Shim can't USB boot on Dell XPS 9343

[David Hewitt]

There appears to have been a regression in newer versions of shim-signed on my hardware.

When using the shimx64.efi file (from 1.40.6 or impish) as the \EFI\BOOT\BOOTx64.efi executable on USB media, the Dell XPS 9343 will fail to boot that media.

The output is as follows (photograph also attached):

Failed to open \EFI\BOOT\*garbled character* - Invalid Parameter
Failed to open \EFI\BOOT\*garbled character*: Invalid Parameter
start_image() returned Invalid Parameter

To reproduce, boot an Impish iso on a Dell XPS 9343 or replace the BOOTx64.efi on an Ubuntu 20.04.2 USB stick with shimx64.efi file from the 1.40.6 version of the package and attempt to boot on Dell XPS 9343.

I suspect when a new 20.04 point release iso is built, it will use the newer version of shim from the focal repos and hence cause this issue with an unmodified iso.

Interestingly, this issue does not occur when booting the same version of the shim binary from the internal SATA storage. shim-signed was recently successfully upgraded from 1.40.4 to 1.40.6 on a running focal install on the same hardware with no issues experienced after reboot.

i.e. I can copy the successfully booting shimx64.efi binary from my ESP on my internal SATA drive onto a previously successfully booting unmodified 20.04 USB stick, and break it with the newer verison of the shim. Equally, I can copy the older version of shim from an unmodified 20.04 iso and replace the shim on an Impish iso and get it to boot.

Secure boot is disabled, but enabling it results in the same error. BIOS revision is latest available (A20).