From: Ard Biesheuvel <ardb@kernel.org>

The EFI libstub performs some redundant conversions between UTF-16 and
UTF-8 and back again, which includes dealing with surrogate pairs, which
UEFI implementations themselves simply ignore.

So drop all the homegrown code, and use the existing UCS-2 (== UTF-16
without surrogate pairs) library code where conversion to UTF-8 is
actually needed (the kernel command line).

The remaining handling involves the EFI console, which supports wide
characters natively, so just use those directly.

Changes since v1 [0]:
- drop size limit from ucs2_strscpy() instead of just the WARN()
- suppress modinfo sections from ucs2_string when __DISABLE_EXPORTS is
  defined
- allow the input limit and max output size to be passed separately to
  ucs2_to_utf8()
- reimplement efi_convert_cmdline() to optimize the common case, and
  only process the input character by character if its size exceeds
  COMMAND_LINE_SIZE
- use memcpy() to avoid strscpy() semantics in handling of %ls
- incorporate Vincent's patch (which inspired this work) for
  completeness
	
Cc: Vincent Mailhol <mailhol@kernel.org>
Cc: x86@kernel.org

[0] https://lore.kernel.org/all/20260906130817.1151961-9-ardb@kernel.org

Ard Biesheuvel (9):
  x86/boot: Drop pointless re-implementation of panic()
  lib/ucs2_string: Drop arbitrary input size limit and associated WARN()
  lib/ucs2_string: Suppress modinfo when __DISABLE_EXPORTS is set
  lib/ucs2_string: Split out ucs2_as_utf8_l() taking a separate limit
  efi/libstub: Use ucs2_string library for UTF-16 to UTF-8 conversion
  efi/libstub: Avoid efi_puts() for compile time constant strings
  efi/libstub: Output UTF-16 directly from vsnprintf()
  efi/libstub: Add support for printing human readable GUIDs
  efi/libstub: Add efi_snprintf() to construct wide strings

Vincent Mailhol (1):
  efi/libstub: add initial Boot Loader Interface support

 arch/x86/boot/compressed/error.c               |  19 ---
 arch/x86/boot/compressed/error.h               |   1 -
 arch/x86/boot/compressed/mem.c                 |   2 +-
 drivers/firmware/efi/libstub/Makefile          |   3 +-
 drivers/firmware/efi/libstub/bli.c             |  87 ++++++++++++
 drivers/firmware/efi/libstub/efi-stub-helper.c | 100 +++++--------
 drivers/firmware/efi/libstub/efi-stub.c        |   1 +
 drivers/firmware/efi/libstub/efistub.h         |   8 +-
 drivers/firmware/efi/libstub/gop.c             |   6 +-
 drivers/firmware/efi/libstub/printk.c          |  95 ++-----------
 drivers/firmware/efi/libstub/vsprintf.c        | 150 +++++++-------------
 drivers/firmware/efi/libstub/x86-stub.c        |   1 +
 include/linux/efi.h                            |  22 +++
 include/linux/ucs2_string.h                    |  11 +-
 lib/ucs2_string.c                              |  10 +-
 15 files changed, 241 insertions(+), 275 deletions(-)
 create mode 100644 drivers/firmware/efi/libstub/bli.c

-- 
2.55.0.1003.g10538fe699-goog

On 09/09/2026 at 13:55, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> The EFI libstub performs some redundant conversions between UTF-16 and
> UTF-8 and back again, which includes dealing with surrogate pairs, which
> UEFI implementations themselves simply ignore.
> 
> So drop all the homegrown code, and use the existing UCS-2 (== UTF-16
> without surrogate pairs) library code where conversion to UTF-8 is
> actually needed (the kernel command line).
> 
> The remaining handling involves the EFI console, which supports wide
> characters natively, so just use those directly.
> 
> Changes since v1 [0]:
> - drop size limit from ucs2_strscpy() instead of just the WARN()
> - suppress modinfo sections from ucs2_string when __DISABLE_EXPORTS is
>   defined
> - allow the input limit and max output size to be passed separately to
>   ucs2_to_utf8()
> - reimplement efi_convert_cmdline() to optimize the common case, and
>   only process the input character by character if its size exceeds
>   COMMAND_LINE_SIZE
> - use memcpy() to avoid strscpy() semantics in handling of %ls
> - incorporate Vincent's patch (which inspired this work) for
>   completeness
> 	
> Cc: Vincent Mailhol <mailhol@kernel.org>

Sorry for the late feedback. I just finished my review. Aside from the
few comments I left, the runtime tests were all OK. My BLI patch works
fine on top of your changes.

Thanks again for the time spent to accommodate for my patch!


Yours sincerely,
Vincent Mailhol