[Devel] [PATCH vz10] selftests/uevent: give the netlink socket a usable receive buffer

[Devel] [PATCH vz10] selftests/uevent: give the netlink socket a usable receive buffer

[Eva Kurchatova]

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

https://virtuozzo.atlassian.net/browse/VSTOR-139674
Feature: fix selftests
Signed-off-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
---
 tools/testing/selftests/uevent/uevent_filtering.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/uevent/uevent_filtering.c b/tools/testing/selftests/uevent/uevent_filtering.c
index 8062804ff759..3b504c9e645d 100644
--- a/tools/testing/selftests/uevent/uevent_filtering.c
+++ b/tools/testing/selftests/uevent/uevent_filtering.c
@@ -22,7 +22,7 @@
 #include "../kselftest_harness.h"
 
 #define __DEV_FULL "/sys/devices/virtual/mem/full/uevent"
-#define __UEVENT_BUFFER_SIZE (2048 * 2)
+#define __UEVENT_BUFFER_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
-- 
2.55.0

Re: [Devel] [PATCH vz10] selftests/uevent: give the netlink socket a usable receive buffer

[Vasileios Almpanis]

On Fri, 21 Aug 2026 18:03:58 +0300, Eva Kurchatova <eva.kurchatova@virtuozzo.com> wrote:
> 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
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-139674
> 
> [...]

Reviewed-by: Vasileios Almpanis <vasileios.almpanis@virtuozzo.com>

-- 
Vasileios Almpanis <vasileios.almpanis@virtuozzo.com>

[Devel] [PATCH vz10 v2] selftests/uevent: do not fail on a netlink receive buffer overrun

[Konstantin Khorenko]

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>
---
Changes in v2:
- keep __UEVENT_BUFFER_SIZE at 4KB and give SO_RCVBUF its own
  __UEVENT_RCVBUF_SIZE.  v1 raised the shared macro, which sized the
  socket queue correctly but also turned the per message buffer into a
  128KB zero initialized array on the stack, while the kernel caps a
  single uevent at UEVENT_BUFFER_SIZE, 2048 bytes.
- retry recvmsg() on ENOBUFS instead of failing.  v1 only made the
  overrun less likely; the test still died on the first one, even
  though do_test() triggers ten uevents precisely so that drops are
  tolerated.
- subject and commit message updated accordingly.

 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 8062804ff759..735eb8138c44 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;
-- 
2.47.1

Re: [Devel] [PATCH vz10 v2] selftests/uevent: do not fail on a netlink receive buffer overrun

[Konstantin Khorenko]

a kind ping

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team

On 8/24/26 15:09, Konstantin Khorenko wrote:
> 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>
> ---
> Changes in v2:
> - keep __UEVENT_BUFFER_SIZE at 4KB and give SO_RCVBUF its own
>   __UEVENT_RCVBUF_SIZE.  v1 raised the shared macro, which sized the
>   socket queue correctly but also turned the per message buffer into a
>   128KB zero initialized array on the stack, while the kernel caps a
>   single uevent at UEVENT_BUFFER_SIZE, 2048 bytes.
> - retry recvmsg() on ENOBUFS instead of failing.  v1 only made the
>   overrun less likely; the test still died on the first one, even
>   though do_test() triggers ten uevents precisely so that drops are
>   tolerated.
> - subject and commit message updated accordingly.
> 
>  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 8062804ff759..735eb8138c44 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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Re: [Devel] [PATCH vz10 v2] selftests/uevent: do not fail on a netlink receive buffer overrun

[Eva Kurchatova (Virtuozzo)]

On 8/24/26 16:09, Konstantin Khorenko wrote:
> 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>
> ---
> Changes in v2:
> - keep __UEVENT_BUFFER_SIZE at 4KB and give SO_RCVBUF its own
>    __UEVENT_RCVBUF_SIZE.  v1 raised the shared macro, which sized the
>    socket queue correctly but also turned the per message buffer into a
>    128KB zero initialized array on the stack, while the kernel caps a
>    single uevent at UEVENT_BUFFER_SIZE, 2048 bytes.
> - retry recvmsg() on ENOBUFS instead of failing.  v1 only made the
>    overrun less likely; the test still died on the first one, even
>    though do_test() triggers ten uevents precisely so that drops are
>    tolerated.
> - subject and commit message updated accordingly.
>
>   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 8062804ff759..735eb8138c44 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;

I was originally concerned about spinning on recvmsg(), but considering 
the socket is blocking, and recvmsg() should consume the error once, 
this should be fine.

Otherwise, this is a good improvement over the v1.


Acknowledged-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
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Re: [Devel] [PATCH RHEL10 COMMIT] selftests/uevent: do not fail on a netlink receive buffer overrun

[Konstantin Khorenko]

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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