > > assignment.  What do the maintainers think?
> > > >
> > >
> > > That's fine with me, I tend to explicitly cast if it is only one
> > > location but it certainly makes sense in this case as well. I'll
> > > wait for the maintainers to weigh in before sending a v2.
> > 
> > Yeah, I mean my opinion on this might seem arbitrary, but based on the
> > pattern and the comment in ib_qp_create_flags, those enum values are
> > reserved to be "subclassed" in a sense, so they should always be in
> > sync or this code will have bigger problems.
> 
> One should not use an 'enum' type name for bitfield storage, as once
> you start or'ing things together the values no longer fall on the
> enum. Some compilers and tools even give warnings in this case, ie
> 
>    enum x foo = X_A | X_B;
> 
> Is an assignment from 'int' to an 'enum x' with an implicit cast.
> 
> For this reason, usually bitfield enum declarations should be
> anonymous.
> 
> Jason

Hi Jason,

I apologize for not understanding but how should I adjust my patch so
that it is acceptable? Do you want the explicit casts like Nick
suggested?

Thanks for the comments,
Nathan