[ipxe-devel] EfiRom vs. iPXE usability note

Laszlo Ersek lersek at redhat.com
Fri Feb 19 17:33:22 UTC 2021

Hi All,

this is just a small note on the usability of the EfiRom utility from
BaseTools, in particular in combination with iPXE option ROMs.

The PCI Firmware Spec does not seem to specify a particular "checksum
byte" in the option ROM format, it only seems to state that the bytes in
the option ROM must sum to zero.

This (apparently) allows option ROM providers to implement different
schemes for placing the checksum byte.

When talking about traditional BIOS ROMs, EfiRom considers the last byte
in the image the checksum byte. The assumption is that the last byte is
padding anyway, so it can be repurposed as a checksum byte.

On the other hand, iPXE's "util/catrom.pl", or more precisely,
"util/Option/ROM.pm", considers byte#6 -- a reserved byte -- in the PCI
Expansion ROM Header the checksum byte.

iPXE's choice is arguably safer, because it does not assume any
particular padding at the end of the traditional ROM BIOS image that
could be stolen as checksum byte.

We've recently had a weird (difficult) bug [*] where, in the iPXE build,
the virtio-net BIOS option ROM ended up a whole multiple of 512 bytes,
*without* padding. Therefore, the "util/padimg.pl" utility, invoked by
the iPXE build process, would not append any padding to the end of the
BIOS ROM file.

Subsequently, when we combined the BIOS option ROM and the EFI option
ROM into a combined image, as follows:

  EfiRom -f "$vid" -i "$did" --pci23 \
         -b  bin/${rom}.rom \
         -ec bin-x86_64-efi/${rom}.efidrv \
         -o  bin-combined/${rom}.rom

EfiRom would *corrupt* the BIOS ROM, by changing its last byte (which
was *not* a padding byte, in this rare case). EfiRom justifiedly needed
to update the "last image indicator" in the BIOS ROM, but the checksum
compensation for that happened in the wrong place (the last byte was not
a padding byte).

Consequently, iPXE's self-decompression (LZMA) and/or CRC32 check would
fail during traditional BIOS boot, in the following call tree:

install_prealloc                 [arch/x86/prefix/libprefix.S]
  install_block (...)
  install_block (...)
  install_block (...)
  install_block (.textdata)
    process_bytes (decompress16)

This was difficult to debug because even small changes to the assembly
code (let alone enabling the existent, feature-ful, debugging
infrastructure) would increase the LZMA-compressed stream's size,
trigger padding during the build, and hide the corruption.

[*] https://bugzilla.redhat.com/show_bug.cgi?id=1926561

We found the following solution:

  EfiRom -f "$vid" -i "$did" --pci23 \
         -ec bin-x86_64-efi/${rom}.efidrv \
         -o  bin-combined/${rom}.eficrom
  util/catrom.pl \
      bin/${rom}.rom \
      bin-combined/${rom}.eficrom \
      > bin-combined/${rom}.rom

I.e., continue invoking EfiRom, but only for compressing the EFI driver
image, stand-alone. And the combining of the BIOS and EFI ROMs is now
performed with iPXE's "util/catrom.pl" program, which has the safer
checksum byte placement.


Like Michael has recommended before, it's also possible to eliminate
EfiRom from the equation completely, and just build "*.efirom" in place
of "*.efidrv". That would give us an EFI ROM image, passable to
"catrom.pl", unlike a "naked" EFI driver executable (*.efidrv).

However, iPXE's "util/efirom" tool, which converts *.efidrv to *.efirom,
doesn't seem to offer "EFI compression", while EfiRom does (-ec option).
For QEMU live-migration compatibility, we further pad the *combined* ROM
images, currently to 256 KB. Abandoning EFI compression would eat up
approx. 80KB suddenly, and nearly exhaust our current padding. Hence the
above "hybrid" approach, where we retain EfiRom for the EFI
compression's sake, but use "util/catrom.pl" for combining the images.


Assuming my reading of the PCI Firmware Spec is correct, I think that
not specifying a particular checksum byte, in the various headers, was a
mistake in the spec. It's difficult to combine ROMs of different origins
into a multi-ROM image, like this.

I'm sending this note because it could prove useful to other distros
that use EfiRom similarly.


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