Subject: [PATCH net-next v2 2/4] tools: ynl: allow setting recv() size

From: Jakub Kicinski <[email-masked]>
Date: Mon, 4 Mar 2024 21:33:08 -0800

Make the size of the buffer we use for recv() configurable.
The details of the buffer sizing in netlink are somewhat
arcane, we could spend a lot of time polishing this API.
Let's just leave some hopefully helpful comments for now.
This is a for-developers-only feature, anyway.

Signed-off-by: Jakub Kicinski <[email-masked]>
---
v2:
 - move the handling somewhere a bit more sensible
 - use 0 as default, and if to set it
---
 tools/net/ynl/lib/ynl.py | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/tools/net/ynl/lib/ynl.py b/tools/net/ynl/lib/ynl.py
index 92ade9105f31..c3ff5be33e4e 100644
--- a/tools/net/ynl/lib/ynl.py
+++ b/tools/net/ynl/lib/ynl.py
@@ -84,6 +84,10 @@ from .nlspec import SpecFamily
     return f"Netlink error: {os.strerror(-self.nl_msg.error)}\n{self.nl_msg}"

+class ConfigError(Exception):
+    pass
+
+
 class NlAttr:
     ScalarFormat = namedtuple('ScalarFormat', ['native', 'big', 'little'])
     type_formats = {
@@ -400,7 +404,8 @@ genl_family_name_to_id = None

 class YnlFamily(SpecFamily):
-    def __init__(self, def_path, schema=None, process_unknown=False):
+    def __init__(self, def_path, schema=None, process_unknown=False,
+                 recv_size=0):
         super().__init__(def_path, schema)
 
         self.include_raw = False
@@ -415,6 +420,16 @@ genl_family_name_to_id = None
         except KeyError:
             raise Exception(f"Family '{self.yaml['name']}' not supported by the kernel")
 
+        # Note that netlink will use conservative (min) message size for
+        # the first dump recv() on the socket, our setting will only matter
+        # from the second recv() on.
+        self._recv_size = recv_size if recv_size else 131072
+        # Netlink will always allocate at least PAGE_SIZE - sizeof(skb_shinfo)
+        # for a message, so smaller receive sizes will lead to truncation.
+        # Note that the min size for other families may be larger than 4k!
+        if self._recv_size < 4000:
+            raise ConfigError()
+
         self.sock = socket.socket(socket.AF_NETLINK, socket.SOCK_RAW, self.nlproto.proto_num)
         self.sock.setsockopt(Netlink.SOL_NETLINK, Netlink.NETLINK_CAP_ACK, 1)
         self.sock.setsockopt(Netlink.SOL_NETLINK, Netlink.NETLINK_EXT_ACK, 1)
@@ -799,7 +814,7 @@ genl_family_name_to_id = None
     def check_ntf(self):
         while True:
             try:
-                reply = self.sock.recv(128 * 1024, socket.MSG_DONTWAIT)
+                reply = self.sock.recv(self._recv_size, socket.MSG_DONTWAIT)
             except BlockingIOError:
                 return
 
@@ -854,7 +869,7 @@ genl_family_name_to_id = None
         done = False
         rsp = []
         while not done:
-            reply = self.sock.recv(128 * 1024)
+            reply = self.sock.recv(self._recv_size)
             nms = NlMsgs(reply, attr_space=op.attr_set)
             for nl_msg in nms:
                 if nl_msg.extack:
-- 
2.44.0