The utf16_le_to_7bit function claims to, naively, convert a UTF-16
string to a 7-bit ASCII string. By naively, we mean that it:
* drops the first byte of every character in the original UTF-16 string
* checks if all characters are printable, and otherwise replaces them
by exclamation mark "!".
This means that theoretically, all characters outside the 7-bit ASCII
range should be replaced by another character. Examples:
* lower-case alpha (ɒ) 0x0252 becomes 0x52 (R)
* ligature OE (œ) 0x0153 becomes 0x53 (S)
* hangul letter pieup (ㅂ) 0x3142 becomes 0x42 (B)
* upper-case gamma (Ɣ) 0x0194 becomes 0x94 (not printable) so gets
replaced by "!"
The result of this conversion for the GPT partition name is passed to
user-space as PARTNAME via udev, which is confusing and feels questionable.
However, there is a flaw in the conversion function itself. By dropping
one byte of each character and using isprint() to check if the remaining
byte corresponds to a printable character, we do not actually guarantee
that the resulting character is 7-bit ASCII.
This happens because we pass 8-bit characters to isprint(), which
in the kernel returns 1 for many values > 0x7f - as defined in ctype.c.
This results in many values which should be replaced by "!" to be kept
as-is, despite not being valid 7-bit ASCII. Examples:
* e with acute accent (é) 0x00E9 becomes 0xE9 - kept as-is because
isprint(0xE9) returns 1.
* euro sign (€) 0x20AC becomes 0xAC - kept as-is because isprint(0xAC)
returns 1.
Fixed by using a mask of 7 bits instead of 8 bits before calling
isprint.
Signed-off-by: Olivier Gayot <olivier.gayot@xxxxxxxxxxxxx>
---
V1 -> V2: No change - resubmitted with subsystem maintainers in CC
block/partitions/efi.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/block/partitions/efi.c b/block/partitions/efi.c
index 5e9be13a56a8..7acba66eed48 100644
--- a/block/partitions/efi.c
+++ b/block/partitions/efi.c
@@ -682,7 +682,7 @@ static void utf16_le_to_7bit(const __le16 *in, unsigned int size, u8 *out)
out[size] = 0;
while (i < size) {
- u8 c = le16_to_cpu(in[i]) & 0xff;
+ u8 c = le16_to_cpu(in[i]) & 0x7f;
if (c && !isprint(c))
c = '!';
