[Libguestfs] [PATCH supermin] init: Delete initramfs files before chrooting into the appliance.

[Richard W.M. Jones]

After supermin has finished running, the initramfs files sit around
occupying swappable memory but serving no further purpose.

This saves a little memory, at the cost of about 1ms of extra boot
time.
---
 init/init.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 48 insertions(+), 2 deletions(-)

diff --git a/init/init.c b/init/init.c
index 733d66e..5ac53e9 100644
--- a/init/init.c
+++ b/init/init.c
@@ -1,5 +1,5 @@
 /* supermin-helper reimplementation in C.
- * Copyright (C) 2009-2014 Red Hat Inc.
+ * Copyright (C) 2009-2016 Red Hat Inc.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -80,6 +80,7 @@ static void mount_proc (void);
 static void print_uptime (void);
 static void read_cmdline (void);
 static void insmod (const char *filename);
+static void delete_initramfs_files (void);
 static void show_directory (const char *dir);
 
 static char cmdline[1024];
@@ -264,9 +265,12 @@ main ()
     exit (EXIT_FAILURE);
   }
 
+  if (!quiet)
+    fprintf (stderr, "supermin: deleting initramfs files\n");
+  delete_initramfs_files ();
+
   /* Note that pivot_root won't work.  See the note in
    * Documentation/filesystems/ramfs-rootfs-initramfs.txt
-   * We could remove the old initramfs files, but let's not bother.
    */
   if (!quiet)
     fprintf (stderr, "supermin: chroot\n");
@@ -396,6 +400,48 @@ read_cmdline (void)
     cmdline[len-1] = '\0';
 }
 
+/* By deleting the files in the initramfs before we chroot, we save a
+ * little bit of memory (or quite a lot of memory if the user is using
+ * unstripped kmods).
+ *
+ * We only delete files in the root directory.  We don't delete
+ * directories because they only take a tiny amount of space and
+ * because we must not delete any mountpoints, especially not /root
+ * where we are about to chroot.
+ *
+ * We don't recursively look for files because that would be too
+ * complex and risky, and the normal supermin initramfs doesn't have
+ * any files except in the root directory.
+ */
+static void
+delete_initramfs_files (void)
+{
+  DIR *dir;
+  struct dirent *d;
+  struct stat statbuf;
+
+  if (chdir ("/") == -1) {
+    perror ("chdir: /");
+    return;
+  }
+
+  dir = opendir (".");
+  if (!dir) {
+    perror ("opendir: /");
+    return;
+  }
+
+  while ((d = readdir (dir)) != NULL) {
+    /* "." and ".." are directories, so the S_ISREG test ignores them. */
+    if (lstat (d->d_name, &statbuf) >= 0 && S_ISREG (statbuf.st_mode)) {
+      if (unlink (d->d_name) == -1)
+        perror (d->d_name);
+    }
+  }
+
+  closedir (dir);
+}
+
 /* Display a directory on stderr.  This is used for debugging only. */
 static char
 dirtype (int dt)
-- 
2.7.4