[PATCH] selftests/net: test tcp_rmem lower bound

[Sahaj Chaudhari <sahaj123.sc@gmail.com>]

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 <sahaj123.sc@gmail.com>
---
 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)) {
+		ksft_test_result_fail("tcp_rmem changed after rejecting invalid input\n");
+		ksft_finished();
+	}
+
+	ksft_test_result_pass("tcp_rmem rejects a default below 4096\n");
+	ksft_finished();
+}
-- 
2.43.0