Re: [PATCH] bcache: check c->root with IS_ERR_OR_NULL() in mca_reserve()

[Coly Li <colyli@suse.de>]

On 2020/9/17 16:13, Dongsheng Yang wrote:
> In mca_reserve(c) macro, we are checking root whether is NULL or not.
> But that's not enough, when we read the root node in run_cache_set(),
> if we got an error in bch_btree_node_read_done(), we will return ERR_PTR(-EIO)
> to c->root.
> 
> And then we will go continue to unregister, but before unregister_shrinker(&c->shrink);
> there is a possibility to call bch_mca_count(), and we would get a crash with call trace like that:
> 
> [ 2149.876008] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000b5
> ... ...
> [ 2150.598931] Call trace:
> [ 2150.606439]  bch_mca_count+0x58/0x98 [escache]
> [ 2150.615866]  do_shrink_slab+0x54/0x310
> [ 2150.624429]  shrink_slab+0x248/0x2d0
> [ 2150.632633]  drop_slab_node+0x54/0x88
> [ 2150.640746]  drop_slab+0x50/0x88
> [ 2150.648228]  drop_caches_sysctl_handler+0xf0/0x118
> [ 2150.657219]  proc_sys_call_handler.isra.18+0xb8/0x110
> [ 2150.666342]  proc_sys_write+0x40/0x50
> [ 2150.673889]  __vfs_write+0x48/0x90
> [ 2150.681095]  vfs_write+0xac/0x1b8
> [ 2150.688145]  ksys_write+0x6c/0xd0
> [ 2150.695127]  __arm64_sys_write+0x24/0x30
> [ 2150.702749]  el0_svc_handler+0xa0/0x128
> [ 2150.710296]  el0_svc+0x8/0xc
> 
> Signed-off-by: Dongsheng Yang <dongsheng.yang@easystack.cn>

It looks good to me, added into my test queue.

Thanks.

Coly Li

> ---
>  drivers/md/bcache/btree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> index d45a1dd..36cae5c 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -514,7 +514,7 @@ static void bch_btree_leaf_dirty(struct btree *b, atomic_t *journal_ref)
>   * mca -> memory cache
>   */
>  
> -#define mca_reserve(c)	(((c->root && c->root->level)		\
> +#define mca_reserve(c)	(((!IS_ERR_OR_NULL(c->root) && c->root->level) \
>  			  ? c->root->level : 1) * 8 + 16)
>  #define mca_can_free(c)						\
>  	max_t(int, 0, c->btree_cache_used - mca_reserve(c))
>