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Support new --netns-type=tapfd
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The new value tapfd enables removing slirp4netns features. Rather than
having it figure out where to create a tap network, the opened tap file
descriptor can now be inherited to slirp4netns and used as is. This
removes the need for forking and attaching a namespace as well as
readiness communication, but it also removes the ability to choose the
name of the network interface or configuring it.

Signed-off-by: Helmut Grohne <helmut@subdivi.de>
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helmutg committed Mar 25, 2024
1 parent c76f542 commit 436755d
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Showing 2 changed files with 76 additions and 45 deletions.
114 changes: 71 additions & 43 deletions main.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -355,23 +355,18 @@ static int recvfd(int sock)
return fd;
}

static int parent(int sock, int ready_fd, int exit_fd, const char *api_socket,
static int parent(int tapfd, int ready_fd, int exit_fd, const char *api_socket,
struct slirp4netns_config *cfg, pid_t target_pid)
{
char str[INET6_ADDRSTRLEN];
int rc, tapfd;
int rc;
struct in_addr vdhcp_end = {
#define NB_BOOTP_CLIENTS 16
/* NB_BOOTP_CLIENTS is hard-coded to 16 in libslirp:
https://gitlab.freedesktop.org/slirp/libslirp/-/issues/49 */
.s_addr = htonl(ntohl(cfg->vdhcp_start.s_addr) + NB_BOOTP_CLIENTS - 1),
#undef NB_BOOTP_CLIENTS
};
if ((tapfd = recvfd(sock)) < 0) {
return tapfd;
}
fprintf(stderr, "received tapfd=%d\n", tapfd);
close(sock);
printf("Starting slirp\n");
printf("* MTU: %d\n", cfg->mtu);
printf("* Network: %s\n",
Expand Down Expand Up @@ -442,7 +437,7 @@ static int parent(int sock, int ready_fd, int exit_fd, const char *api_socket,

static void usage(const char *argv0)
{
printf("Usage: %s [OPTION]... PID|PATH [TAPNAME]\n", argv0);
printf("Usage: %s [OPTION]... PID|PATH|FD [TAPNAME]\n", argv0);
printf("User-mode networking for unprivileged network namespaces.\n\n");
printf("-c, --configure bring up the interface\n");
printf("-e, --exit-fd=FD specify the FD for terminating "
Expand All @@ -461,8 +456,8 @@ static void usage(const char *argv0)
printf("--disable-host-loopback prohibit connecting to 127.0.0.1:* on the "
"host namespace\n");
/* v0.4.0 */
printf("--netns-type=TYPE specify network namespace type ([path|pid], "
"default=%s)\n",
printf("--netns-type=TYPE specify network namespace type "
"([path|pid|tapfd], default=%s)\n",
DEFAULT_NETNS_TYPE);
printf("--userns-path=PATH specify user namespace path\n");
printf(
Expand Down Expand Up @@ -532,13 +527,15 @@ struct options {
char *macaddress; // --macaddress
char *target_type; // --target-type
char *bess_socket; // argv[1] (When --target-type="bess")
int tapfd; // argv[1] (When --netns-type="tapfd")
};

static void options_init(struct options *options)
{
memset(options, 0, sizeof(*options));
options->exit_fd = options->ready_fd = -1;
options->mtu = DEFAULT_MTU;
options->tapfd = -1;
}

