[PATCH 6/9] handshake: advertise the session-tag cap to user space

From: Chuck Lever <[email protected]>

The kernel caps the number of session tags it accepts in a DONE
downcall at HANDSHAKE_MAX_SESSIONTAGS. tlshd has no way to learn
this cap today: a daemon built against newer UAPI headers than
the running kernel silently overruns it, and the kernel truncates
the list with one pr_warn_once per boot. Truncation is
recoverable but the underlying misconfiguration is easy to miss.

Carry the cap on every ACCEPT reply as HANDSHAKE_A_ACCEPT_MAX_TAGS,
a u32 attribute populated by the kernel. User space reads the
value at ACCEPT time and can gate its DONE-side tag list against
it, turning over-cap into a user-space policy choice rather than
a silent kernel-side truncation.

Putting the cap in the ACCEPT reply keeps a single source of
truth and lets the kernel raise it in a later release without
bumping the daemon's UAPI header dependency.

Signed-off-by: Chuck Lever <[email protected]>
---
 Documentation/netlink/specs/handshake.yaml | 4 ++++
 Documentation/networking/tls-handshake.rst | 7 +++++++
 include/uapi/linux/handshake.h             | 1 +
 net/handshake/tlshd.c                      | 5 +++++
 4 files changed, 17 insertions(+)

diff --git a/Documentation/netlink/specs/handshake.yaml b/Documentation/netlink/specs/handshake.yaml
index df36ff7da18f..614d31bee656 100644
--- a/Documentation/netlink/specs/handshake.yaml
+++ b/Documentation/netlink/specs/handshake.yaml
@@ -78,6 +78,9 @@ attribute-sets:
       -
         name: keyring
         type: u32
+      -
+        name: max-tags
+        type: u32
   -
     name: done
     attributes:
@@ -123,6 +126,7 @@ operations:
             - certificate
             - peername
             - keyring
+            - max-tags
     -
       name: done
       doc: Handler reports handshake completion
diff --git a/Documentation/networking/tls-handshake.rst b/Documentation/networking/tls-handshake.rst
index 352842a74e6b..ea2e090a1ed8 100644
--- a/Documentation/networking/tls-handshake.rst
+++ b/Documentation/networking/tls-handshake.rst
@@ -273,4 +273,11 @@ empty. The handshake layer always delivers a finalized tagset to
 the callback, so consumers may call tagset_is_member() and
 tagset_intersection() unconditionally without a separate guard.
 
+The tagset delivered to the consumer may contain fewer tags than
+the handshake agent assigned. The kernel caps the per-DONE tag
+count at HANDSHAKE_MAX_SESSIONTAGS, and individual tags within
+the cap may be dropped under memory pressure. The cap rides on
+every ACCEPT reply so the agent can size its DONE-side tag list
+to it; see Documentation/netlink/specs/handshake.yaml.
+
 See Documentation/core-api/tagset.rst for the complete tagset API.
diff --git a/include/uapi/linux/handshake.h b/include/uapi/linux/handshake.h
index 1ed309e475b4..1445983e7369 100644
--- a/include/uapi/linux/handshake.h
+++ b/include/uapi/linux/handshake.h
@@ -49,6 +49,7 @@ enum {
 	HANDSHAKE_A_ACCEPT_CERTIFICATE,
 	HANDSHAKE_A_ACCEPT_PEERNAME,
 	HANDSHAKE_A_ACCEPT_KEYRING,
+	HANDSHAKE_A_ACCEPT_MAX_TAGS,
 
 	__HANDSHAKE_A_ACCEPT_MAX,
 	HANDSHAKE_A_ACCEPT_MAX = (__HANDSHAKE_A_ACCEPT_MAX - 1)
diff --git a/net/handshake/tlshd.c b/net/handshake/tlshd.c
index 9bcaeba74f8c..eae4a4a0a9ef 100644
--- a/net/handshake/tlshd.c
+++ b/net/handshake/tlshd.c
@@ -238,6 +238,11 @@ static int tls_handshake_accept(struct handshake_req *req,
 			goto out_cancel;
 	}
 
+	ret = nla_put_u32(msg, HANDSHAKE_A_ACCEPT_MAX_TAGS,
+			  HANDSHAKE_MAX_SESSIONTAGS);
+	if (ret < 0)
+		goto out_cancel;
+
 	ret = nla_put_u32(msg, HANDSHAKE_A_ACCEPT_AUTH_MODE,
 			  treq->th_auth_mode);
 	if (ret < 0)

-- 
2.54.0