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core: add namespace_fork() helper, that forks, joins a set of namespaces and forks again

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This helper is useful to ensure pidns/userns joining is properly
executed (as that requires a fork after the setns()). This is
particularly important when it comes to /proc/self/ access or
SCM_CREDENTIALS, but is generally the safer mode of operation.
This commit is contained in:
Lennart Poettering 2018-11-12 23:37:13 +01:00
parent 8724defeae
commit 2709698279
2 changed files with 56 additions and 0 deletions
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54
src/basic/process-util.c
View file

@ -1409,6 +1409,60 @@ int safe_fork_full(
return 0;
}
int namespace_fork(
const char *outer_name,
const char *inner_name,
const int except_fds[],
size_t n_except_fds,
ForkFlags flags,
int pidns_fd,
int mntns_fd,
int netns_fd,
int userns_fd,
int root_fd,
pid_t *ret_pid) {
int r;
/* This is much like safe_fork(), but forks twice, and joins the specified namespaces in the middle
* process. This ensures that we are fully a member of the destination namespace, with pidns an all, so that
* /proc/self/fd works correctly. */
r = safe_fork_full(outer_name, except_fds, n_except_fds, (flags|FORK_DEATHSIG) & ~(FORK_REOPEN_LOG|FORK_NEW_MOUNTNS|FORK_MOUNTNS_SLAVE), ret_pid);
if (r < 0)
return r;
if (r == 0) {
pid_t pid;
/* Child */
r = namespace_enter(pidns_fd, mntns_fd, netns_fd, userns_fd, root_fd);
if (r < 0) {
log_full_errno(FLAGS_SET(flags, FORK_LOG) ? LOG_ERR : LOG_DEBUG, r, "Failed to join namespace: %m");
_exit(EXIT_FAILURE);
}
/* We mask a few flags here that either make no sense for the grandchild, or that we don't have to do again */
r = safe_fork_full(inner_name, except_fds, n_except_fds, flags & ~(FORK_WAIT|FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS|FORK_NULL_STDIO), &pid);
if (r < 0)
_exit(EXIT_FAILURE);
if (r == 0) {
/* Child */
if (ret_pid)
*ret_pid = pid;
return 0;
}
r = wait_for_terminate_and_check(inner_name, pid, FLAGS_SET(flags, FORK_LOG) ? WAIT_LOG : 0);
if (r < 0)
_exit(EXIT_FAILURE);
_exit(r);
}
return 1;
}
int fork_agent(const char *name, const int except[], size_t n_except, pid_t *ret_pid, const char *path, ...) {
bool stdout_is_tty, stderr_is_tty;
size_t n, i;

2
src/basic/process-util.h
View file

@ -159,6 +159,8 @@ static inline int safe_fork(const char *name, ForkFlags flags, pid_t *ret_pid) {
return safe_fork_full(name, NULL, 0, flags, ret_pid);
}
int namespace_fork(const char *outer_name, const char *inner_name, const int except_fds[], size_t n_except_fds, ForkFlags flags, int pidns_fd, int mntns_fd, int netns_fd, int userns_fd, int root_fd, pid_t *ret_pid);
int fork_agent(const char *name, const int except[], size_t n_except, pid_t *pid, const char *path, ...) _sentinel_;
int set_oom_score_adjust(int value);