Re: [PATCH] btrfs: add a sanity check for btrfs root in btrfs_next_old_leaf()

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2025/2/24 18:29, Ma Ke 写道:
> btrfs_next_old_leaf() doesn't check if the target root is NULL or not,
> resulting the null-ptr-deref. Add sanity check for btrfs root before
> using it in btrfs_next_old_leaf().

Please provide a real world call trace when this is triggered.

There is a prerequisite, the extent tree can only be NULL if
rescue=ibadroots is provided and the extent root is corrupted.

And "rescue=" mount options can only be specified for a fully read-only
fs (no internal log replay or any other thing to write even a bit into
the fs).

Previously read-only scrub can still be triggered on such fs, but
6aecd91a5c5b ("btrfs: avoid NULL pointer dereference if no valid extent
tree") fixed that.

And if you hit such a case in real world, please provide the call trace
so that we know we're not missing some critical situations that extent
tree is accessed for read-only operations.

Thanks,
Qu

>
> Found by code review.
>
> Cc: stable@vger.kernel.org
> Fixes: d96b34248c2f ("btrfs: make send work with concurrent block group relocation")
> Signed-off-by: Ma Ke <make24@iscas.ac.cn>
> ---
>   fs/btrfs/ctree.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index 4e2e1c38d33a..1a3fc3863860 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -4794,13 +4794,17 @@ int btrfs_next_old_leaf(struct btrfs_root *root, struct btrfs_path *path,
>   	int level;
>   	struct extent_buffer *c;
>   	struct extent_buffer *next;
> -	struct btrfs_fs_info *fs_info = root->fs_info;
> +	struct btrfs_fs_info *fs_info;
>   	struct btrfs_key key;
>   	bool need_commit_sem = false;
>   	u32 nritems;
>   	int ret;
>   	int i;
>
> +	if (!root)
> +		return -EINVAL;
> +
> +	fs_info = root->fs_info;
>   	/*
>   	 * The nowait semantics are used only for write paths, where we don't
>   	 * use the tree mod log and sequence numbers.