[PATCH v3] lib/bootconfig: guard xbc_node_compose_key_after() buffer size

[Josh Law <objecting@objecting.org>]

xbc_node_compose_key_after() passes a size_t buffer length to
snprintf(), but snprintf() returns int. Guard against size values above
INT_MAX before the loop so the existing truncation check can continue to
compare ret against (int)size safely.

Add a small WARN_ON_ONCE shim for the tools/bootconfig userspace build
so the same source continues to build there.

Changes since v2:
 - Added a comment explaining the INT_MAX guard.

Changes since v1:
 - Removed casting ret to size_t; with the INT_MAX guard, the existing
   ret >= (int)size check is sufficient, per Steven Rostedt.
 - Link to v1:
   https://lore.kernel.org/all/20260317173703.46092-1-objecting@objecting.org/

Signed-off-by: Josh Law <objecting@objecting.org>
---
 lib/bootconfig.c                            | 8 ++++++++
 tools/bootconfig/include/linux/bootconfig.h | 5 +++++
 2 files changed, 13 insertions(+)

diff --git a/lib/bootconfig.c b/lib/bootconfig.c
index 96cbe6738ffe..2a54b51dec5c 100644
--- a/lib/bootconfig.c
+++ b/lib/bootconfig.c
@@ -313,6 +313,14 @@ int __init xbc_node_compose_key_after(struct xbc_node *root,
 	if (!node && root)
 		return -EINVAL;
 
+	/*
+	 * Bootconfig strings never need multi-GB buffers. Reject sizes
+	 * above INT_MAX so snprintf()'s int return value cannot overflow
+	 * the truncation check below.
+	 */
+	if (WARN_ON_ONCE(size > INT_MAX))
+		return -EINVAL;
+
 	while (--depth >= 0) {
 		node = xbc_nodes + keys[depth];
 		ret = snprintf(buf, size, "%s%s", xbc_node_get_data(node),
diff --git a/tools/bootconfig/include/linux/bootconfig.h b/tools/bootconfig/include/linux/bootconfig.h
index 6784296a0692..48383c10e036 100644
--- a/tools/bootconfig/include/linux/bootconfig.h
+++ b/tools/bootconfig/include/linux/bootconfig.h
@@ -8,6 +8,7 @@
 #include <stdbool.h>
 #include <ctype.h>
 #include <errno.h>
+#include <limits.h>
 #include <string.h>
 
 
@@ -19,6 +20,10 @@
 	((cond) ? printf("Internal warning(%s:%d, %s): %s\n",	\
 			__FILE__, __LINE__, __func__, #cond) : 0)
 
+#ifndef WARN_ON_ONCE
+#define WARN_ON_ONCE(cond)	WARN_ON(cond)
+#endif
+
 #define unlikely(cond)	(cond)
 
 /* Copied from lib/string.c */
-- 
2.34.1