Re: lockdep + kasan bug?

[Kent Overstreet]

On Tue, Nov 21, 2023 at 12:41:26PM +0100, Peter Zijlstra wrote:
> > I suspect the dodgy access is to chain_block_buckets[-1], which hits the last 4
> > bytes of the redzone and gets (incorrectly/misleadingly) attributed to
> > nr_large_chain_blocks.
> 
> That would mean @size == 0, at which point size_to_bucket() returns -1
> and the above happens.
> 
> alloc_chain_hlocks() has 'size - req', for the first with the
> precondition 'size >= rq', which allows the 0.
> 
> The second is an iteration with the condition size > req, which does not
> allow the 0 case.
> 
> So the first, thing, IIRC, this is trying to split a block,
> del_chain_block() takes what we need, and add_chain_block() puts back
> the remainder, except in the above case the remainder is 0 sized and
> things go sideways or so.
> 
> Does the below help?
> 
> ---
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index e85b5ad3e206..151bd3de5936 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3497,7 +3497,8 @@ static int alloc_chain_hlocks(int req)
>  		size = chain_block_size(curr);
>  		if (likely(size >= req)) {
>  			del_chain_block(0, size, chain_block_next(curr));
> -			add_chain_block(curr + req, size - req);
> +			if (size > req)
> +				add_chain_block(curr + req, size - req);
>  			return curr;
>  		}
>  	}
> 

Yep, no kasan splats with that patch