On Thu, Feb 10, 2022 at 06:50:11PM +0800, Cheng Jui Wang wrote:

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 01bb6f0af992 ("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: f611e8cf98ec ("lockdep: Take read/write status in consideration when generate chainkey")

Signed-off-by: Cheng Jui Wang cheng-jui.wang@mediatek.com

kernel/locking/lockdep.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index 4a882f83aeb9..f8a0212189ca 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -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"); }

}

2.18.0

<formletter>

This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.

</formletter>