Re: [PATCH v2 net 13/17] net: Fix data-races around sysctl_fb_tunnels_only_for_init_net.

Kuniyuki Iwashima
2022-08-18T16:41:34Z

From: Nathan Chancellor <nathan@kernel.org>
Date: Thu, 18 Aug 2022 09:23:19 -0700
> On Thu, Aug 18, 2022 at 08:01:54AM -0700, Kuniyuki Iwashima wrote:
> > From: kernel test robot <lkp@intel.com>
> > Date: Thu, 18 Aug 2022 16:51:37 +0800
> > > Hi Kuniyuki,
> > >
> > > Thank you for the patch! Yet something to improve:
> > >
> > > [auto build test ERROR on net/master]
> > >
> > > url: https://github.com/intel-lab-lkp/linux/commits/Kuniyuki-Iwashima/net-sysctl-Fix-data-races-around-net-core-XXX/20220818-115941
> > > base: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git fc4aaf9fb3c99bcb326d52f9d320ed5680bd1cee
> > > config: riscv-randconfig-r032-20220818 (https://download.01.org/0day-ci/archive/20220818/202208181615.Lu9xjiEv-lkp@intel.com/config)
> > > compiler: clang version 16.0.0 (https://github.com/llvm/llvm-project aed5e3bea138ce581d682158eb61c27b3cfdd6ec)
> > > reproduce (this is a W=1 build):
> > > wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> > > chmod +x ~/bin/make.cross
> > > # install riscv cross compiling tool for clang build
> > > # apt-get install binutils-riscv64-linux-gnu
> > > # https://github.com/intel-lab-lkp/linux/commit/6bc3dfb3dc4862f4e00ba93c45cd5c0251b85d5b
> > > git remote add linux-review https://github.com/intel-lab-lkp/linux
> > > git fetch --no-tags linux-review Kuniyuki-Iwashima/net-sysctl-Fix-data-races-around-net-core-XXX/20220818-115941
> > > git checkout 6bc3dfb3dc4862f4e00ba93c45cd5c0251b85d5b
> > > # save the config file
> > > mkdir build_dir && cp config build_dir/.config
> > > COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=riscv SHELL=/bin/bash
> > >
> > > If you fix the issue, kindly add following tag where applicable
> > > Reported-by: kernel test robot <lkp@intel.com>
> > >
> > > All errors (new ones prefixed by >>):
> > >
> > > >> ld.lld: error: undefined symbol: sysctl_fb_tunnels_only_for_init_net
> > > >>> referenced by ip_tunnel.c
> > > >>> ipv4/ip_tunnel.o:(ip_tunnel_init_net) in archive net/built-in.a
> > > >>> referenced by ip_tunnel.c
> > > >>> ipv4/ip_tunnel.o:(ip_tunnel_init_net) in archive net/built-in.a
> >
> > Hmm... I tested allmodconfig with x86_64 but it seems not enough...
> >
> > I don't think just using READ_ONCE() causes regression.
> > Is this really regression or always-broken stuff in some arch,
> > or ... clang 16?
> >
> > Anyway, I'll take a look.
>
> You'll see the same error with the same configuration and GCC:
>
> riscv64-linux-gnu-ld: net/ipv4/ip_tunnel.o: in function `.L536':
> ip_tunnel.c:(.text+0x1d4a): undefined reference to `sysctl_fb_tunnels_only_for_init_net'
> riscv64-linux-gnu-ld: ip_tunnel.c:(.text+0x1d4e): undefined reference to `sysctl_fb_tunnels_only_for_init_net'
>
> $ scripts/config --file build/riscv/.config -s SYSCTL
> undef
>
> Prior to your change, the '!IS_ENABLED(CONFIG_SYSCTL)' would cause
> net_has_fallback_tunnels() to unconditionally 'return 1' in the
> CONFIG_SYSCTL=n case,

Yes, you are right.
I've just noticed it and this fixed the error.
Also, I did the same mistake in the 14th patch...
Thank you, Nathan!

---8<---
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 2563d30736e9..78dd63a5c7c8 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -640,9 +640,14 @@ extern int sysctl_devconf_inherit_init_net;
*/
static inline bool net_has_fallback_tunnels(const struct net *net)
{
- return !IS_ENABLED(CONFIG_SYSCTL) ||
- !sysctl_fb_tunnels_only_for_init_net ||
- (net == &init_net && sysctl_fb_tunnels_only_for_init_net == 1);
+#if IS_ENABLED(CONFIG_SYSCTL)
+ int fb_tunnels_only_for_init_net = READ_ONCE(sysctl_fb_tunnels_only_for_init_net);
+
+ return !fb_tunnels_only_for_init_net ||
+ (net_eq(net, &init_net) && fb_tunnels_only_for_init_net == 1);
+#else
+ return true;
+#endif
}

static inline int netdev_queue_numa_node_read(const struct netdev_queue *q)
---8<---

> meaning 'fb_tunnels_only_for_init_net' was never
> emitted in the final assembly so the linker would not complain about it
> never being defined (the kernel relies on this trick a lot, which is why
> you cannot build the kernel with -O0).
>
> After your change, sysctl_fb_tunnels_only_for_init_net is
> unconditionally used but it is only defined in sysctl_net_core.c, which
> is only built when CONFIG_SYSCTL=y, hence the link error.
>
> I suspect hoisting '!IS_ENABLED(CONFIG_SYSCTL)' out of the return into
> its own conditional would fix the error:
>
> static inline bool net_has_fallback_tunnels(const struct net *net)
> {
> int fb_tunnels_only_for_init_net;
>
> if (!IS_ENABLED(CONFIG_SYSCTL))
> return true;
>
> fb_tunnels_only_for_init_net = READ_ONCE(sysctl_fb_tunnels_only_for_init_net);
>
> return !fb_tunnels_only_for_init_net ||
> (net == &init_net && fb_tunnels_only_for_init_net == 1)
> }
>
> Cheers,
> Nathan