[PATCH 1/4] bootconfig: return 0 from xbc_snprint_cmdline() for a leaf root

[Breno Leitao <leitao@debian.org>]

Returning -EINVAL when @root has no descendant key nodes is a quirky
result for a renderer: "nothing to render" is not an error. The only
existing caller, xbc_make_cmdline(), papers over it with a `len <= 0`
check, so the misbehavior is harmless today. The new -C user in
tools/bootconfig added by the follow-up patches propagates the error
and turns an empty "kernel {}" subtree into a build failure.

Short-circuit the leaf-root case and return 0 so the rendered length
matches the rendered content.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 lib/bootconfig.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/lib/bootconfig.c b/lib/bootconfig.c
index f445b7703fdd..3a102c9122f7 100644
--- a/lib/bootconfig.c
+++ b/lib/bootconfig.c
@@ -431,6 +431,16 @@ int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 	const char *val, *q;
 	int ret;
 
+	/*
+	 * A leaf @root (e.g. an empty "kernel {}" subtree, or a key whose
+	 * only child is a value node) has no descendant key/value pairs to
+	 * render. The leaf-finding iterator below would otherwise return
+	 * @root itself, which xbc_node_compose_key_after() rejects with
+	 * -EINVAL.
+	 */
+	if (root && xbc_node_is_leaf(root))
+		return 0;
+
 	xbc_node_for_each_key_value(root, knode, val) {
 		ret = xbc_node_compose_key_after(root, knode,
 					xbc_namebuf, XBC_KEYLEN_MAX);

-- 
2.54.0