linux-kernel - [PATCH net] tools: ynl: fix string attribute length to include null terminator

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Message-ID: <20251024132438.351290-1-poros@redhat.com>
Date: Fri, 24 Oct 2025 15:24:38 +0200
From: Petr Oros <poros@...hat.com>
To: Donald Hunter <donald.hunter@...il.com>,
	Jakub Kicinski <kuba@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Jacob Keller <jacob.e.keller@...el.com>,
	Petr Oros <poros@...hat.com>,
	Asbjørn Sloth Tønnesen <ast@...erby.net>,
	netdev@...r.kernel.org (open list:NETWORKING [GENERAL]),
	linux-kernel@...r.kernel.org (open list)
Cc: ivecera@...hat.com,
	mschmidt@...hat.com
Subject: [PATCH net] tools: ynl: fix string attribute length to include null terminator

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 <poros@...hat.com>
---
 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);
 }
-- 
2.51.0