bpf: Handle bpf_mprog_query with NULL entry

Improve consistency for bpf_mprog_query() API and let the latter also handle
a NULL entry as can be the case for tcx. Instead of returning -ENOENT, we
copy a count of 0 and revision of 1 to user space, so that this can be fed
into a subsequent bpf_mprog_attach() call as expected_revision. A BPF self-
test as part of this series has been added to assert this case.

Suggested-by: Lorenz Bauer <lmb@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/mprog.c

@@ -401,14 +401,16 @@ int bpf_mprog_query(const union bpf_attr *attr, union bpf_attr __user *uattr,
 	struct bpf_mprog_cp *cp;
 	struct bpf_prog *prog;
 	const u32 flags = 0;
+	u32 id, count = 0;
+	u64 revision = 1;
 	int i, ret = 0;
-	u32 id, count;
-	u64 revision;
 
 	if (attr->query.query_flags || attr->query.attach_flags)
 		return -EINVAL;
-	revision = bpf_mprog_revision(entry);
-	count = bpf_mprog_total(entry);
+	if (entry) {
+		revision = bpf_mprog_revision(entry);
+		count = bpf_mprog_total(entry);
+	}
 	if (copy_to_user(&uattr->query.attach_flags, &flags, sizeof(flags)))
 		return -EFAULT;
 	if (copy_to_user(&uattr->query.revision, &revision, sizeof(revision)))

kernel/bpf/tcx.c

@@ -123,7 +123,6 @@ int tcx_prog_query(const union bpf_attr *attr, union bpf_attr __user *uattr)
 {
 	bool ingress = attr->query.attach_type == BPF_TCX_INGRESS;
 	struct net *net = current->nsproxy->net_ns;
-	struct bpf_mprog_entry *entry;
 	struct net_device *dev;
 	int ret;
 
@@ -133,12 +132,7 @@ int tcx_prog_query(const union bpf_attr *attr, union bpf_attr __user *uattr)
 		ret = -ENODEV;
 		goto out;
 	}
-	entry = tcx_entry_fetch(dev, ingress);
-	if (!entry) {
-		ret = -ENOENT;
-		goto out;
-	}
-	ret = bpf_mprog_query(attr, uattr, entry);
+	ret = bpf_mprog_query(attr, uattr, tcx_entry_fetch(dev, ingress));
 out:
 	rtnl_unlock();
 	return ret;