On 10/7/26 07:45, Sahaj Chaudhari wrote:
Exercise a tcp_rmem default below 4096 and verify that it is rejected.
A tiny default can make TCP receive-window accounting space zero and
trigger a divide-by-zero when the receive buffer grows.
Run the test in a fresh network namespace and verify that rejection
leaves the setting unchanged.
Signed-off-by: Sahaj Chaudhari <[email protected]>
---
tools/testing/selftests/net/.gitignore | 1 +
tools/testing/selftests/net/Makefile | 1 +
tools/testing/selftests/net/tcp_rmem_min.c | 78 ++++++++++++++++++++++
3 files changed, 80 insertions(+)
create mode 100644 tools/testing/selftests/net/tcp_rmem_min.c
diff --git a/tools/testing/selftests/net/.gitignore
b/tools/testing/selftests/net/.gitignore
index dacd36ed8455..44de061e3eef 100644
--- a/tools/testing/selftests/net/.gitignore
+++ b/tools/testing/selftests/net/.gitignore
@@ -48,6 +48,7 @@ tcp_fastopen_backup_key
tcp_inq
tcp_mmap
tcp_port_share
+tcp_rmem_min
tfo
timestamping
tls
diff --git a/tools/testing/selftests/net/Makefile
b/tools/testing/selftests/net/Makefile
index 590b33e16d9f..595f0e6c38b0 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -198,6 +198,7 @@ TEST_GEN_PROGS := \
so_incoming_cpu \
tap \
tcp_port_share \
+ tcp_rmem_min \
tls \
udp_splice_checksum \
# end of TEST_GEN_PROGS
diff --git a/tools/testing/selftests/net/tcp_rmem_min.c
b/tools/testing/selftests/net/tcp_rmem_min.c
new file mode 100644
index 000000000000..dd2305d7e599
--- /dev/null
+++ b/tools/testing/selftests/net/tcp_rmem_min.c
@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <fcntl.h>
+#include <sched.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "kselftest.h"
+
+#define TCP_RMEM_PATH "/proc/sys/net/ipv4/tcp_rmem"
+
+static int read_tcp_rmem(char *buf, size_t size)
+{
+ int fd = open(TCP_RMEM_PATH, O_RDONLY | O_CLOEXEC);
+ ssize_t len;
+
+ if (fd < 0)
+ return -1;
+
+ len = read(fd, buf, size - 1);
+ close(fd);
+ if (len < 0)
+ return -1;
+
+ buf[len] = '\0';
+ return 0;
+}
+
+int main(void)
+{
+ /* tcp_rmem is min, default, max; keep min valid and lower the default.
*/
+ static const char invalid_default[] = "4096 1 6291456";
+ char before[128], after[128];
+ ssize_t len;
+ int fd, write_errno;
+
+ ksft_print_header();
+ ksft_set_plan(1);
+
+ if (unshare(CLONE_NEWNET)) {
+ if (errno == EPERM || errno == EACCES)
+ ksft_exit_skip("network namespace unavailable\n");
+ ksft_exit_fail_perror("unshare(CLONE_NEWNET)");
+ }
+
+ if (read_tcp_rmem(before, sizeof(before)))
+ ksft_exit_fail_perror("read tcp_rmem before write");
+
+ fd = open(TCP_RMEM_PATH, O_WRONLY | O_CLOEXEC);
+ if (fd < 0)
+ ksft_exit_fail_perror("open tcp_rmem");
+
+ len = write(fd, invalid_default, sizeof(invalid_default) - 1);
+ write_errno = errno;
+ close(fd);
+
+ if (len >= 0) {
+ ksft_test_result_fail("tcp_rmem accepted a default below
4096\n");
+ ksft_finished();
+ }
+ if (write_errno != EINVAL && write_errno != ERANGE) {
+ errno = write_errno;
+ ksft_exit_fail_perror("write tcp_rmem");
+ }
+
+ if (read_tcp_rmem(after, sizeof(after)))
+ ksft_exit_fail_perror("read tcp_rmem after rejected write");
+ if (strcmp(before, after)) {
I do not think we want one selftest per sysctl range check.
net/ has more than 500 sysctls, most of them with bounds. A test
asserting that tcp_rmem's lower bound is 4096 only restates the table,
and proc_dointvec_minmax() itself is already covered by
tools/testing/selftests/sysctl.
This does not scale: each of these adds a binary, a CI slot and
something to maintain. This one already breaks if tcp_rmem[0] is not
4096 (PAGE_SIZE > 4K, or tuned host), because elements are stored one
by one and the rejected write still changes the min.
What would be useful is a single generic test: in a fresh netns, walk
the writable /proc/sys/net entries, set each to the extreme values the
kernel accepts, run some loopback TCP/UDP traffic, and check that
nothing splats. No bounds to duplicate, new sysctls covered for free,
and it would have caught the divide by zero fixed in commit
83a945a529d6 ("tcp: do not let tcp_rmem be set below 4096").
Thanks.
pw-bot: cr
