Re: [Linux-kernel-mentees] [PATCH net 2/2 RESEND] ipmr: Add lockdep expression to ipmr_for_each_table macro

[Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>]

On Sat, May 09, 2020 at 02:19:38PM -0700, Jakub Kicinski wrote:
> On Sat,  9 May 2020 12:52:44 +0530 Amol Grover wrote:
> > ipmr_for_each_table() uses list_for_each_entry_rcu() for
> > traversing outside of an RCU read-side critical section but
> > under the protection of pernet_ops_rwsem. Hence add the
> > corresponding lockdep expression to silence the following
> > false-positive warning at boot:
> 
> Thanks for the fix, the warning has been annoying me as well!
> 
> > [    0.645292] =============================
> > [    0.645294] WARNING: suspicious RCU usage
> > [    0.645296] 5.5.4-stable #17 Not tainted
> > [    0.645297] -----------------------------
> > [    0.645299] net/ipv4/ipmr.c:136 RCU-list traversed in non-reader section!!
> 
> please provide a fuller stack trace, it would have helped the review
> 
> > Signed-off-by: Amol Grover <frextrite@gmail.com>
> > ---
> >  net/ipv4/ipmr.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
> > index 99c864eb6e34..950ffe9943da 100644
> > --- a/net/ipv4/ipmr.c
> > +++ b/net/ipv4/ipmr.c
> > @@ -109,9 +109,10 @@ static void mroute_clean_tables(struct mr_table *mrt, int flags);
> >  static void ipmr_expire_process(struct timer_list *t);
> >  
> >  #ifdef CONFIG_IP_MROUTE_MULTIPLE_TABLES
> > -#define ipmr_for_each_table(mrt, net) \
> > -	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
> > -				lockdep_rtnl_is_held())
> > +#define ipmr_for_each_table(mrt, net)					\
> > +	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list,	\
> > +				lockdep_rtnl_is_held() ||		\
> > +				lockdep_is_held(&pernet_ops_rwsem))
> 
> This is a strange condition, IMHO. How can we be fine with either
> lock.. This is supposed to be the writer side lock, one can't have 
> two writer side locks..
> 
> I think what is happening is this:
> 
> ipmr_net_init() -> ipmr_rules_init() -> ipmr_new_table()
> 
> ipmr_new_table() returns an existing table if there is one, but
> obviously none can exist at init.  So a better fix would be:
> 
> #define ipmr_for_each_table(mrt, net)					\
> 	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list,	\
> 				lockdep_rtnl_is_held() ||		\
> 				list_empty(&net->ipv4.mr_tables))
>
(adding Stephen)

Hi Jakub,

Thank you for your suggestion about this patch.
Here is a stack trace for ipmr.c:

[    1.515015] TCP: Hash tables configured (established 8192 bind 8192)
[    1.516790] UDP hash table entries: 512 (order: 3, 49152 bytes, linear)
[    1.518177] UDP-Lite hash table entries: 512 (order: 3, 49152 bytes, linear)
[    1.519805]
[    1.520178] =============================
[    1.520982] WARNING: suspicious RCU usage
[    1.521798] 5.7.0-rc2-00006-gb35af6a26b7c6f #1 Not tainted
[    1.522910] -----------------------------
[    1.523671] net/ipv4/ipmr.c:136 RCU-list traversed in non-reader section!!
[    1.525218]
[    1.525218] other info that might help us debug this:
[    1.525218]
[    1.526731]
[    1.526731] rcu_scheduler_active = 2, debug_locks = 1
[    1.528004] 1 lock held by swapper/1:
[    1.528714]  #0: c20be1d8 (pernet_ops_rwsem){+.+.}-{3:3}, at: register_pernet_subsys+0xd/0x30
[    1.530433]
[    1.530433] stack backtrace:
[    1.531262] CPU: 0 PID: 1 Comm: swapper Not tainted 5.7.0-rc2-00006-gb35af6a26b7c6f #1
[    1.532729] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
[    1.534305] Call Trace:
[    1.534758]  ? ipmr_get_table+0x3c/0x70
[    1.535430]  ? ipmr_new_table+0x1c/0x60
[    1.536173]  ? ipmr_net_init+0x7b/0x170
[    1.536923]  ? register_pernet_subsys+0xd/0x30
[    1.537810]  ? ops_init+0x1a0/0x1e0
[    1.538518]  ? kmem_cache_create_usercopy+0x28a/0x350
[    1.539752]  ? register_pernet_operations+0xc9/0x1c0
[    1.540630]  ? ipv4_offload_init+0x65/0x65
[    1.541451]  ? register_pernet_subsys+0x19/0x30
[    1.542357]  ? ip_mr_init+0x28/0xff
[    1.543079]  ? inet_init+0x17b/0x249
[    1.543773]  ? do_one_initcall+0xc5/0x240
[    1.544532]  ? parse_args+0x192/0x350
[    1.545266]  ? rcu_read_lock_sched_held+0x2f/0x60
[    1.546180]  ? trace_initcall_level+0x61/0x93
[    1.547061]  ? kernel_init_freeable+0x112/0x18a
[    1.547978]  ? kernel_init_freeable+0x12b/0x18a
[    1.548974]  ? rest_init+0x220/0x220
[    1.549792]  ? kernel_init+0x8/0x100
[    1.550548]  ? rest_init+0x220/0x220
[    1.551288]  ? schedule_tail_wrapper+0x6/0x8
[    1.552136]  ? rest_init+0x220/0x220
[    1.552873]  ? ret_from_fork+0x2e/0x38

ALso, there is a similar warning for ip6mr.c :

=============================
WARNING: suspicious RCU usage
5.7.0-rc4-next-20200507-syzkaller #0 Not tainted
-----------------------------
net/ipv6/ip6mr.c:124 RCU-list traversed in non-reader section!!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by swapper/0/1:
#0: ffffffff8a7aae30 (pernet_ops_rwsem){+.+.}-{3:3}, at: register_pernet_subsys+0x16/0x40 net/core/net_namespace.c:1257

stack backtrace:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.7.0-rc4-next-20200507-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x18f/0x20d lib/dump_stack.c:118
ip6mr_get_table+0x153/0x180 net/ipv6/ip6mr.c:124
ip6mr_new_table+0x1b/0x70 net/ipv6/ip6mr.c:382
ip6mr_rules_init net/ipv6/ip6mr.c:236 [inline]
ip6mr_net_init+0x133/0x3f0 net/ipv6/ip6mr.c:1310
ops_init+0xaf/0x420 net/core/net_namespace.c:151
__register_pernet_operations net/core/net_namespace.c:1140 [inline]
register_pernet_operations+0x346/0x840 net/core/net_namespace.c:1217
register_pernet_subsys+0x25/0x40 net/core/net_namespace.c:1258
ip6_mr_init+0x49/0x152 net/ipv6/ip6mr.c:1363
inet6_init+0x1d7/0x6dc net/ipv6/af_inet6.c:1037
do_one_initcall+0x10a/0x7d0 init/main.c:1159
do_initcall_level init/main.c:1232 [inline]
do_initcalls init/main.c:1248 [inline]
do_basic_setup init/main.c:1268 [inline]
kernel_init_freeable+0x501/0x5ae init/main.c:1454
kernel_init+0xd/0x1bb init/main.c:1359
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:351
Segment Routing with IPv6
mip6: Mobile IPv6
sit: IPv6, IPv4 and MPLS over IPv4 tunneling driver
ip6_gre: GRE over IPv6 tunneling driver

> Thoughts?

Do you think a similar fix (the one you suggested) is also applicable
in the ip6mr case.

Thank you,
Madhuparna
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