Re: [Devel] [PATCH RHEL10 COMMIT] selftests/uevent: do not fail on a netlink receive buffer overrun

[Konstantin Khorenko <khorenko@virtuozzo.com>]

The commit is pushed to "branch-rh10-6.12.0-211.39.1.16.x.vz10-ovz" and will appear at git@bitbucket.org:openvz/vzkernel.git
after rh10-6.12.0-211.39.1.16.11.vz10
------>
commit dd27dfa9341d66afa10abd7e082b2c32dfd160d0
Author: Konstantin Khorenko <khorenko@virtuozzo.com>
Date:   Mon Aug 24 15:09:22 2026 +0200

    selftests/uevent: do not fail on a netlink receive buffer overrun
    
    SO_RCVBUF is set to __UEVENT_BUFFER_SIZE, 4KB, which a busy machine
    overruns while the test is listening:
    
      No buffer space available - Failed to receive uevent
    
    Two things are wrong here.
    
    The socket queue is sized after a single message, while do_test()
    deliberately triggers ten uevents "to account for the case where the
    kernel might drop some", so the queue has to hold more than one.
    
    Give it its own size and leave the message buffer alone: the kernel caps
    a single uevent at UEVENT_BUFFER_SIZE, 2048 bytes, so 4KB per message is
    already generous.
    
    The receive loop then treats every error as fatal, ENOBUFS included,
    which defeats those ten uevents. Netlink clears the error after
    reporting it once, so the copies still queued, or still on their way,
    are perfectly receivable. Retry instead.
    
    do_test() bounds the listener with a two second sigtimedwait(), so a
    retry cannot hang the test.
    
    https://virtuozzo.atlassian.net/browse/VSTOR-139674
    Feature: fix selftests
    Reported-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
    Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
    Reviewed-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
---
 tools/testing/selftests/uevent/uevent_filtering.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/uevent/uevent_filtering.c b/tools/testing/selftests/uevent/uevent_filtering.c
index 8062804ff759a..735eb8138c448 100644
--- a/tools/testing/selftests/uevent/uevent_filtering.c
+++ b/tools/testing/selftests/uevent/uevent_filtering.c
@@ -23,6 +23,11 @@
 
 #define __DEV_FULL "/sys/devices/virtual/mem/full/uevent"
 #define __UEVENT_BUFFER_SIZE (2048 * 2)
+/*
+ * The socket queue has to hold more than a single message: the test
+ * triggers ten uevents and a busy machine overruns a small buffer.
+ */
+#define __UEVENT_RCVBUF_SIZE (2048 * 64)
 #define __UEVENT_HEADER "add@/devices/virtual/mem/full"
 #define __UEVENT_HEADER_LEN sizeof("add@/devices/virtual/mem/full")
 #define __UEVENT_LISTEN_ALL -1
@@ -78,7 +83,7 @@ static int uevent_listener(unsigned long post_flags, bool expect_uevent,
 {
 	int sk_fd, ret;
 	socklen_t sk_addr_len;
-	int rcv_buf_sz = __UEVENT_BUFFER_SIZE;
+	int rcv_buf_sz = __UEVENT_RCVBUF_SIZE;
 	uint64_t sync_add = 1;
 	struct sockaddr_nl sk_addr = { 0 }, rcv_addr = { 0 };
 	char buf[__UEVENT_BUFFER_SIZE] = { 0 };
@@ -158,6 +163,12 @@ static int uevent_listener(unsigned long post_flags, bool expect_uevent,
 		ssize_t r;
 
 		r = recvmsg(sk_fd, &hdr, 0);
+		/*
+		 * The queue overran.  The kernel clears the error after
+		 * reporting it once and more uevents are on their way.
+		 */
+		if (r < 0 && errno == ENOBUFS)
+			continue;
 		if (r <= 0) {
 			fprintf(stderr, "%s - Failed to receive uevent\n", strerror(errno));
 			ret = -1;
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