[PATCH 5.15 179/570] lib/bootconfig: fix snprintf truncation check in xbc_node_compose_key_after()

Greg Kroah-Hartman
2026-04-13T16:25:48Z

From: Josh Law <objecting@objecting.org>

commit 1120a36bb1e9b9e22de75ecb4ef0b998f73a97f1 upstream.

snprintf() returns the number of characters that would have been
written excluding the NUL terminator. Output is truncated when the
return value is >= the buffer size, not just > the buffer size.

When ret == size, the current code takes the non-truncated path,
advancing buf by ret and reducing size to 0. This is wrong because
the output was actually truncated (the last character was replaced by
NUL). Fix by using >= so the truncation path is taken correctly.

Link: https://lore.kernel.org/all/20260312191143.28719-4-objecting@objecting.org/

Fixes: 76db5a27a827 ("bootconfig: Add Extra Boot Config support")
Cc: stable@vger.kernel.org
Signed-off-by: Josh Law <objecting@objecting.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
lib/bootconfig.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/lib/bootconfig.c
+++ b/lib/bootconfig.c
@@ -250,7 +250,7 @@ int __init xbc_node_compose_key_after(st
depth ? "." : "");
if (ret < 0)
return ret;
- if (ret > size) {
+ if (ret >= size) {
size = 0;
} else {
size -= ret;