[PATCH] lockdep: Correct lock_classes index mapping

[Boqun Feng]

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 at mediatek.com>

Hmm.. this means that the /proc/lockdep_chains has been incorrect since
commit f611e8cf89ec..

Nice catch!

Reviewed-by: Boqun Feng <boqun.feng at gmail.com>

Regards,
Boqun

> ---
>  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
> 
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