unshare: make pidfd_open() use more portable and robust

* use #ifdef UL_HAVE_PIDFD for better portability

* open and close fd_parent_pid only when --kill-child specified

Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/sys-utils/unshare.c b/sys-utils/unshare.c
index d35f1df27412ef9d637dc4d6b320a4788fec58c6..755631ff4d75b1c37540370e00114dc9a309d9b8 100644 (file)
--- a/sys-utils/unshare.c
+++ b/sys-utils/unshare.c
@@ -766,7 +766,9 @@ int main(int argc, char *argv[])
        const char *newdir = NULL;
        pid_t pid_bind = 0, pid_idmap = 0;
        pid_t pid = 0;
+#ifdef UL_HAVE_PIDFD
        int fd_parent_pid = -1;
+#endif
        int fd_idmap, fd_bind = -1;
        sigset_t sigset, oldsigset;
        int status;
@@ -956,13 +958,16 @@ int main(int argc, char *argv[])
                        sigaddset(&sigset, SIGTERM) != 0 ||
                        sigprocmask(SIG_BLOCK, &sigset, &oldsigset) != 0)
                        err(EXIT_FAILURE, _("sigprocmask block failed"));
-
-               /* force child forking before mountspace binding
-                * so pid_for_children is populated */
-               fd_parent_pid = pidfd_open(getpid(), 0);
-               if (0 > fd_parent_pid)
-                       err(EXIT_FAILURE, _("pidfd_open failed"));
-
+#ifdef UL_HAVE_PIDFD
+               if (kill_child_signo != 0) {
+                       /* make a connection to the original process (parent) */
+                       fd_parent_pid = pidfd_open(getpid(), 0);
+                       if (0 > fd_parent_pid)
+                               err(EXIT_FAILURE, _("pidfd_open failed"));
+               }
+#endif
+               /* force child forking before mountspace binding so
+                * pid_for_children is populated */
                pid = fork();
 
                switch(pid) {
@@ -1017,29 +1022,30 @@ int main(int argc, char *argv[])
        if (kill_child_signo != 0) {
                if (prctl(PR_SET_PDEATHSIG, kill_child_signo) < 0)
                        err(EXIT_FAILURE, "prctl failed");
-
-               struct pollfd pollfds[1] = {
-                       {
-                               .fd = fd_parent_pid,
-                               .events = POLLIN,
-                       }
-                };
-
-               int nfds = poll(pollfds, 1, 0);
-               if (0 > nfds)
-                       err(EXIT_FAILURE, "poll parent pidfd failed");
-
-               /* If the child was re-parented before prctl(2) was called,
-                * the new parent will likely not be interested in the
-                 * precise exit status of the orphan.
-                */
-               if (nfds)
-                       exit(EXIT_FAILURE);
+#ifdef UL_HAVE_PIDFD
+               /* Use poll() to check that there is still the original parent. */
+               if (fd_parent_pid != -1) {
+                       struct pollfd pollfds[1] = {
+                               { .fd = fd_parent_pid, .events = POLLIN }
+                       };
+                       int nfds = poll(pollfds, 1, 0);
+
+                       if (0 > nfds)
+                               err(EXIT_FAILURE, "poll parent pidfd failed");
+
+                       /* If the child was re-parented before prctl(2) was called, the
+                        * new parent will likely not be interested in the precise exit
+                        * status of the orphan.
+                        */
+                       if (nfds)
+                               exit(EXIT_FAILURE);
+
+                       close(fd_parent_pid);
+                       fd_parent_pid = -1;
+               }
+#endif
        }
 
-       close(fd_parent_pid);
-       fd_parent_pid = -1;
-
         if (mapuser != (uid_t) -1 && !usermap)
                map_id(_PATH_PROC_UIDMAP, mapuser, real_euid);