Introduce netns API

Write a new API and implementation to manage network namespace related
operations.

This will be used by the upcoming tundev module.

Change-Id: I0f2fba2fa42250a07211a7b7f479498f27c529da

5 files changed, 235 insertions, 0 deletions

diff --git a/TODO-RELEASE b/TODO-RELEASE
index 86363e14..f9b31a0f 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -9,3 +9,4 @@
 #library	what			description / commit summary line
 libosmocore new API osmo_sockaddr_is_any(), osmo_sockaddr_netmask_to_prefixlen()
 libosmocore	ABI breakage		OSMO_NUM_DLIB change affecting internal_cat[]
+libosmocore new API osmo_netns_*()
diff --git a/include/osmocom/core/Makefile.am b/include/osmocom/core/Makefile.am
index 1b0a1ce5..e88ce899 100644
--- a/include/osmocom/core/Makefile.am
+++ b/include/osmocom/core/Makefile.am
@@ -35,6 +35,7 @@ osmocore_HEADERS = \
 	stats.h \
 	macaddr.h \
 	msgb.h \
+	netns.h \
 	panic.h \
 	prbs.h \
 	prim.h \
diff --git a/include/osmocom/core/netns.h b/include/osmocom/core/netns.h
new file mode 100644
index 00000000..5bbf2242
--- /dev/null
+++ b/include/osmocom/core/netns.h
@@ -0,0 +1,24 @@
+/*! \file netns.h
+ *  Network namespace convenience functions. */
+
+#pragma once
+#if (!EMBEDDED)
+
+#if defined(__linux__)
+
+#include <signal.h>
+
+struct osmo_netns_switch_state {
+	sigset_t prev_sigmask;
+	int prev_nsfd;
+};
+
+int osmo_netns_open_fd(const char *name);
+int osmo_netns_switch_enter(int nsfd, struct osmo_netns_switch_state *state);
+int osmo_netns_switch_exit(struct osmo_netns_switch_state *state);
+
+
+#endif /* defined(__linux__) */
+
+#endif /* (!EMBEDDED) */
+/*! @} */
diff --git a/src/core/Makefile.am b/src/core/Makefile.am
index 9b812cef..216330a8 100644
--- a/src/core/Makefile.am
+++ b/src/core/Makefile.am
@@ -48,6 +48,7 @@ libosmocore_la_SOURCES = \
 	loggingrb.c \
 	macaddr.c \
 	msgb.c \
+	netns.c \
 	panic.c \
 	prbs.c \
 	prim.c \
diff --git a/src/core/netns.c b/src/core/netns.c
new file mode 100644
index 00000000..56b0c3ae
--- /dev/null
+++ b/src/core/netns.c
@@ -0,0 +1,208 @@
+
+/* Network namespace convenience functions
+ * (C) 2023 by sysmocom - s.m.f.c. GmbH <info@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ */
+
+#include "config.h"
+
+/*! \addtogroup netns
+ *  @{
+ *  Network namespace convenience functions
+ *
+ * \file netns.c */
+
+#if defined(__linux__)
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sched.h>
+#include <signal.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/socket.h>
+#include <sys/mount.h>
+#include <sys/param.h>
+#include <fcntl.h>
+#include <errno.h>
+
+#include <osmocom/core/utils.h>
+#include <osmocom/core/netns.h>
+
+#define NETNS_PREFIX_PATH "/var/run/netns"
+#define NETNS_CURRENT_PATH "/proc/self/ns/net"
+
+/*! Open a file descriptor for the current network namespace.
+ *  \returns fd of the current network namespace on success; negative in case of error
+ */
+static int netns_open_current_fd(void)
+{
+	int fd;
+	/* store the default namespace for later reference */
+	if ((fd = open(NETNS_CURRENT_PATH, O_RDONLY)) < 0)
+		return -errno;
+	return fd;
+}
+
+/*! switch to a (non-default) namespace, store existing signal mask in oldmask.
+ *  \param[in] nsfd file descriptor representing the namespace to which we shall switch
+ *  \param[out] state caller-provided memory location to which state of previous netns is stored
+ *  \returns 0 on success; negative on error */
+int osmo_netns_switch_enter(int nsfd, struct osmo_netns_switch_state *state)
+{
