Re: [PATCH] proc_sysctl: Fix up ->is_seen() handling

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Jun 13, 2025 at 02:54:21AM +0100, Al Viro wrote:
> On Fri, Jun 13, 2025 at 10:37:58AM +1000, NeilBrown wrote:
> > 
> > Some sysctl tables can provide an is_seen() function which reports if
> > the sysctl should be visible to the current process.  This is currently
> > used to cause d_compare to fail for invisible sysctls.
> > 
> > This technique might have worked in 2.6.26 when it was implemented, but
> > it cannot work now.  In particular if ->d_compare always fails for a
> > particular name, then d_alloc_parallel() will always create a new dentry
> > and pass it to lookup() resulting in a new inode for every lookup.  I
> > tested this by changing sysctl_is_seen() to always return 0.  When
> > all sysctls were still visible and repeated lookups (ls -li) reported
> > different inode numbers.
> 
> What do you mean, "name"?

The whole fucking point of that thing is that /proc/sys/net contents for
processes in different netns is not the same.  And such processes should
not screw each other into the ground by doing lookups in that area.

Yes, it means multiple children of the same dentry having the same name
*and* staying hashed at the same time.