linux-kernel - Re: [PATCH] net/mlx5e: Return in default case statement in tx_post_resync

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Message-ID: <20190710042453.GZ7034@mtr-leonro.mtl.com>
Date:   Wed, 10 Jul 2019 07:24:53 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        Boris Pismenny <borisp@...lanox.com>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] net/mlx5e: Return in default case statement in
 tx_post_resync_params

On Tue, Jul 09, 2019 at 04:10:24PM -0700, Nathan Chancellor wrote:
> On Tue, Jul 09, 2019 at 03:44:59PM -0700, Nick Desaulniers wrote:
> > On Mon, Jul 8, 2019 at 4:13 PM Nathan Chancellor
> > <natechancellor@...il.com> wrote:
> > >
> > > clang warns:
> > >
> > > drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls_tx.c:251:2:
> > > warning: variable 'rec_seq_sz' is used uninitialized whenever switch
> > > default is taken [-Wsometimes-uninitialized]
> > >         default:
> > >         ^~~~~~~
> > > drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls_tx.c:255:46: note:
> > > uninitialized use occurs here
> > >         skip_static_post = !memcmp(rec_seq, &rn_be, rec_seq_sz);
> > >                                                     ^~~~~~~~~~
> > > drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls_tx.c:239:16: note:
> > > initialize the variable 'rec_seq_sz' to silence this warning
> > >         u16 rec_seq_sz;
> > >                       ^
> > >                        = 0
> > > 1 warning generated.
> > >
> > > This case statement was clearly designed to be one that should not be
> > > hit during runtime because of the WARN_ON statement so just return early
> > > to prevent copying uninitialized memory up into rn_be.
> > >
> > > Fixes: d2ead1f360e8 ("net/mlx5e: Add kTLS TX HW offload support")
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/590
> > > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > > ---
> > >  drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls_tx.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls_tx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls_tx.c
> > > index 3f5f4317a22b..5c08891806f0 100644
> > > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls_tx.c
> > > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls_tx.c
> > > @@ -250,6 +250,7 @@ tx_post_resync_params(struct mlx5e_txqsq *sq,
> > >         }
> > >         default:
> > >                 WARN_ON(1);
> > > +               return;
> > >         }
> >
> > hmm...a switch statement with a single case is a code smell.  How
> > about a single conditional with early return?  Then the "meat" of the
> > happy path doesn't need an additional level of indentation.
> > --
> > Thanks,
> > ~Nick Desaulniers
>
> I assume that the reason for this is there may be other cipher types
> added in the future? I suppose the maintainers can give more clarity to
> that.

Our devices supports extra ciphers, for example TLS_CIPHER_AES_GCM_256.
So I assume this was the reason for switch<->case, but because such
implementation doesn't exist in any driver, I recommend to rewrite the
code to have "if" statement and return early.

>
> Furthermore, if they want the switch statements to remain, it looks like
> fill_static_params_ctx also returns in the default statement so it seems
> like this is the right fix.
>
> Cheers,
> Nathan