linux-kernel - Re: [PATCH] btrfs: add missing ASSERT for system space_info in reserve_chunk

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Message-ID: <3a9648fa-49c2-4f07-84ee-50a1c7d7196f@suse.com>
Date: Fri, 9 Jan 2026 10:02:00 +1030
From: Qu Wenruo <wqu@...e.com>
To: Jiasheng Jiang <jiashengjiangcool@...il.com>, Chris Mason <clm@...com>,
 David Sterba <dsterba@...e.com>
Cc: linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: add missing ASSERT for system space_info in
 reserve_chunk_space



在 2026/1/9 06:15, Jiasheng Jiang 写道:
> In reserve_chunk_space(), btrfs_find_space_info() is called to retrieve
> the space_info for the SYSTEM block group. However, the returned pointer
> is immediately dereferenced by spin_lock(&info->lock) without validation.
> 
> While the SYSTEM space_info is expected to be present during normal
> operations, direct dereference without checking creates a risk of a
> null pointer dereference. This deviates from the coding pattern seen
> in peer functions like btrfs_chunk_alloc() where such returns are
> validated with an ASSERT.
> 
> Add an ASSERT() to ensure the pointer is valid before access, improving
> code robustness and consistency with the rest of the block-group logic.

If you know how system chunk works, you must understand without a system 
chunk a btrfs will never be mounted.

The ASSERT() looks unnecessary to me.

Thanks,
Qu

> 
> Signed-off-by: Jiasheng Jiang <jiashengjiangcool@...il.com>
> ---
>   fs/btrfs/block-group.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index 08b14449fabe..f4229ee4f573 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -4313,6 +4313,7 @@ static void reserve_chunk_space(struct btrfs_trans_handle *trans,
>   	lockdep_assert_held(&fs_info->chunk_mutex);
>   
>   	info = btrfs_find_space_info(fs_info, BTRFS_BLOCK_GROUP_SYSTEM);
> +	ASSERT(info);
>   	spin_lock(&info->lock);
>   	left = info->total_bytes - btrfs_space_info_used(info, true);
>   	spin_unlock(&info->lock);