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@xxxxxxxxxxxx>
> ---
> 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>
