Re: [PATCH v4] lib/bootconfig: guard xbc_node_compose_key_after() buffer size

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

On Tue, 17 Mar 2026 20:44:03 +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.

NACK.

Don't do such over engineering effort.

Thanks,

> 
> Signed-off-by: Josh Law <objecting@objecting.org>
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> Changes since v3:
>  - Moved the revision history below the --- separator so it does not
>    become part of the git commit.
>  - Added Reviewed-by from Steven Rostedt.
> 
> 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/
> 
>  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
> 


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