cmd/snap-confine: start cwd in outer namespace

Inspect the current working directory in the initial mount namespace.
While this doesn't change anything it is more natural and more resilient
to breakage in case kernel changes semantics of fstatfs invoked across
unshare/setns.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

cmd/snap-confine/snap-confine.c

@@ -106,6 +106,7 @@ static void sc_maybe_fixup_udev(void)
 typedef struct sc_preserved_process_state {
 	mode_t orig_umask;
 	int orig_cwd_fd;
+	struct stat file_info_orig_cwd;
 } sc_preserved_process_state;
 
 /**
@@ -128,10 +129,14 @@ static void sc_preserve_and_sanitize_process_state(sc_preserved_process_state *
 	 * directory. This is an O_PATH file descriptor. The descriptor is
 	 * used as explained below. */
 	proc_state->orig_cwd_fd =
-	    openat(AT_FDCWD, ".", O_PATH | O_DIRECTORY | O_CLOEXEC | O_NOFOLLOW);
+	    openat(AT_FDCWD, ".",
+		   O_PATH | O_DIRECTORY | O_CLOEXEC | O_NOFOLLOW);
 	if (proc_state->orig_cwd_fd < 0) {
 		die("cannot open path of the current working directory");
 	}
+	if (fstat(proc_state->orig_cwd_fd, &proc_state->file_info_orig_cwd) < 0) {
+		die("cannot stat path of the current working directory");
+	}
 	if (chdir("/") < 0) {
 		die("cannot move to /");
 	}
@@ -180,7 +185,8 @@ static void sc_restore_process_state(const sc_preserved_process_state *
 	 * execution environment. This may normally fail if the path no longer
 	 * exists here, this is not a fatal error. */
 	int inner_cwd_fd SC_CLEANUP(sc_cleanup_close) = -1;
-	inner_cwd_fd = open(orig_cwd, O_PATH | O_DIRECTORY | O_CLOEXEC | O_NOFOLLOW);
+	inner_cwd_fd =
+	    open(orig_cwd, O_PATH | O_DIRECTORY | O_CLOEXEC | O_NOFOLLOW);
 	if (inner_cwd_fd < 0 && errno != ENOENT) {
 		die("cannot open path of the original working directory %s (%d)", orig_cwd, errno);
 	}
@@ -195,19 +201,19 @@ static void sc_restore_process_state(const sc_preserved_process_state *
 		}
 		debug("cannot represent original working directory %s",
 		      orig_cwd);
-		debug("the process has been placed in the special void directory");
+		debug
+		    ("the process has been placed in the special void directory");
 	} else {
 		/* The original working directory exists in the execution environment
 		 * which lets us check if it points to the same inode as before. */
-		struct stat file_info_outer, file_info_inner;
-		if (fstat(proc_state->orig_cwd_fd, &file_info_outer) < 0) {
-			die("cannot stat path of working directory in the host environment");
-		}
+		struct stat file_info_inner;
 		if (fstat(inner_cwd_fd, &file_info_inner) < 0) {
 			die("cannot stat path of working directory in the execution environment");
 		}
-		if (file_info_outer.st_dev == file_info_inner.st_dev &&
-		    file_info_outer.st_ino == file_info_inner.st_ino) {
+		if (proc_state->file_info_orig_cwd.st_dev ==
+		    file_info_inner.st_dev
+		    && proc_state->file_info_orig_cwd.st_ino ==
+		    file_info_inner.st_ino) {
 			/* The path of the original working directory points to the same
 			 * inode as before. Use fchdir to change to that directory.
 			 *