exec: support --preserve-fds

Allow to pass additional FDs to the process being executed.

Closes: https://github.com/containers/libpod/issues/2372

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/pkg/rootless/rootless_linux.go b/pkg/rootless/rootless_linux.go
index 98692707f..55fba900e 100644
--- a/pkg/rootless/rootless_linux.go
+++ b/pkg/rootless/rootless_linux.go
@@ -102,7 +102,7 @@ func tryMappingTool(tool string, pid int, hostID int, mappings []idtools.IDMap)
 
 // JoinNS re-exec podman in a new userNS and join the user namespace of the specified
 // PID.
-func JoinNS(pid uint) (bool, int, error) {
+func JoinNS(pid uint, preserveFDs int) (bool, int, error) {
 	if os.Geteuid() == 0 || os.Getenv("_LIBPOD_USERNS_CONFIGURED") != "" {
 		return false, -1, nil
 	}
@@ -117,6 +117,13 @@ func JoinNS(pid uint) (bool, int, error) {
 	if int(pidC) < 0 {
 		return false, -1, errors.Errorf("cannot re-exec process")
 	}
+	if preserveFDs > 0 {
+		for fd := 3; fd < 3+preserveFDs; fd++ {
+			// These fds were passed down to the runtime.  Close them
+			// and not interfere
+			os.NewFile(uintptr(fd), fmt.Sprintf("fd-%d", fd)).Close()
+		}
+	}
 
 	ret := C.reexec_in_user_namespace_wait(pidC)
 	if ret < 0 {