switch_root: improve behavior on error; improve help text

*: make "can't execute '%s'" message uniform

1 files changed, 34 insertions, 28 deletions

diff --git a/util-linux/switch_root.c b/util-linux/switch_root.c
index 08aa725..f9e3444 100644
--- a/util-linux/switch_root.c
+++ b/util-linux/switch_root.c
@@ -5,11 +5,10 @@
  *
  * Licensed under GPL version 2, see file LICENSE in this tarball for details.
  */
-
 #include "libbb.h"
 #include <sys/vfs.h>
 
-// Make up for header deficiencies.
+// Make up for header deficiencies
 #ifndef RAMFS_MAGIC
 #define RAMFS_MAGIC ((unsigned)0x858458f6)
 #endif
@@ -22,7 +21,7 @@
 #define MS_MOVE     8192
 #endif
 
-// Recursively delete contents of rootfs.
+// Recursively delete contents of rootfs
 static void delete_contents(const char *directory, dev_t rootdev)
 {
 	DIR *dir;
@@ -33,7 +32,7 @@ static void delete_contents(const char *directory, dev_t rootdev)
 	if (lstat(directory, &st) || st.st_dev != rootdev)
 		return;
 
-	// Recursively delete the contents of directories.
+	// Recursively delete the contents of directories
 	if (S_ISDIR(st.st_mode)) {
 		dir = opendir(directory);
 		if (dir) {
@@ -51,42 +50,47 @@ static void delete_contents(const char *directory, dev_t rootdev)
 			}
 			closedir(dir);
 
-			// Directory should now be empty.  Zap it.
+			// Directory should now be empty, zap it
 			rmdir(directory);
 		}
-
-	// It wasn't a directory.  Zap it.
-	} else unlink(directory);
+	} else {
+		// It wasn't a directory, zap it
+		unlink(directory);
+	}
 }
 
 int switch_root_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int switch_root_main(int argc UNUSED_PARAM, char **argv)
 {
 	char *newroot, *console = NULL;
-	struct stat st1, st2;
+	struct stat st;
 	struct statfs stfs;
 	dev_t rootdev;
 
 	// Parse args (-c console)
 	opt_complementary = "-2"; // minimum 2 params
-	getopt32(argv, "+c:", &console); // '+': stop parsing at first non-option
+	getopt32(argv, "+c:", &console); // '+': stop at first non-option
 	argv += optind;
-
-	// Change to new root directory and verify it's a different fs.
 	newroot = *argv++;
 
+	// Change to new root directory and verify it's a different fs
 	xchdir(newroot);
-	if (lstat(".", &st1) || lstat("/", &st2) || st1.st_dev == st2.st_dev) {
-		bb_error_msg_and_die("bad newroot %s", newroot);
+	xstat("/", &st);
+	rootdev = st.st_dev;
+	xstat(".", &st);
+	if (st.st_dev == rootdev || getpid() != 1) {
+		// Show usage, it says new root must be a mountpoint
+		// and we must be PID 1
+		bb_show_usage();
 	}
-	rootdev = st2.st_dev;
-
-	// Additional sanity checks: we're about to rm -rf /,  so be REALLY SURE
-	// we mean it.  (I could make this a CONFIG option, but I would get email
-	// from all the people who WILL eat their filesystems.)
-	if (lstat("/init", &st1) || !S_ISREG(st1.st_mode) || statfs("/", &stfs)
-	 || (((unsigned)stfs.f_type != RAMFS_MAGIC) && ((unsigned)stfs.f_type != TMPFS_MAGIC))
-	 || (getpid() != 1)
+
+	// Additional sanity checks: we're about to rm -rf /, so be REALLY SURE
+	// we mean it. I could make this a CONFIG option, but I would get email
+	// from all the people who WILL destroy their filesystems.
+	statfs("/", &stfs); // this never fails
+	if (lstat("/init", &st) != 0 || !S_ISREG(st.st_mode)
+	 || ((unsigned)stfs.f_type != RAMFS_MAGIC
+	     && (unsigned)stfs.f_type != TMPFS_MAGIC)
 	) {
 		bb_error_msg_and_die("not rootfs");
 	}
@@ -94,14 +98,16 @@ int switch_root_main(int argc UNUSED_PARAM, char **argv)
 	// Zap everything out of rootdev
 	delete_contents("/", rootdev);
 
-	// Overmount / with newdir and chroot into it.  The chdir is needed to
-	// recalculate "." and ".." links.
-	if (mount(".", "/", NULL, MS_MOVE, NULL))
+	// Overmount / with newdir and chroot into it
+	if (mount(".", "/", NULL, MS_MOVE, NULL)) {
+		// For example, fails when newroot is not a mountpoint
 		bb_perror_msg_and_die("error moving root");
+	}
+	// The chdir is needed to recalculate "." and ".." links
 	xchroot(".");
 	xchdir("/");
 
-	// If a new console specified, redirect stdin/stdout/stderr to that.
+	// If a new console specified, redirect stdin/stdout/stderr to it
 	if (console) {
 		close(0);
 		xopen(console, O_RDWR);
@@ -109,7 +115,7 @@ int switch_root_main(int argc UNUSED_PARAM, char **argv)
 		xdup2(0, 2);
 	}
 
-	// Exec real init.  (This is why we must be pid 1.)
+	// Exec real init
 	execv(argv[0], argv);
-	bb_perror_msg_and_die("bad init %s", argv[0]);
+	bb_perror_msg_and_die("can't execute '%s'", argv[0]);
 }