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

14 Jun 2016

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

    

Richard W.M. Jones

Richard W.M. Jones

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