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drm/panic: Better binary encoding in QR code

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The current encoding, is done by converting 13bits of input into 4
decimal digits, that are then encoded efficiently using the numeric
encoding of the QR code specification.
The Fido v2.2 specification [1] uses a similar approach for its
QR-initiated authentication. The only difference is that it converts
7 bytes (56bits) of input into 17 decimal digits. The benefit is that
the algorithm doesn't require to split input bytes into 13bits chunk,
and the ratio is a bit better.
This improvement was proposed by Jó Ágila Bitsch in [2].
drm_panic is still young, and the QR code feature is not widely used,
so it's still time to switch to a common algorithm, shared with a
widely used standard.
I also changed the name of the url parameter, from zl= to z=, so the
website can keep backward compatibility if needed.

[1] https://fidoalliance.org/specs/fido-v2.2-rd-20230321/fido-client-to-authenticator-protocol-v2.2-rd-20230321.html#hybrid-qr-initiated
[2] https://github.com/kdj0c/panic_report/issues/2

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250128165254.893204-1-jfalempe@redhat.com
This commit is contained in:
Jocelyn Falempe 2025-01-28 17:52:10 +01:00
parent 0590c94c35
commit dbed4a797e
2 changed files with 36 additions and 74 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
drivers/gpu/drm/drm_panic.c
View File

@ -499,7 +499,7 @@ static int drm_panic_get_qr_code_url(u8 **qr_image)
char *kmsg;
int max_qr_data_size, url_len;
url_len = snprintf(url, sizeof(url), CONFIG_DRM_PANIC_SCREEN_QR_CODE_URL "?a=%s&v=%s&zl=",
url_len = snprintf(url, sizeof(url), CONFIG_DRM_PANIC_SCREEN_QR_CODE_URL "?a=%s&v=%s&z=",
utsname()->machine, utsname()->release);
max_qr_data_size = drm_panic_qr_max_data_size(panic_qr_version, url_len);

