lockdep: Correct lock_classes index mapping
authorCheng Jui Wang <cheng-jui.wang@mediatek.com>
Thu, 10 Feb 2022 10:50:11 +0000 (18:50 +0800)
committerPeter Zijlstra <peterz@infradead.org>
Fri, 11 Feb 2022 22:30:02 +0000 (23:30 +0100)
A kernel exception was hit when trying to dump /proc/lockdep_chains after
lockdep report "BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!":

Unable to handle kernel paging request at virtual address 00054005450e05c3
...
00054005450e05c3] address between user and kernel address ranges
...
pc : [0xffffffece769b3a8] string+0x50/0x10c
lr : [0xffffffece769ac88] vsnprintf+0x468/0x69c
...
 Call trace:
  string+0x50/0x10c
  vsnprintf+0x468/0x69c
  seq_printf+0x8c/0xd8
  print_name+0x64/0xf4
  lc_show+0xb8/0x128
  seq_read_iter+0x3cc/0x5fc
  proc_reg_read_iter+0xdc/0x1d4

The cause of the problem is the function lock_chain_get_class() will
shift lock_classes index by 1, but the index don't need to be shifted
anymore since commit 02771f570002 ("locking/lockdep: Change the range
of class_idx in held_lock struct") already change the index to start
from 0.

The lock_classes[-1] located at chain_hlocks array. When printing
lock_classes[-1] after the chain_hlocks entries are modified, the
exception happened.

The output of lockdep_chains are incorrect due to this problem too.

Fixes: d3ac0644e08f ("lockdep: Take read/write status in consideration when generate chainkey")
Signed-off-by: Cheng Jui Wang <cheng-jui.wang@mediatek.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20220210105011.21712-1-cheng-jui.wang@mediatek.com
kernel/locking/lockdep.c

index 4a882f83aeb9da4688e0a6af1180373eaf0048d1..f8a0212189cadec93b600f4f2fc3824cc1f9e17b 100644 (file)
@@ -3462,7 +3462,7 @@ struct lock_class *lock_chain_get_class(struct lock_chain *chain, int i)
        u16 chain_hlock = chain_hlocks[chain->base + i];
        unsigned int class_idx = chain_hlock_class_idx(chain_hlock);
 
-       return lock_classes + class_idx - 1;
+       return lock_classes + class_idx;
 }
 
 /*
@@ -3530,7 +3530,7 @@ static void print_chain_keys_chain(struct lock_chain *chain)
                hlock_id = chain_hlocks[chain->base + i];
                chain_key = print_chain_key_iteration(hlock_id, chain_key);
 
-               print_lock_name(lock_classes + chain_hlock_class_idx(hlock_id) - 1);
+               print_lock_name(lock_classes + chain_hlock_class_idx(hlock_id));
                printk("\n");
        }
 }