static void options_destroy(struct options *options)
Expand Down Expand Up @@ -828,13 +825,9 @@ static void parse_args(int argc, char *const argv[], struct options *options)
fprintf(stderr, "--target-type must be either \"netns\" or \"bess\"\n");
goto error;
}
if (argc - optind < 2) {
if (argc - optind < 1) {
goto error;
}
if (argc - optind > 2) {
// not an error, for preventing potential compatibility issue
printf("WARNING: too many arguments\n");
}
if (!options->netns_type ||
strcmp(options->netns_type, DEFAULT_NETNS_TYPE) == 0) {
errno = 0;
Expand All @@ -843,14 +836,36 @@ static void parse_args(int argc, char *const argv[], struct options *options)
fprintf(stderr, "PID must be a positive integer\n");
goto error;
}
} else if (options->netns_type &&
strcmp(options->netns_type, "tapfd") == 0) {
errno = 0;
options->tapfd = strtol(argv[optind], &strtol_e, 10);
if (errno || *strtol_e != '\0' || options->tapfd < 0) {
fprintf(stderr, "TAPFD must a file descriptor\n");
goto error;
}
} else {
options->netns_path = strdup(argv[optind]);
if (access(options->netns_path, F_OK) == -1) {
perror("existing path expected when --netns-type=path");
goto error;
}
}
options->tapname = strdup(argv[optind + 1]);
if (options->tapfd >= 0) {
if (argc - optind > 1) {
// not an error, for preventing potential compatibility issue
printf("WARNING: too many arguments\n");
}
} else {
if (argc - optind < 2) {
goto error;
}
if (argc - optind > 2) {
// not an error, for preventing potential compatibility issue
printf("WARNING: too many arguments\n");
}
options->tapname = strdup(argv[optind + 1]);
}
return;
error:
usage(argv[0]);
Expand Down Expand Up @@ -1114,7 +1129,7 @@ static int slirp4netns_config_from_options(struct slirp4netns_config *cfg,
int main(int argc, char *const argv[])
{
int sv[2];
pid_t child_pid;
pid_t child_pid = -1;
struct options options;
struct slirp4netns_config slirp4netns_config;
int exit_status = 0;
Expand All @@ -1129,11 +1144,12 @@ int main(int argc, char *const argv[])
exit_status = EXIT_FAILURE;
goto finish;
}
if ((child_pid = fork()) < 0) {
perror("fork");
exit_status = EXIT_FAILURE;
goto finish;
}
if (options.tapfd < 0)
if ((child_pid = fork()) < 0) {
perror("fork");
exit_status = EXIT_FAILURE;
goto finish;
}
if (child_pid == 0) {
int ret;
if (options.target_type != NULL &&
Expand All @@ -1149,28 +1165,40 @@ int main(int argc, char *const argv[])
goto finish;
}
} else {
int ret, child_wstatus, child_status;
do
ret = waitpid(child_pid, &child_wstatus, 0);
while (ret < 0 && errno == EINTR);
if (ret < 0) {
perror("waitpid");
exit_status = EXIT_FAILURE;
goto finish;
}
if (!WIFEXITED(child_wstatus)) {
fprintf(stderr, "child failed(wstatus=%d, !WIFEXITED)\n",
child_wstatus);
exit_status = EXIT_FAILURE;
goto finish;
}
child_status = WEXITSTATUS(child_wstatus);
if (child_status != 0) {
fprintf(stderr, "child failed(%d)\n", child_status);
exit_status = child_status;
goto finish;
int tapfd;
if (options.tapfd >= 0) {
tapfd = options.tapfd;
} else {
int ret, child_wstatus, child_status;
do
ret = waitpid(child_pid, &child_wstatus, 0);
while (ret < 0 && errno == EINTR);
if (ret < 0) {
perror("waitpid");
exit_status = EXIT_FAILURE;
goto finish;
}
if (!WIFEXITED(child_wstatus)) {
fprintf(stderr, "child failed(wstatus=%d, !WIFEXITED)\n",
child_wstatus);
exit_status = EXIT_FAILURE;
goto finish;
}
child_status = WEXITSTATUS(child_wstatus);
if (child_status != 0) {
fprintf(stderr, "child failed(%d)\n", child_status);
exit_status = child_status;
goto finish;
}
if ((tapfd = recvfd(sv[0])) < 0) {
fprintf(stderr, "failed to receive tapfd from child\n");
exit_status = EXIT_FAILURE;
goto finish;
}
fprintf(stderr, "received tapfd=%d\n", tapfd);
close(sv[0]);
}
if (parent(sv[0], options.ready_fd, options.exit_fd, options.api_socket,
if (parent(tapfd, options.ready_fd, options.exit_fd, options.api_socket,
&slirp4netns_config, options.target_pid) < 0) {
fprintf(stderr, "parent failed\n");
exit_status = EXIT_FAILURE;
Expand Down
7 changes: 5 additions & 2 deletions slirp4netns.1
View file
Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@ slirp4netns \- User\-mode networking for unprivileged network namespaces

.SH SYNOPSIS
.PP
slirp4netns [OPTION]... PID|PATH [TAPNAME]
slirp4netns [OPTION]... PID|PATH|FD [TAPNAME]


.SH DESCRIPTION
Expand Down Expand Up @@ -84,7 +84,10 @@ prohibit connecting to 127.0.0.1:* on the host namespace

.PP
\fB\fC\-\-netns\-type=TYPE\fR (since v0.4.0)
specify network namespace type ([path|pid], default=pid)
specify network namespace type ([path|pid|tapfd], default=pid)

If the namespace type is tapfd, the first positional argument is expected to be an inherited file descriptor that corresponds to a \fB\fC/dev/net/tun\fR connection.
This conflicts with \fB\fC\-\-configure\fR and a \fB\fcTAPNAME\fR is ignored.

.PP
\fB\fC\-\-userns\-path=PATH\fR (since v0.4.0)
Expand Down

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