+	sigset_t intmask;
+	int rc;
+
+	state->prev_nsfd = -1;
+
+	if (sigfillset(&intmask) < 0)
+		return -errno;
+	if ((rc = sigprocmask(SIG_BLOCK, &intmask, &state->prev_sigmask)) != 0)
+		return -rc;
+	state->prev_nsfd = netns_open_current_fd();
+
+	if (setns(nsfd, CLONE_NEWNET) < 0) {
+		/* restore old mask if we couldn't switch the netns */
+		sigprocmask(SIG_SETMASK, &state->prev_sigmask, NULL);
+		close(state->prev_nsfd);
+		state->prev_nsfd = -1;
+		return -errno;
+	}
+	return 0;
+}
+
+/*! switch back to the previous namespace, restoring signal mask.
+ *  \param[in] state information about previous netns, filled by osmo_netns_switch_enter()
+ *  \returns 0 on successs; negative on error */
+int osmo_netns_switch_exit(struct osmo_netns_switch_state *state)
+{
+	if (state->prev_nsfd < 0)
+		return -EINVAL;
+
+	int rc;
+	if (setns(state->prev_nsfd, CLONE_NEWNET) < 0)
+		return -errno;
+
+	close(state->prev_nsfd);
+	state->prev_nsfd = -1;
+
+	if ((rc = sigprocmask(SIG_SETMASK, &state->prev_sigmask, NULL)) != 0)
+		return -rc;
+	return 0;
+}
+
+static int create_netns(const char *name)
+{
+	char path[MAXPATHLEN];
+	sigset_t intmask, oldmask;
+	int fd, prev_nsfd;
+	int rc, rc2;
+
+	/* create /var/run/netns, if it doesn't exist already */
+	rc = mkdir(NETNS_PREFIX_PATH, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH);
+	if (rc < 0 && errno != EEXIST)
+		return rc;
+
+	/* create /var/run/netns/[name], if it doesn't exist already */
+	rc = snprintf(path, sizeof(path), "%s/%s", NETNS_PREFIX_PATH, name);
+	if (rc >= sizeof(path))
+		return -ENAMETOOLONG;
+	fd = open(path, O_RDONLY|O_CREAT|O_EXCL, 0);
+	if (fd < 0)
+		return -errno;
+	if (close(fd) < 0)
+		return -errno;
+
+	/* mask off all signals, store old signal mask */
+	if (sigfillset(&intmask) < 0)
+		return -errno;
+	if ((rc = sigprocmask(SIG_BLOCK, &intmask, &oldmask)) != 0)
+		return -rc;
+
+	prev_nsfd = netns_open_current_fd();
+	if (prev_nsfd < 0)
+		return prev_nsfd;
+
+	/* create a new network namespace */
+	if (unshare(CLONE_NEWNET) < 0) {
+		rc = -errno;
+		goto restore_sigmask;
+	}
+	if (mount(NETNS_CURRENT_PATH, path, "none", MS_BIND, NULL) < 0) {
+		rc = -errno;
+		goto restore_sigmask;
+	}
+
+	/* switch back to previous namespace */
+	if (setns(prev_nsfd, CLONE_NEWNET) < 0) {
+		rc = -errno;
+		goto restore_sigmask;
+	}
+
+restore_sigmask:
+	close(prev_nsfd);
+
+	/* restore process mask */
+	if ((rc2 = sigprocmask(SIG_SETMASK, &oldmask, NULL)) != 0)
+		return -rc2;
+
+	/* might have been set above in case mount fails */
+	if (rc < 0)
+		return rc;
+
+	/* finally, open the created namespace file descriptor from previous ns */
+	if ((fd = open(path, O_RDONLY)) < 0)
+		return -errno;
+
+	return fd;
+}
+
+/*! Open a file descriptor for the network namespace with provided name.
+ *  Creates /var/run/netns/ directory if it doesn't exist already.
+ *  \param[in] name Name of the network namespace (in /var/run/netns/)
+ *  \returns File descriptor of network namespace; negative in case of error
+ */
+int osmo_netns_open_fd(const char *name)
+{
+	int rc;
+	int fd;
+	char path[MAXPATHLEN];
+
+	/* path = /var/run/netns/[name] */
+	rc = snprintf(path, sizeof(path), "%s/%s", NETNS_PREFIX_PATH, name);
+	if (rc >= sizeof(path))
+		return -ENAMETOOLONG;
+
+	/* If netns already exists, simply open it: */
+	fd = open(path, O_RDONLY);
+	if (fd >= 0)
+		return fd;
+
+	/* The netns doesn't exist yet, let's create it: */
+	fd = create_netns(name);
+	return fd;
+}
+
+#endif /* defined(__linux__) */
+
+/*! @} */