Fallback to default loader if parsed one does not exist

[julian-klode]

If the specified second stage loader does not exist (invalid
parameter), fall back to the DEFAULT_LOADER. This avoids failing
the boot on any garbage that made it through the load option parser
as a second stage loader name.

Bug-Ubuntu: https://bugs.launchpad.net/bugs/1937115

[julian-klode]

Arguably I have not tested this. Seems to make some sense. Might want to reset load_options to NULL though? I guess I should setup a non-existing second stage loader and then see if it falls back. Maybe here it should sleep like 5s so you see the error message.

Though, eek, shouldn't it return "Not found" and not "Invalid parameter".

[julian-klode]

Updated with changes for load_options and EFI_NOT_FOUND. I believe in the bug in question, EFI_INVALID_PARAMETER was returned because the given bytes were simply not valid in filenames, but can't be sure.

[julian-klode]

Moved it out of the wrong if, added the msleep and a message, and tested in qemu by running shimx64.efi in efi shell with non-existing second-stage as argument :)

[julian-klode]

Also added code to dump the load options in debug mode

[haobinnan]

Exception occurred after using patches 393-1 / 393-2 / 399-1 / 399-2. And I got following messages after replacing \EFI\Microsoft\Boot\bootmgfw.efi with shimx64.efi:
Falled to open \EFI\Microsoft\Boot\S - Invalid Parameter
Failed to load image \EFI\Microsoft\Boot\S: Invalid Parameter
start_image() returned Invalid Parameter, falling back to default loader

@julian-klode

[vathpela]

Exception occurred after using patches 393-1 / 393-2 / 399-1 / 399-2. And I got following messages after replacing \EFI\Microsoft\Boot\bootmgfw.efi with shimx64.efi:
Falled to open \EFI\Microsoft\Boot\S - Invalid Parameter
Failed to load image \EFI\Microsoft\Boot\S: Invalid Parameter
start_image() returned Invalid Parameter, falling back to default loader

This is pretty clearly not caused by this change, so I'm going to merge this and let you all discuss that issue in the thread for #399.