108
drivers/gpu/drm/drm_panic_qr.rs
View File

@ -13,12 +13,13 @@
//! The binary data must be a valid URL parameter, so the easiest way is
//! to use base64 encoding. But this wastes 25% of data space, so the
//! whole stack trace won't fit in the QR code. So instead it encodes
//! every 13bits of input into 4 decimal digits, and then uses the
//! every 7 bytes of input into 17 decimal digits, and then uses the
//! efficient numeric encoding, that encode 3 decimal digits into
//! 10bits. This makes 39bits of compressed data into 12 decimal digits,
//! into 40bits in the QR code, so wasting only 2.5%. And the numbers are
//! 10bits. This makes 168bits of compressed data into 51 decimal digits,
//! into 170bits in the QR code, so wasting only 1.17%. And the numbers are
//! valid URL parameter, so the website can do the reverse, to get the
//! binary data.
//! binary data. This is the same algorithm used by Fido v2.2 QR-initiated
//! authentication specification.
//!
//! Inspired by these 3 projects, all under MIT license:
//!
@ -26,7 +27,6 @@
//! * <https://github.com/erwanvivien/fast_qr>
//! * <https://github.com/bjguillot/qr>
use core::cmp;
use kernel::str::CStr;
#[derive(Debug, Clone, Copy, PartialEq, Eq, Ord, PartialOrd)]
@ -296,35 +296,11 @@ const MODE_BINARY: u16 = 4;
/// Padding bytes.
const PADDING: [u8; 2] = [236, 17];
/// Get the next 13 bits of data, starting at specified offset (in bits).
fn get_next_13b(data: &[u8], offset: usize) -> Option<(u16, usize)> {
if offset < data.len() * 8 {
let size = cmp::min(13, data.len() * 8 - offset);
let byte_off = offset / 8;
let bit_off = offset % 8;
// `b` is 20 at max (`bit_off` <= 7 and `size` <= 13).
let b = (bit_off + size) as u16;
let first_byte = (data[byte_off] << bit_off >> bit_off) as u16;
let number = match b {
0..=8 => first_byte >> (8 - b),
9..=16 => (first_byte << (b - 8)) + (data[byte_off + 1] >> (16 - b)) as u16,
_ => {
(first_byte << (b - 8))
+ ((data[byte_off + 1] as u16) << (b - 16))
+ (data[byte_off + 2] >> (24 - b)) as u16
}
};
Some((number, size))
} else {
None
}
}
/// Number of bits to encode characters in numeric mode.
const NUM_CHARS_BITS: [usize; 4] = [0, 4, 7, 10];
const POW10: [u16; 4] = [1, 10, 100, 1000];
/// Number of decimal digits required to encode n bytes of binary data.
/// eg: you need 15 decimal digits to fit 6 bytes of binary data.
const BYTES_TO_DIGITS: [usize; 8] = [0, 3, 5, 8, 10, 13, 15, 17];
enum Segment<'a> {
Numeric(&'a [u8]),
@ -360,13 +336,9 @@ impl Segment<'_> {
match self {
Segment::Binary(data) => data.len(),
Segment::Numeric(data) => {
let data_bits = data.len() * 8;
let last_chars = match data_bits % 13 {
1 => 1,
k => (k + 1) / 3,
};
// 4 decimal numbers per 13bits + remainder.
4 * (data_bits / 13) + last_chars
let last_chars = BYTES_TO_DIGITS[data.len() % 7];
// 17 decimal numbers per 7bytes + remainder.
17 * (data.len() / 7) + last_chars
}
}
}
@ -403,7 +375,7 @@ impl Segment<'_> {
struct SegmentIterator<'a> {
segment: &'a Segment<'a>,
offset: usize,
carry: u16,
carry: u64,
carry_len: usize,
}
@ -422,40 +394,30 @@ impl Iterator for SegmentIterator<'_> {
}
}
Segment::Numeric(data) => {
if self.carry_len == 3 {
let out = (self.carry, NUM_CHARS_BITS[self.carry_len]);
self.carry_len = 0;
self.carry = 0;
Some(out)
} else if let Some((bits, size)) = get_next_13b(data, self.offset) {
self.offset += size;
let new_chars = match size {
1 => 1,
k => (k + 1) / 3,
};
if self.carry_len + new_chars > 3 {
self.carry_len = new_chars + self.carry_len - 3;
let out = (
self.carry * POW10[new_chars - self.carry_len]
+ bits / POW10[self.carry_len],
NUM_CHARS_BITS[3],
);
self.carry = bits % POW10[self.carry_len];
Some(out)
} else {
let out = (
self.carry * POW10[new_chars] + bits,
NUM_CHARS_BITS[self.carry_len + new_chars],
);
self.carry_len = 0;
Some(out)
if self.carry_len < 3 && self.offset < data.len() {
// If there are less than 3 decimal digits in the carry,
// take the next 7 bytes of input, and add them to the carry.
let mut buf = [0u8; 8];
let len = 7.min(data.len() - self.offset);
buf[..len].copy_from_slice(&data[self.offset..self.offset + len]);
let chunk = u64::from_le_bytes(buf);
let pow = u64::pow(10, BYTES_TO_DIGITS[len] as u32);
self.carry = chunk + self.carry * pow;
self.offset += len;
self.carry_len += BYTES_TO_DIGITS[len];
}
match self.carry_len {
0 => None,
len => {
// take the next 3 decimal digits of the carry
// and return 10bits of numeric data.
let out_len = 3.min(len);
self.carry_len -= out_len;
let pow = u64::pow(10, self.carry_len as u32);
let out = (self.carry / pow) as u16;
self.carry = self.carry % pow;
Some((out, NUM_CHARS_BITS[out_len]))
}
} else if self.carry_len > 0 {
let out = (self.carry, NUM_CHARS_BITS[self.carry_len]);
self.carry_len = 0;
Some(out)
} else {
None
}
}
}