linux-kernel - [RFC PATCH kernel] block: initialize block_device::bd_bdi for bdev

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Date:   Wed,  6 Jan 2021 20:29:00 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Christoph Hellwig <hch@....de>
Cc:     Alexey Kardashevskiy <aik@...abs.ru>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Hannes Reinecke <hare@...e.de>, Jan Kara <jack@...e.cz>,
        Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [RFC PATCH kernel] block: initialize block_device::bd_bdi for bdev_cache

This is a workaround to fix a null derefence crash:

[c00000000b01f840] c00000000b01f880 (unreliable)
[c00000000b01f880] c000000000769a3c bdev_evict_inode+0x21c/0x370
[c00000000b01f8c0] c00000000070bacc evict+0x11c/0x230
[c00000000b01f900] c00000000070c138 iput+0x2a8/0x4a0
[c00000000b01f970] c0000000006ff030 dentry_unlink_inode+0x220/0x250
[c00000000b01f9b0] c0000000007001c0 __dentry_kill+0x190/0x320
[c00000000b01fa00] c000000000701fb8 dput+0x5e8/0x860
[c00000000b01fa80] c000000000705848 shrink_dcache_for_umount+0x58/0x100
[c00000000b01fb00] c0000000006cf864 generic_shutdown_super+0x54/0x200
[c00000000b01fb80] c0000000006cfd48 kill_anon_super+0x38/0x60
[c00000000b01fbc0] c0000000006d12cc deactivate_locked_super+0xbc/0x110
[c00000000b01fbf0] c0000000006d13bc deactivate_super+0x9c/0xc0
[c00000000b01fc20] c00000000071a340 cleanup_mnt+0x1b0/0x250
[c00000000b01fc80] c000000000278fa8 task_work_run+0xf8/0x180
[c00000000b01fcd0] c00000000002b4ac do_notify_resume+0x4dc/0x5d0
[c00000000b01fda0] c00000000004ba0c syscall_exit_prepare+0x28c/0x370
[c00000000b01fe10] c00000000000e06c system_call_common+0xfc/0x27c
--- Exception: c00 (System Call) at 0000000010034890

Is this fixed properly already somewhere? Thanks,

Fixes: e6cb53827ed6 ("block: initialize struct block_device in bdev_alloc")
---
 fs/block_dev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 3e5b02f6606c..86fdc28d565e 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -792,8 +792,10 @@ static void bdev_free_inode(struct inode *inode)
 static void init_once(void *data)
 {
 	struct bdev_inode *ei = data;
+	struct block_device *bdev = &ei->bdev;
 
 	inode_init_once(&ei->vfs_inode);
+	bdev->bd_bdi = &noop_backing_dev_info;
 }
 
 static void bdev_evict_inode(struct inode *inode)
-- 
2.17.1