On Mon, 30 Jun 2025, Al Viro wrote:
> [In #fixes, I'll send a pull request in a few days unless anybody objects]
>
> There's one case where ->d_compare() can be called for an in-lookup
> dentry; usually that's nothing special from ->d_compare() point of
> view, but proc_sys_compare() is... unique.
>
> The thing is, /proc/sys subdirectories can look differently for
> different processes. Up to and including having the same name
> resolve to different dentries - all of them hashed.
>
> The way it's done is ->d_compare() refusing to admit a match unless
> this dentry is supposed to be visible to this caller. The information
> needed to discriminate between them is stored in inode; it is set
> during proc_sys_lookup() and until it's done d_splice_alias() we really
> can't tell who should that dentry be visible for.
>
> Normally there's no negative dentries in /proc/sys; we can run into
> a dying dentry in RCU dcache lookup, but those can be safely rejected.
>
> However, ->d_compare() is also called for in-lookup dentries, before
> they get positive - or hashed, for that matter. In case of match
> we will wait until dentry leaves in-lookup state and repeat ->d_compare()
> afterwards. In other words, the right behaviour is to treat the
> name match as sufficient for in-lookup dentries; if dentry is not
> for us, we'll see that when we recheck once proc_sys_lookup() is
> done with it.
>
> Fixes: d9171b934526 ("parallel lookups machinery, part 4 (and last)")
> Reported-by: NeilBrown <neilb@xxxxxxxxxx>
^should be "neil" :-(
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Reviewed-by: NeilBrown <neil@xxxxxxxxxx>
Checking the name first is definitely cleaner. The fact that
d_in_lookup() allows the rest to be short-circuited is neat but
certainly deserves the comment.
NeilBrown
> ---
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index cc9d74a06ff0..b0ff2d21a3d9 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -918,16 +918,20 @@ static int proc_sys_compare(const struct dentry *dentry,
> struct ctl_table_header *head;
> struct inode *inode;
>
> - /* Although proc doesn't have negative dentries, rcu-walk means
> - * that inode here can be NULL */
> - /* AV: can it, indeed? */
> - inode = d_inode_rcu(dentry);
> - if (!inode)
> - return 1;
> if (name->len != len)
> return 1;
> if (memcmp(name->name, str, len))
> return 1;
> +
> + // false positive is fine here - we'll recheck anyway
> + if (d_in_lookup(dentry))
> + return 0;
> +
> + inode = d_inode_rcu(dentry);
> + // we just might have run into dentry in the middle of __dentry_kill()
> + if (!inode)
> + return 1;
> +
> head = rcu_dereference(PROC_I(inode)->sysctl);
> return !head || !sysctl_is_seen(head);
> }
>
