rootless: support a custom arg to the new process

let the process running as euid != 0 pass down an argument to the
process running in the user namespace.  This will be useful for
commands like rm -a that needs to join different namespaces, so that
we can re-exec separately for each of them.

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

3 files changed, 71 insertions, 8 deletions

diff --git a/pkg/rootless/rootless.go b/pkg/rootless/rootless.go
new file mode 100644
index 000000000..a531e43ce
--- /dev/null
+++ b/pkg/rootless/rootless.go
@@ -0,0 +1,9 @@
+package rootless
+
+// Opts allows to customize how re-execing to a rootless process is done
+type Opts struct {
+	// Argument overrides the arguments on the command line
+	// for the re-execed process.  The process in the namespace
+	// must use rootless.Argument() to read its value.
+	Argument string
+}
diff --git a/pkg/rootless/rootless_linux.go b/pkg/rootless/rootless_linux.go
index 933cfa2c2..69614cefc 100644
--- a/pkg/rootless/rootless_linux.go
+++ b/pkg/rootless/rootless_linux.go
@@ -61,6 +61,11 @@ func SkipStorageSetup() bool {
 	return skipStorageSetup
 }
 
+// Argument returns the argument that was set for the rootless session.
+func Argument() string {
+	return os.Getenv("_LIBPOD_ROOTLESS_ARG")
+}
+
 // GetRootlessUID returns the UID of the user in the parent userNS
 func GetRootlessUID() int {
 	uidEnv := os.Getenv("_LIBPOD_ROOTLESS_UID")
@@ -135,8 +140,16 @@ func JoinNS(pid uint, preserveFDs int) (bool, int, error) {
 
 // JoinDirectUserAndMountNS re-exec podman in a new userNS and join the user and mount
 // namespace of the specified PID without looking up its parent.  Useful to join directly
-// the conmon process.
+// the conmon process.  It is a convenience function for JoinDirectUserAndMountNSWithOpts
+// with a default configuration.
 func JoinDirectUserAndMountNS(pid uint) (bool, int, error) {
+	return JoinDirectUserAndMountNSWithOpts(pid, nil)
+}
+
+// JoinDirectUserAndMountNSWithOpts re-exec podman in a new userNS and join the user and
+// mount namespace of the specified PID without looking up its parent.  Useful to join
+// directly the conmon process.
+func JoinDirectUserAndMountNSWithOpts(pid uint, opts *Opts) (bool, int, error) {
 	if os.Geteuid() == 0 || os.Getenv("_LIBPOD_USERNS_CONFIGURED") != "" {
 		return false, -1, nil
 	}
@@ -153,6 +166,12 @@ func JoinDirectUserAndMountNS(pid uint) (bool, int, error) {
 	}
 	defer userNS.Close()
 
+	if opts != nil && opts.Argument != "" {
+		if err := os.Setenv("_LIBPOD_ROOTLESS_ARG", opts.Argument); err != nil {
+			return false, -1, err
+		}
+	}
+
 	pidC := C.reexec_userns_join(C.int(userNS.Fd()), C.int(mountNS.Fd()))
 	if int(pidC) < 0 {
 		return false, -1, errors.Errorf("cannot re-exec process")
@@ -211,8 +230,16 @@ func getMinimumIDs(p string) int {
 // BecomeRootInUserNS re-exec podman in a new userNS.  It returns whether podman was re-executed
 // into a new user namespace and the return code from the re-executed podman process.
 // If podman was re-executed the caller needs to propagate the error code returned by the child
-// process.
+// process.  It is a convenience function for BecomeRootInUserNSWithOpts with a default configuration.
 func BecomeRootInUserNS() (bool, int, error) {
+	return BecomeRootInUserNSWithOpts(nil)
+}
+
+// BecomeRootInUserNSWithOpts re-exec podman in a new userNS.  It returns whether podman was
+// re-execute into a new user namespace and the return code from the re-executed podman process.
+// If podman was re-executed the caller needs to propagate the error code returned by the child
+// process.
+func BecomeRootInUserNSWithOpts(opts *Opts) (bool, int, error) {
 	if os.Geteuid() == 0 || os.Getenv("_LIBPOD_USERNS_CONFIGURED") != "" {
 		if os.Getenv("_LIBPOD_USERNS_CONFIGURED") == "init" {
 			return false, 0, runInUser()
@@ -231,6 +258,12 @@ func BecomeRootInUserNS() (bool, int, error) {
 	defer w.Close()
 	defer w.Write([]byte("0"))
 
