Re: [PATCH v2] lib/bootconfig: fix undefined behavior involving NULL pointer arithmetic

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

On Tue, 30 Jun 2026 17:47:46 +0000
Bradley Morgan <include@grrlz.net> wrote:

> When xbc_snprint_cmdline() is called during the size-probing phase
> (with buf = NULL and size = 0), the function computes the end pointer
> as 'buf + size' (NULL + 0) and repeatedly advances 'buf' via 'buf += ret'.
> 
> Under the C standard, performing pointer arithmetic on a NULL pointer is
> undefined behavior. While harmless inside the kernel, this code is also
> compiled into the userspace host tool 'tools/bootconfig', where host
> compilers with UBSan or FORTIFY_SOURCE enabled abort the build when they
> detect NULL pointer arithmetic.
> 
> Fix this by guarding the pointer arithmetic so 'buf' is only advanced when
> non-NULL, and track the running written length in a separate 'len' counter
> for the return value (which cannot be recovered from pointer math when
> 'buf' is NULL). The rest() helper and snprintf call sites are unchanged.
> 
> Fixes: 51887d03aca1 ("bootconfig: init: Allow admin to use bootconfig for kernel command line")
> Cc: stable@vger.kernel.org
> Assisted-by: GLM:glm-5.2
> Signed-off-by: Bradley Morgan <include@grrlz.net>

Thanks for the fix!
Let me pick this to bootconfig/fixes.

Thank you,

> ---
>  lib/bootconfig.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> Changes since v1:
> - Got the big guns out! :) (see Assisted-by).
> - Addressed review from Masami Hiramatsu and Breno Leitao.
> 
> diff --git a/lib/bootconfig.c b/lib/bootconfig.c
> index f445b7703fdd..c913259c80ce 100644
> --- a/lib/bootconfig.c
> +++ b/lib/bootconfig.c
> @@ -427,8 +427,9 @@ static char xbc_namebuf[XBC_KEYLEN_MAX] __initdata;
>  int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
>  {
>  	struct xbc_node *knode, *vnode;
> -	char *end = buf + size;
> +	char *end = buf ? buf + size : NULL;
>  	const char *val, *q;
> +	size_t len = 0;
>  	int ret;
>  
>  	xbc_node_for_each_key_value(root, knode, val) {
> @@ -442,7 +443,9 @@ int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
>  			ret = snprintf(buf, rest(buf, end), "%s ", xbc_namebuf);
>  			if (ret < 0)
>  				return ret;
> -			buf += ret;
> +			len += ret;
> +			if (buf)
> +				buf += ret;
>  			continue;
>  		}
>  		xbc_array_for_each_value(vnode, val) {
> @@ -456,11 +459,13 @@ int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
>  				       xbc_namebuf, q, val, q);
>  			if (ret < 0)
>  				return ret;
> -			buf += ret;
> +			len += ret;
> +			if (buf)
> +				buf += ret;
>  		}
>  	}
>  
> -	return buf - (end - size);
> +	return len;
>  }
>  #undef rest
>  
> -- 
> 2.53.0
> 


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