[PATCH] rcu: fix crash when reading rcudata debugfs node

[Leon Yang]

With CONFIG_RCU_TRACE=y set, reading the rcu/<rcu_flavor>/rcudata file
in debugfs will leads to a segmentation fault.

dmesg reports general protection fault with the following message:

task: ffffa0e715959d00 task.stack: ffffc117c7390000
RIP: 0010:show_rcudata+0x4e/0x1e0
RSP: 0018:ffffc117c7393d50 EFLAGS: 00010282
RAX: ffffa0e8d7212098 RBX: ffffa0e8d721cd00 RCX: ffffa0e8d7200000
RDX: 0000000000000000 RSI: 0000000000000e6c RDI: 0000000000000000
RBP: ffffc117c7393d90 R08: 0000000000000000 R09: 0000000000000fff
R10: ffffa0e8bf464000 R11: ffffa0e715959d00 R12: ffffa0e81ecf4100
R13: ffffa0e8c51fcd00 R14: ffffa0e8d721cd00 R15: ffffa0e81ecf4100
FS:  00007fea5a064700(0000) GS:ffffa0e8d7440000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fea5a041008 CR3: 00000006035f1000 CR4: 00000000000406e0
Call Trace:
 seq_read+0x9e/0x440
 ? lru_cache_add_active_or_unevictable+0x36/0xb0
 full_proxy_read+0x54/0x90
 __vfs_read+0x37/0x150
 ? security_file_permission+0x9b/0xc0
 vfs_read+0x93/0x130
 SyS_read+0x55/0xc0
 entry_SYSCALL_64_fastpath+0x1e/0xa9
RIP: 0033:0x7fea59b89230
RSP: 002b:00007ffca86f0418 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00007fea59e56b20 RCX: 00007fea59b89230
RDX: 0000000000020000 RSI: 00007fea5a042000 RDI: 0000000000000003
RBP: 0000000000021010 R08: ffffffffffffffff R09: 0000000000000000
R10: 000000000000037b R11: 0000000000000246 R12: 0000000000022000
R13: 00007fea59e56b78 R14: 0000000000001000 R15: 0000000000020000
Code: f3 48 83 ec 18 48 63 8e 70 01 00 00 48 8b 86 c8 00 00 00 44 0f
b6 46 1a 48 8b 76 10 48 83 c0 18 48 89 ca 48 8b 0c cd e0 f3 54 90 <48>
39 34 01 0f b6 73 18 89 d0 40 0f 94 c7 48 0f a3 05 5c 0f e8

In print_one_rcu_data(), the argument rdp is already a per_cpu pointer
(see r_start():66). Therefore, calling per_cpu() on
rdp->dynticks->rcu_qs_ctr will dereference invalid memory address.
Since rdp->dynticks points to the per_cpu struct rcu_dynticks, using
rdp->dynticks->rcu_qs_ctr directly will fix this problem.

Fixes: 9577df9a3122af08fff84b8a1a60dccf524a3891 ("rcu: Pull rcu_qs_ctr
into rcu_dynticks structure")
Signed-off-by: Leon Yang <leon.gh.yang@xxxxxxxxx>
---
--- kernel/rcu/tree_trace.c.orig
+++ kernel/rcu/tree_trace.c
@@ -121,7 +121,7 @@ static void print_one_rcu_data(struct se
    cpu_is_offline(rdp->cpu) ? '!' : ' ',
    ulong2long(rdp->completed), ulong2long(rdp->gpnum),
    rdp->cpu_no_qs.b.norm,
-   rdp->rcu_qs_ctr_snap == per_cpu(rdp->dynticks->rcu_qs_ctr, rdp->cpu),
+   rdp->rcu_qs_ctr_snap == rdp->dynticks->rcu_qs_ctr,
    rdp->core_needs_qs);
  seq_printf(m, " dt=%d/%llx/%d df=%lu",
    rcu_dynticks_snap(rdp->dynticks),