From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Bergmann Subject: Re: [PATCH] [net] net/mlx5e: fix another -Wmaybe-uninitialized warning Date: Thu, 12 Jan 2017 17:04:43 +0100 Message-ID: <2981112.7jTLlX72ae@wuerfel> References: <20170111211451.2705705-1-arnd@arndb.de> <46a85790-2cfe-a8d9-f764-4f736fbd1af7@mellanox.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Cc: Saeed Mahameed , Hadar Hen Zion , "David S . Miller" , netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: Or Gerlitz Return-path: In-Reply-To: <46a85790-2cfe-a8d9-f764-4f736fbd1af7@mellanox.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Thursday, January 12, 2017 5:21:49 PM CET Or Gerlitz wrote: > On 1/11/2017 11:14 PM, Arnd Bergmann wrote: > > As found by Olof's build bot, today's mainline kernel gained a harmless > > warning about a potential uninitalied variable reference: > > > > drivers/net/ethernet/mellanox/mlx5/core/en_tc.c: In function 'parse_tc_fdb_actions': > > drivers/net/ethernet/mellanox/mlx5/core/en_tc.c:769:13: warning: 'out_dev' may be used uninitialized in this function [-Wmaybe-uninitialized] > > drivers/net/ethernet/mellanox/mlx5/core/en_tc.c:811:21: note: 'out_dev' was declared here > > > > This was introduced through the addition of an 'IS_ERR/PTR_ERR' pair that > > gcc is unfortunately unable to completely figure out. Replacing it with > > PTR_ERR_OR_ZERO makes the code more understandable to gcc so it no longer > > warns. > > can you elaborate on this a little further? The problem is static int mlx5e_route_lookup_ipv4(struct net_device **out_dev, ...) { ... if (IS_ERR(rt)) return PTR_ERR(rt); *out_dev = ...; ... } static int mlx5e_create_encap_header_ipv4(...) { ... err = mlx5e_route_lookup_ipv4(..., out_dev, ...); if (err) goto out; e->out_dev = *out_dev; ... } I've seen several examples of this, the problem every time is that gcc cannot tell that if(IS_ERR()) in the first function is equivalent to if(err) in the second, so it assumes that 'out_dev' is used here after the first 'return PTR_ERR(rt)'. The PTR_ERR_OR_ZERO() case by comparison is fairly easy to detect by gcc, so it can't get that wrong here. > > Hadar Hen Zion already attempted to fix the warning earlier by adding > > fake initializations, but that ended up just making the code worse without > > fully addressing all warnings, so I'm reverting it now that it is no longer needed. > > ok, so if your approach eliminates the warning on out_dev and also on > the variables for which Hadar added the faked initializers, I guess we > should be fine with this change (saw your reply on my other comment), Ok. > just another question: > > > In order to avoid pulling a variable declaration into the #ifdef, I'm > > removing it in favor of a more readable 'if()' statement here that has the same effect. > > When I build here without CONFIG_INET in my system, the build goes fine > with this approach. However, we're pretty sure that in the past we got > 0-day report from the kbuild test robot where he was unhappy that we > make the ip_route_output_key call without being wrapped with that #if > IS_ENABLED(CONFIG_INET) -- so, we don't want to go there again... thoughts? I went back and forth between the two versions, either leaving the #if in place, or using the if(IS_ENABLED()) check to be really sure that we can't get compile error here. I did check that ip_route_output_key() is always declared, but now I see that net/route.h might not always be included from en_tc.c if CONFIG_INET is disabled (I don't see how it gets included, but it obviously is when CONFIG_INET is turned on). Adding an explicit include of that file should probably avoid the case you ran into earlier, but for I agree it's safer to not rely on that here for a bugfix, and just leave the #ifdef. Do you want to modify it yourself, or should I spin a new version with that? Arnd