[v3] efi/libstub: Avoid UTF-16 conversion busywork

[PATCH v3 0/9] efi/libstub: Avoid UTF-16 conversion busywork

Posted by Ard Biesheuvel 1 week, 1 day ago

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 v2 [1]:
- drop x86 specific patch, it has been split out into a separate series
  [2]
- handle truncated output of ucs2_as_utf8() in the middle of a multibyte
  character correctly
- improve ucs2_as_utf8() documentation
- tweaks to the commit logs

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>

[0] https://lore.kernel.org/all/20260906130817.1151961-9-ardb@kernel.org
[1] http://lore.kernel.org/r/20260909115530.1924665-12-ardb+git@google.com
[2] http://lore.kernel.org/r/20260914183745.37538-5-ardb@kernel.org

Ard Biesheuvel (8):
  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

 drivers/firmware/efi/libstub/Makefile          |   3 +-
 drivers/firmware/efi/libstub/bli.c             |  87 ++++++++++++
 drivers/firmware/efi/libstub/efi-stub-helper.c |  98 +++++--------
 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        | 149 +++++++-------------
 drivers/firmware/efi/libstub/x86-stub.c        |   1 +
 include/linux/efi.h                            |  22 +++
 include/linux/ucs2_string.h                    |  11 +-
 lib/ucs2_string.c                              |  19 +--
 12 files changed, 242 insertions(+), 258 deletions(-)
 create mode 100644 drivers/firmware/efi/libstub/bli.c

-- 
2.55.0.1032.g73a4cd73de-goog

Re: [PATCH v3 0/9] efi/libstub: Avoid UTF-16 conversion busywork

Posted by Vincent Mailhol 1 week ago

On 16/09/2026 at 16:46, 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.

This v3 looks good to me and runs fine.

Patch 8 still uses spaces instead of tabulation for indentations. But,
notwithstanding of that and for the full series:

Reviewed-by: Vincent Mailhol <mailhol@kernel.org>


Yours sincerely,
Vincent Mailhol

Re: [PATCH v3 0/9] efi/libstub: Avoid UTF-16 conversion busywork

Posted by Ard Biesheuvel 1 week ago


On Thu, 17 Sep 2026, at 08:21, Vincent Mailhol wrote:
> On 16/09/2026 at 16:46, 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.
>
> This v3 looks good to me and runs fine.
>
> Patch 8 still uses spaces instead of tabulation for indentations. But,
> notwithstanding of that and for the full series:
>
> Reviewed-by: Vincent Mailhol <mailhol@kernel.org>
>

Thanks. I've fixed patch #8 and pushed the set (minus the x86 boot)
to the bootloader-info topic branch. I'll merge that into efi/next
once the x86 changes land in tip.