[PATCH v3 1/2] unshare: mount binfmt_misc

[Laurent Vivier <laurent@vivier.eu>]

add --mount-binfmt[=<dir>] to mount binfmt_misc filesystem,
this results in clearing inherited interpreters from the previous namespace

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
---

Notes:
    v2:
      - remove definition of _PATH_PROC_BINFMT_MISC_REGISTER

 include/pathnames.h      |  1 +
 sys-utils/unshare.1.adoc |  3 +++
 sys-utils/unshare.c      | 19 +++++++++++++++++++
 3 files changed, 23 insertions(+)

diff --git a/include/pathnames.h b/include/pathnames.h
index 81fa405f63c7..569bef17f982 100644
--- a/include/pathnames.h
+++ b/include/pathnames.h
@@ -204,6 +204,7 @@
 /* sysctl fs paths */
 #define _PATH_PROC_SYS_FS	"/proc/sys/fs"
 #define _PATH_PROC_PIPE_MAX_SIZE	_PATH_PROC_SYS_FS "/pipe-max-size"
+#define _PATH_PROC_BINFMT_MISC	_PATH_PROC_SYS_FS "/binfmt_misc"
 
 /* irqtop paths */
 #define _PATH_PROC_INTERRUPTS	"/proc/interrupts"
diff --git a/sys-utils/unshare.1.adoc b/sys-utils/unshare.1.adoc
index e6201e28fffd..48d1a5579282 100644
--- a/sys-utils/unshare.1.adoc
+++ b/sys-utils/unshare.1.adoc
@@ -90,6 +90,9 @@ When *unshare* terminates, have _signame_ be sent to the forked child process. C
 *--mount-proc*[**=**__mountpoint__]::
 Just before running the program, mount the proc filesystem at _mountpoint_ (default is _/proc_). This is useful when creating a new PID namespace. It also implies creating a new mount namespace since the _/proc_ mount would otherwise mess up existing programs on the system. The new proc filesystem is explicitly mounted as private (with *MS_PRIVATE*|*MS_REC*).
 
+*--mount-binfmt*[**=**__mountpoint__]::
+Just before running the program, mount the binfmt_misc filesystem at _mountpoint_ (default is /proc/sys/fs/binfmt_misc).  It also implies creating a new mount namespace since the binfmt_misc mount would otherwise mess up existing programs on the system.  The new binfmt_misc filesystem is explicitly mounted as private (with *MS_PRIVATE*|*MS_REC*).
+
 **--map-user=**__uid|name__::
 Run the program only after the current effective user ID has been mapped to _uid_. If this option is specified multiple times, the last occurrence takes precedence. This option implies *--user*.
 
diff --git a/sys-utils/unshare.c b/sys-utils/unshare.c
index 57f3b8744fb5..d79aa1125955 100644
--- a/sys-utils/unshare.c
+++ b/sys-utils/unshare.c
@@ -760,6 +760,7 @@ static void __attribute__((__noreturn__)) usage(void)
 	fputs(_(" --kill-child[=<signame>]  when dying, kill the forked child (implies --fork)\n"
 		"                             defaults to SIGKILL\n"), out);
 	fputs(_(" --mount-proc[=<dir>]      mount proc filesystem first (implies --mount)\n"), out);
+	fputs(_(" --mount-binfmt[=<dir>]    mount binfmt filesystem first (implies --user and --mount)\n"), out);
 	fputs(_(" --propagation slave|shared|private|unchanged\n"
 	        "                           modify mount propagation in mount namespace\n"), out);
 	fputs(_(" --setgroups allow|deny    control the setgroups syscall in user namespaces\n"), out);
@@ -783,6 +784,7 @@ int main(int argc, char *argv[])
 {
 	enum {
 		OPT_MOUNTPROC = CHAR_MAX + 1,
+		OPT_MOUNTBINFMT,
 		OPT_PROPAGATION,
 		OPT_SETGROUPS,
 		OPT_KILLCHILD,
@@ -811,6 +813,7 @@ int main(int argc, char *argv[])
 		{ "fork",          no_argument,       NULL, 'f'             },
 		{ "kill-child",    optional_argument, NULL, OPT_KILLCHILD   },
 		{ "mount-proc",    optional_argument, NULL, OPT_MOUNTPROC   },
+		{ "mount-binfmt",  optional_argument, NULL, OPT_MOUNTBINFMT },
 		{ "map-user",      required_argument, NULL, OPT_MAPUSER     },
 		{ "map-users",     required_argument, NULL, OPT_MAPUSERS    },
 		{ "map-group",     required_argument, NULL, OPT_MAPGROUP    },
@@ -839,6 +842,7 @@ int main(int argc, char *argv[])
 	struct map_range *groupmap = NULL;
 	int kill_child_signo = 0; /* 0 means --kill-child was not used */
 	const char *procmnt = NULL;
+	const char *binfmt_mnt = NULL;
 	const char *newroot = NULL;
 	const char *newdir = NULL;
 	pid_t pid_bind = 0, pid_idmap = 0;
@@ -913,6 +917,15 @@ int main(int argc, char *argv[])
 			unshare_flags |= CLONE_NEWNS;
 			procmnt = optarg ? optarg : "/proc";
 			break;
+		case OPT_MOUNTBINFMT:
+			unshare_flags |= CLONE_NEWNS | CLONE_NEWUSER;
+			binfmt_mnt = optarg;
+			if (!binfmt_mnt) {
+				if (!procmnt)
+					procmnt = "/proc";
+				binfmt_mnt = _PATH_PROC_BINFMT_MISC;
+			}
+			break;
 		case OPT_MAPUSER:
 			unshare_flags |= CLONE_NEWUSER;
 			mapuser = get_user(optarg, _("failed to parse uid"));
@@ -1178,6 +1191,12 @@ int main(int argc, char *argv[])
 			err(EXIT_FAILURE, _("mount %s failed"), procmnt);
 	}
 
+	if (binfmt_mnt) {
+		if (mount("binfmt_misc", binfmt_mnt, "binfmt_misc",
+			  MS_NOSUID|MS_NOEXEC|MS_NODEV, NULL) != 0)
+			err(EXIT_FAILURE, _("mount %s failed"), binfmt_mnt);
+	}
+
 	if (force_gid) {
 		if (setgroups(0, NULL) != 0)	/* drop supplementary groups */
 			err(EXIT_FAILURE, _("setgroups failed"));
-- 
2.45.2