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

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Mar 26, 2026 at 10:28:46PM -0700, Christoph Hellwig wrote:
> 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.

Seconded.  It's a pity that dax_writeback_mapping_range makes nearly the
same mistake...

--D