README.fallback

   1 Somebody mailed me this:
   2
   3 > I am trying to understand how fallback.efi works.  I have been reading
   4 > shim.c and see that fallback is called when
   5 > should_use_fallback(EFI_HANDLE image_handle) returns a 1.  But in
   6 > should_use_fallback(EFI_HANDLE image_handle), there is a comparison of
   7 > bootpath to \\EFI\\BOOT\\BOOT.  Why is it compare to \\EFI\\BOOT\\BOOT
   8 > since bootpath always return \EFI\Boot\shim.efi?
   9
  10 And it seems like a common enough question that we need some
  11 documentation of how to properly use shim and fallback.  Here's the
  12 basics of it:
  13
  14 It doesn't always return \EFI\boot\shim.efi - in fact, not ever if
  15 installed correctly.  The FS layouts are like this:
  16
  17 for removable media:
  18 \EFI\BOOT\BOOTX64.EFI <-- shim
  19 \EFI\BOOT\MokManager.efi
  20 \EFI\BOOT\grubx64.efi
  21 \EFI\BOOT\grub.cfg
  22
  23 for an installed system:
  24 \EFI\BOOT\BOOTX64.EFI    <-- shim
  25 \EFI\BOOT\MokManager.efi
  26 \EFI\BOOT\fallback.efi
  27 \EFI\fedora\BOOT.CSV
  28 \EFI\fedora\shim.efi
  29 \EFI\fedora\MokManager.efi
  30 \EFI\fedora\grubx64.efi
  31 \EFI\fedora\grub.cfg
  32
  33 When you boot removable media, it'll be in \EFI\BOOT , but fallback.efi
  34 won't be there, so it goes ahead and boots the normal bootloader
  35 (grubx64.efi).  When you boot a normal system through a boot variable,
  36 the boot variable is configured to start \EFI\fedora\shim.efi (or
  37 whatever your distro's EFI directory is.)  In that case it won't try to
  38 invoke fallback.  But if the boot variables are missing or corrupted,
  39 the firmware will eventually try to boot the hard disk as removable
  40 media.  In that case, it'll invoke \EFI\BOOT\BOOTX64.EFI (or whatever
  41 filename is right for your architecture.)  In that case it'll be in
  42 \EFI\BOOT, so it'll check for fallback.efi , and it'll find it and run
  43 it.  When it runs, fallback will look for every directory in \EFI\ with
  44 a BOOT${ARCH}.CSV in it, or BOOT.CSV if that's not found.  It'll parse that,
  45 and create new boot variables from what it finds.  Then it'll try to boot one
  46 of them.
  47
  48 BOOT.CSV is a UCS-2 LE formatted CSV file.  So it has the LE byte order
  49 marker, and after that it's just a series of lines, each having
  50 comma-separated date.  It looks like this on Fedora:
  51
  52 shim.efi,Fedora,,This is the boot entry for Fedora
  53
  54 so basically it's:
  55
  56 $FILENAME,$LABEL,$LOADER_DATA,$COMMENT0[,$COMMENT1[,...]]
  57
  58 Where $FILENAME has to be the name of a file in the same directory as
  59 BOOT.CSV .
  60