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 08aa72597..f9e3444d5 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]);
 }