Somebody mailed me this:

> I am trying to understand how fallback.efi works.  I have been reading
> shim.c and see that fallback is called when
> should_use_fallback(EFI_HANDLE image_handle) returns a 1.  But in
> should_use_fallback(EFI_HANDLE image_handle), there is a comparison of
> bootpath to \\EFI\\BOOT\\BOOT.  Why is it compare to \\EFI\\BOOT\\BOOT
> since bootpath always return \EFI\Boot\shim.efi?

And it seems like a common enough question that we need some
documentation of how to properly use shim and fallback.  Here's the
basics of it:

It doesn't always return \EFI\boot\shim.efi - in fact, not ever if
installed correctly.  The FS layouts are like this:

for removable media:
\EFI\BOOT\BOOTX64.EFI <-- shim
\EFI\BOOT\MokManager.efi
\EFI\BOOT\grubx64.efi
\EFI\BOOT\grub.cfg

for an installed system:
\EFI\BOOT\BOOTX64.EFI    <-- shim
\EFI\BOOT\MokManager.efi
\EFI\BOOT\fallback.efi
\EFI\fedora\BOOT.CSV
\EFI\fedora\shim.efi
\EFI\fedora\MokManager.efi
\EFI\fedora\grubx64.efi
\EFI\fedora\grub.cfg

When you boot removable media, it'll be in \EFI\BOOT , but fallback.efi
won't be there, so it goes ahead and boots the normal bootloader
(grubx64.efi).  When you boot a normal system through a boot variable,
the boot variable is configured to start \EFI\fedora\shim.efi (or
whatever your distro's EFI directory is.)  In that case it won't try to
invoke fallback.  But if the boot variables are missing or corrupted,
the firmware will eventually try to boot the hard disk as removable
media.  In that case, it'll invoke \EFI\BOOT\BOOTX64.EFI (or whatever
filename is right for your architecture.)  In that case it'll be in
\EFI\BOOT, so it'll check for fallback.efi , and it'll find it and run
it.  When it runs, fallback will look for every directory in \EFI\ with
a BOOT${ARCH}.CSV in it, or BOOT.CSV if that's not found.  It'll parse that,
and create new boot variables from what it finds.  Then it'll try to boot one
of them.

BOOT.CSV is a UCS-2 LE formatted CSV file.  So it has the LE byte order
marker, and after that it's just a series of lines, each having
comma-separated date.  It looks like this on Fedora:

shim.efi,Fedora,,This is the boot entry for Fedora

so basically it's:

$FILENAME,$LABEL,$LOADER_DATA,$COMMENT0[,$COMMENT1[,...]]

Where $FILENAME has to be the name of a file in the same directory as
BOOT.CSV .