From: Deepanshu Kartikey <kartikey406@gmail.com>

The DAT inode's btree node cache (i_assoc_inode) is initialized lazily
during btree operations. However, nilfs_mdt_save_to_shadow_map()
assumes i_assoc_inode is already initialized when copying dirty pages
to the shadow map during GC.

If NILFS_IOCTL_CLEAN_SEGMENTS is called immediately after mount before
any btree operation has occurred on the DAT inode, i_assoc_inode is
NULL leading to a general protection fault.

Fix this by calling nilfs_attach_btree_node_cache() on the DAT inode
in nilfs_dat_read() at mount time, ensuring i_assoc_inode is always
initialized before any GC operation can use it.

Reported-by: syzbot+4b4093b1f24ad789bf37@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=4b4093b1f24ad789bf37
Tested-by: syzbot+4b4093b1f24ad789bf37@syzkaller.appspotmail.com
Fixes: e897be17a441 ("nilfs2: fix lockdep warnings in page operations for btree nodes")
Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: stable@vger.kernel.org
---
Hi Viacheslav,

Please queue this patch first.

It fixes a potential NULL pointer dereference defect that could be
triggered (likely most cases are intentionally) via the GC ioctl.

Thanks,
Ryusuke Konishi

 fs/nilfs2/dat.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/nilfs2/dat.c b/fs/nilfs2/dat.c
index 674380837ab9..888dc1831c86 100644
--- a/fs/nilfs2/dat.c
+++ b/fs/nilfs2/dat.c
@@ -524,6 +524,9 @@ int nilfs_dat_read(struct super_block *sb, size_t entry_size,
 	if (err)
 		goto failed;
 
+	err = nilfs_attach_btree_node_cache(dat);
+	if (err)
+		goto failed;
 	err = nilfs_read_inode_common(dat, raw_inode);
 	if (err)
 		goto failed;
-- 
2.43.0

Hi Ryusuke,

On Tue, 2026-03-31 at 09:47 +0900, Ryusuke Konishi wrote:
> From: Deepanshu Kartikey <kartikey406@gmail.com>
> 
> The DAT inode's btree node cache (i_assoc_inode) is initialized lazily
> during btree operations. However, nilfs_mdt_save_to_shadow_map()
> assumes i_assoc_inode is already initialized when copying dirty pages
> to the shadow map during GC.
> 
> If NILFS_IOCTL_CLEAN_SEGMENTS is called immediately after mount before
> any btree operation has occurred on the DAT inode, i_assoc_inode is
> NULL leading to a general protection fault.
> 
> Fix this by calling nilfs_attach_btree_node_cache() on the DAT inode
> in nilfs_dat_read() at mount time, ensuring i_assoc_inode is always
> initialized before any GC operation can use it.
> 
> Reported-by: syzbot+4b4093b1f24ad789bf37@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=4b4093b1f24ad789bf37
> Tested-by: syzbot+4b4093b1f24ad789bf37@syzkaller.appspotmail.com
> Fixes: e897be17a441 ("nilfs2: fix lockdep warnings in page operations for btree nodes")
> Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
> Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
> Cc: stable@vger.kernel.org
> ---
> Hi Viacheslav,
> 
> Please queue this patch first.
> 
> It fixes a potential NULL pointer dereference defect that could be
> triggered (likely most cases are intentionally) via the GC ioctl.
> 
> Thanks,
> Ryusuke Konishi
> 
>  fs/nilfs2/dat.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/nilfs2/dat.c b/fs/nilfs2/dat.c
> index 674380837ab9..888dc1831c86 100644
> --- a/fs/nilfs2/dat.c
> +++ b/fs/nilfs2/dat.c
> @@ -524,6 +524,9 @@ int nilfs_dat_read(struct super_block *sb, size_t entry_size,
>  	if (err)
>  		goto failed;
>  
> +	err = nilfs_attach_btree_node_cache(dat);
> +	if (err)
> +		goto failed;
>  	err = nilfs_read_inode_common(dat, raw_inode);
>  	if (err)
>  		goto failed;

Applied on for-next branch of NILFS2 git tree.

Thanks,
Slava.