Re: [PATCH v2 3/3] lib/bootconfig: fix snprintf truncation check in xbc_node_compose_key_after()

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 12 Mar 2026 19:11:43 +0000
Josh Law <hlcj1234567@gmail.com> wrote:

> From: Josh Law <objecting@objecting.org>
> 
> 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.
> 

OK, but this one is a minor issue, because either way, remaining
size becomes 0.

		ret = snprintf(buf, size, "%s%s", xbc_node_get_data(node),
			       depth ? "." : "");
		if (ret < 0)
			return ret;
		if (ret > size) {
			size = 0;
		} else {
			size -= ret; // if ret == size, the size becomes 0
			buf += ret;
		}

Anyway, to be clear the correct error case handling, this
should be applied.

Thank you!

> Signed-off-by: Josh Law <objecting@objecting.org>
> ---
>  lib/bootconfig.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/bootconfig.c b/lib/bootconfig.c
> index 62b4ed7a0ba6..b0ef1e74e98a 100644
> --- a/lib/bootconfig.c
> +++ b/lib/bootconfig.c
> @@ -316,7 +316,7 @@ int __init xbc_node_compose_key_after(struct xbc_node *root,
>  			       depth ? "." : "");
>  		if (ret < 0)
>  			return ret;
> -		if (ret > size) {
> +		if (ret >= size) {
>  			size = 0;
>  		} else {
>  			size -= ret;
> -- 
> 2.34.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>