An upcoming change will update ucs2_as_utf8() to expose a strscpy()
style API where the size argument is the destination buffer size,
including space for the final NUL terminator.

Some EFI callers currently pass the exact number of UTF-8 payload bytes
that they expect to copy and add the terminator themselves afterwards.
Extend those sizes to include the final NUL terminator so the upcoming
contract change does not truncate the converted output by one byte.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
Hi Ard,

I saw that you pushed on efi-libstub-native-utf16 WIP branch [1] and did
some testing, despite those changes not yet submitted for review.

There is an off-by-one error following your ucs2_as_utf8() code
refactor. This patch prevents the issue. It should be cherry-picked just
before your "efi/libstub: Use ucs2_string library for UTF-16 to UTF-8
conversion" commit.

There is one final off-by-one in "efi/libstub: Use ucs2_string library
for UTF-16 to UTF-8 conversion" itself. This is the fix:

	---8<---
	diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
	index e9b714ca811db..db0bde514f7fd 100644
	--- a/drivers/firmware/efi/libstub/efi-stub-helper.c
	+++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
	@@ -379,8 +379,7 @@ char *efi_convert_cmdline(efi_loaded_image_t *image)
	 	if (status != EFI_SUCCESS)
	 		return NULL;

	-	ucs2_as_utf8(cmdline_addr, options, options_bytes - 1);
	-	cmdline_addr[options_bytes - 1] = '\0';
	+	ucs2_as_utf8(cmdline_addr, options, options_bytes);

	 	return cmdline_addr;
	 }
	---8<---

[1] https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=efi-libstub-native-utf16
---
 drivers/firmware/efi/efi.c | 2 +-
 fs/efivarfs/vars.c         | 5 ++---
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 6d987d7f97781..221804e7d5390 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -304,7 +304,7 @@ static __init int efivar_ssdt_load(void)
 		}
 
 		limit = min(EFIVAR_SSDT_NAME_MAX, name_size);
-		ucs2_as_utf8(utf8_name, name, limit - 1);
+		ucs2_as_utf8(utf8_name, name, limit);
 		if (strncmp(utf8_name, efivar_ssdt, limit) != 0)
 			continue;
 
diff --git a/fs/efivarfs/vars.c b/fs/efivarfs/vars.c
index 6833c3d24b541..1ddc89e13518f 100644
--- a/fs/efivarfs/vars.c
+++ b/fs/efivarfs/vars.c
@@ -237,7 +237,7 @@ efivar_get_utf8name(const efi_char16_t *name16, efi_guid_t *vendor)
 	if (!name)
 		return NULL;
 
-	ucs2_as_utf8(name, name16, len);
+	ucs2_as_utf8(name, name16, len + 1);
 
 	name[len] = '-';
 
@@ -264,8 +264,7 @@ efivar_validate(efi_guid_t vendor, efi_char16_t *var_name, u8 *data,
 	if (!utf8_name)
 		return false;
 
-	ucs2_as_utf8(utf8_name, var_name, utf8_size);
-	utf8_name[utf8_size] = '\0';
+	ucs2_as_utf8(utf8_name, var_name, utf8_size + 1);
 
 	for (i = 0; variable_validate[i].name[0] != '\0'; i++) {
 		const char *name = variable_validate[i].name;
-- 
2.43.0

(drop x86@ from cc)

Hi Vincent,

On Tue, 8 Sep 2026, at 19:24, Vincent Mailhol wrote:
> An upcoming change will update ucs2_as_utf8() to expose a strscpy()
> style API where the size argument is the destination buffer size,
> including space for the final NUL terminator.
>
> Some EFI callers currently pass the exact number of UTF-8 payload bytes
> that they expect to copy and add the terminator themselves afterwards.
> Extend those sizes to include the final NUL terminator so the upcoming
> contract change does not truncate the converted output by one byte.
>
> Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
> ---
> Hi Ard,
>
> I saw that you pushed on efi-libstub-native-utf16 WIP branch [1] and did
> some testing, despite those changes not yet submitted for review.
>


Thanks for the review, but I am going to drop that change. I am also going
to drop the ucs2_strscpy() call from my series, and use memcpy() instead.
The guaranteed NUL terminator is not really needed for a printf() style
function, and it actually interferes with the precision handling.

Instead, I'm adding a ucs2_to_utf8() that takes a separate limit argument,
and putting efi_convert_cmdline() on its head entirely so we don't go
through the input character by character twice.

I'll have v2 out shortly, including your v3 with my tweaks on top.
Please take the time to review on list if you have the bandwidth. Thanks.