[PATCH v2] lib/bootconfig: guard xbc_node_compose_key_after() buffer size

[PATCH v2] lib/bootconfig: guard xbc_node_compose_key_after() buffer size

[Josh Law]

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.

Signed-off-by: Josh Law <objecting@objecting.org>
---
 lib/bootconfig.c                            | 3 +++
 tools/bootconfig/include/linux/bootconfig.h | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/lib/bootconfig.c b/lib/bootconfig.c
index 96cbe6738ffe..730209c83e62 100644
--- a/lib/bootconfig.c
+++ b/lib/bootconfig.c
@@ -313,6 +313,9 @@ int __init xbc_node_compose_key_after(struct xbc_node *root,
 	if (!node && root)
 		return -EINVAL;
 
+	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

Re: [PATCH v2] lib/bootconfig: guard xbc_node_compose_key_after() buffer size

[Steven Rostedt]

On Tue, 17 Mar 2026 18:06:05 +0000
Josh Law <objecting@objecting.org> wrote:

> 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.
> 
> Signed-off-by: Josh Law <objecting@objecting.org>
> ---

BTW, you can add here:

Changes since v1: https://lore.kernel.org/all/20260317173703.46092-1-objecting@objecting.org/

- Removed typecasting ret to size_t, as it is not needed (Steven Rostedt)

>  lib/bootconfig.c                            | 3 +++
>  tools/bootconfig/include/linux/bootconfig.h | 5 +++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/lib/bootconfig.c b/lib/bootconfig.c
> index 96cbe6738ffe..730209c83e62 100644
> --- a/lib/bootconfig.c
> +++ b/lib/bootconfig.c
> @@ -313,6 +313,9 @@ int __init xbc_node_compose_key_after(struct xbc_node *root,
>  	if (!node && root)
>  		return -EINVAL;
>  

I wonder if this should have a comment here:

	/*
	 * Greater than 2G isn't needed for the bootconfig. Warn if it is
	 * bigger as to not need to worry about overruns of snprintf()
	 * return value.
	 */

> +	if (WARN_ON_ONCE(size > INT_MAX))
> +		return -EINVAL;
> +

-- Steve

>  	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 */