Subject: Re: [PATCH net] tools: ynl: fix string attribute length to include null terminator

From: Jakub Kicinski <[email-masked]>
Date: Fri, 24 Oct 2025 17:03:47 -0700

On Fri, 24 Oct 2025 15:24:38 +0200 Petr Oros wrote:
> The ynl_attr_put_str() function was not including the null terminator
> in the attribute length calculation. This caused kernel to reject
> CTRL_CMD_GETFAMILY requests with EINVAL:
> "Attribute failed policy validation".
> 
> For a 4-character family name like "dpll":
> - Sent: nla_len=8 (4 byte header + 4 byte string without null)
> - Expected: nla_len=9 (4 byte header + 5 byte string with null)
> 
> The bug was introduced in commit 15d2540e0d62 ("tools: ynl: check for
> overflow of constructed messages") when refactoring from stpcpy() to
> strlen(). The original code correctly included the null terminator:
> 
>   end = stpcpy(ynl_attr_data(attr), str);
>   attr->nla_len = NLA_HDRLEN + NLA_ALIGN(end -
>                                 (char *)ynl_attr_data(attr));
> 
> Since stpcpy() returns a pointer past the null terminator, the length
> included it. The refactored version using strlen() omitted the +1.
> 
> The fix also removes NLA_ALIGN() from nla_len calculation, since
> nla_len should contain actual attribute length, not aligned length.
> Alignment is only for calculating next attribute position. This makes
> the code consistent with ynl_attr_put().
> 
> CTRL_ATTR_FAMILY_NAME uses NLA_NUL_STRING policy which requires
> null terminator. Kernel validates with memchr() and rejects if not
> found.
> 
> Fixes: 15d2540e0d62 ("tools: ynl: check for overflow of constructed messages")
> Signed-off-by: Petr Oros <[email-masked]>
> ---
>  tools/net/ynl/lib/ynl-priv.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/net/ynl/lib/ynl-priv.h b/tools/net/ynl/lib/ynl-priv.h
> index 29481989ea7662..ced7dce44efb43 100644
> --- a/tools/net/ynl/lib/ynl-priv.h
> +++ b/tools/net/ynl/lib/ynl-priv.h
> @@ -313,7 +313,7 @@ ynl_attr_put_str(struct nlmsghdr *nlh, unsigned int attr_type, const char *str)
>  	struct nlattr *attr;
>  	size_t len;
>  
> -	len = strlen(str);
> +	len = strlen(str) + 1;
>  	if (__ynl_attr_put_overflow(nlh, len))
>  		return;
>  
> @@ -321,7 +321,7 @@ ynl_attr_put_str(struct nlmsghdr *nlh, unsigned int attr_type, const char *str)
>  	attr->nla_type = attr_type;
>  
>  	strcpy((char *)ynl_attr_data(attr), str);
> -	attr->nla_len = NLA_HDRLEN + NLA_ALIGN(len);
> +	attr->nla_len = NLA_HDRLEN + len;
>  
>  	nlh->nlmsg_len += NLMSG_ALIGN(attr->nla_len);
>  }

looks familiar...

Link: https://lore.kernel.org/[email-masked]