mdev: increase netlink buffer sizes

The socket receive buffer turned out to be too small for real world
systems. Use the same size as udevd to be on the safe side. As this is
just a limit and the memory is not allocated by the kernel until really
needed there is actually no memory wasted.

Signed-off-by: Jan Klötzke <jan@kloetzke.net>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 17 insertions, 9 deletions

diff --git a/util-linux/mdev.c b/util-linux/mdev.c
index 59dbcf0..8b06d34 100644
--- a/util-linux/mdev.c
+++ b/util-linux/mdev.c
@@ -269,10 +269,6 @@
 # define dbg3s(msg) ((void)0)
 #endif
 
-
-#ifndef SO_RCVBUFFORCE
-#define SO_RCVBUFFORCE 33
-#endif
 static const char keywords[] ALIGN1 = "add\0remove\0"; // "change\0"
 enum { OP_add, OP_remove };
 
@@ -1152,15 +1148,27 @@ static void initial_scan(char *temp)
 
 #if ENABLE_FEATURE_MDEV_DAEMON
 
-/* uevent applet uses 16k buffer, and mmaps it before every read */
-# define BUFFER_SIZE (2 * 1024)
-# define RCVBUF (2 * 1024 * 1024)
+/*
+ * The kernel (as of v5.4) will pass up to 32 environment variables with a
+ * total of 2kiB on each event. On top of that the action string and device
+ * path are added. Using a 3kiB buffer for the event should suffice in any
+ * case.
+ *
+ * As far as the socket receive buffer size is concerned 2MiB proved to be too
+ * small (see [1]). Udevd seems to use a whooping 128MiB. The socket receive
+ * buffer size is just a resource limit. The buffers are allocated lazily so
+ * the memory is not wasted.
+ *
+ * [1] http://lists.busybox.net/pipermail/busybox/2019-December/087665.html
+ */
+# define USER_RCVBUF (3 * 1024)
+# define KERN_RCVBUF (128 * 1024 * 1024)
 # define MAX_ENV 32
 
 static void daemon_loop(char *temp, int fd)
 {
 	for (;;) {
-		char netbuf[BUFFER_SIZE];
+		char netbuf[USER_RCVBUF];
 		char *env[MAX_ENV];
 		char *s, *end;
 		ssize_t len;
@@ -1237,7 +1245,7 @@ int mdev_main(int argc UNUSED_PARAM, char **argv)
 		 *	mdev -d
 		 *	find /sys -name uevent -exec sh -c 'echo add >"{}"' ';'
 		 */
-		fd = create_and_bind_to_netlink(NETLINK_KOBJECT_UEVENT, 1 << 0, RCVBUF);
+		fd = create_and_bind_to_netlink(NETLINK_KOBJECT_UEVENT, 1 << 0, KERN_RCVBUF);
 
 		/*
 		 * Make inital scan after the uevent socket is alive and