+	if opts != nil && opts.Argument != "" {
+		if err := os.Setenv("_LIBPOD_ROOTLESS_ARG", opts.Argument); err != nil {
+			return false, -1, err
+		}
+	}
+
 	pidC := C.reexec_in_user_namespace(C.int(r.Fd()))
 	pid := int(pidC)
 	if pid < 0 {
diff --git a/pkg/rootless/rootless_unsupported.go b/pkg/rootless/rootless_unsupported.go
index 1823c023e..54e70961b 100644
--- a/pkg/rootless/rootless_unsupported.go
+++ b/pkg/rootless/rootless_unsupported.go
@@ -11,10 +11,18 @@ func IsRootless() bool {
 	return false
 }
 
+// BecomeRootInUserNS re-exec podman in a new userNS.  It returns whether podman was re-executed
+// into a new user namespace and the return code from the re-executed podman process.
+// If podman was re-executed the caller needs to propagate the error code returned by the child
+// process.  It is a convenience function for BecomeRootInUserNSWithOpts with a default configuration.
+func BecomeRootInUserNS() (bool, int, error) {
+	return false, -1, errors.New("this function is not supported on this os")
+}
+
 // BecomeRootInUserNS is a stub function that always returns false and an
 // error on unsupported OS's
-func BecomeRootInUserNS() (bool, int, error) {
-	return false, -1, errors.New("this function is not supported on this os1")
+func BecomeRootInUserNSWithOpts(opts *Opts) (bool, int, error) {
+	return false, -1, errors.New("this function is not supported on this os")
 }
 
 // GetRootlessUID returns the UID of the user in the parent userNS
@@ -34,18 +42,31 @@ func SkipStorageSetup() bool {
 // JoinNS re-exec podman in a new userNS and join the user namespace of the specified
 // PID.
 func JoinNS(pid uint) (bool, int, error) {
-	return false, -1, errors.New("this function is not supported on this os2")
+	return false, -1, errors.New("this function is not supported on this os")
 }
 
 // JoinNSPath re-exec podman in a new userNS and join the owner user namespace of the
 // specified path.
 func JoinNSPath(path string) (bool, int, error) {
-	return false, -1, errors.New("this function is not supported on this os3")
+	return false, -1, errors.New("this function is not supported on this os")
+}
+
+// JoinDirectUserAndMountNSWithOpts re-exec podman in a new userNS and join the user and
+// mount namespace of the specified PID without looking up its parent.  Useful to join
+// directly the conmon process.
+func JoinDirectUserAndMountNSWithOpts(pid uint, opts *Opts) (bool, int, error) {
+	return false, -1, errors.New("this function is not supported on this os")
 }
 
 // JoinDirectUserAndMountNS re-exec podman in a new userNS and join the user and mount
 // namespace of the specified PID without looking up its parent.  Useful to join directly
-// the conmon process.
+// the conmon process.  It is a convenience function for JoinDirectUserAndMountNSWithOpts
+// with a default configuration.
 func JoinDirectUserAndMountNS(pid uint) (bool, int, error) {
-	return false, -1, errors.New("this function is not supported on this os4")
+	return false, -1, errors.New("this function is not supported on this os")
+}
+
+// Argument returns the argument that was set for the rootless session.
+func Argument() string {
+	return ""
 }