Re: [PATCH] bpf: add bpf_real_inode() kfunc

[Christoph Hellwig <hch@infradead.org>]

No comment on the code, here, but this caught my eye:

On Thu, Mar 26, 2026 at 05:53:44PM +0100, Christian Brauner wrote:
> This is needed by the dm-verity based execution policy implemented in
> systemd [1] where BPF LSM hooks must resolve a file's backing block
> device via inode->i_sb->s_dev. 

inode->i_sb->s_dev is not a files backing block device.  The only
think inode->i_sb->s_dev is required to be is the lookup key for
finding the super block.  It also happens to be the default backing
block device for simple file systems, but once things get a little more
complicated it often is not.  Examples are btrfs where it never matches,
f2fs additional devices, the XFS RT device, pNFS block layouts and
probably a few more I forgot.

For the more complex cases like btrfs there might not even be a single
block device for a file and/or the mapping can change.  So please do not
encode such an assumption anywhere because it is broken.