Re: [PATCH v5 3/6] lib/base64: rework encode/decode for speed and stricter validation

David Laight
2025-11-14T09:18:34Z

On Fri, 14 Nov 2025 14:01:32 +0800
Guan-Chun Wu <409411716@gms.tku.edu.tw> wrote:

> The old base64 implementation relied on a bit-accumulator loop, which was
> slow for larger inputs and too permissive in validation. It would accept
> extra '=', missing '=', or even '=' appearing in the middle of the input,
> allowing malformed strings to pass. This patch reworks the internals to
> improve performance and enforce stricter validation.
>
> Changes:
> - Encoder:
> * Process input in 3-byte blocks, mapping 24 bits into four 6-bit
> symbols, avoiding bit-by-bit shifting and reducing loop iterations.
> * Handle the final 1-2 leftover bytes explicitly and emit '=' only when
> requested.
> - Decoder:
> * Based on the reverse lookup tables from the previous patch, decode
> input in 4-character groups.
> * Each group is looked up directly, converted into numeric values, and
> combined into 3 output bytes.
> * Explicitly handle padded and unpadded forms:
> - With padding: input length must be a multiple of 4, and '=' is
> allowed only in the last two positions. Reject stray or early '='.
> - Without padding: validate tail lengths (2 or 3 chars) and require
> unused low bits to be zero.
> * Removed the bit-accumulator style loop to reduce loop iterations.
>
> Performance (x86_64, Intel Core i7-10700 @ 2.90GHz, avg over 1000 runs,
> KUnit):
>
> Encode:
> 64B ~90ns -> ~32ns (~2.8x)
> 1KB ~1332ns -> ~510ns (~2.6x)
>
> Decode:
> 64B ~1530ns -> ~35ns (~43.7x)
> 1KB ~27726ns -> ~530ns (~52.3x)
>
> Co-developed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> Co-developed-by: Yu-Sheng Huang <home7438072@gmail.com>
> Signed-off-by: Yu-Sheng Huang <home7438072@gmail.com>
> Signed-off-by: Guan-Chun Wu <409411716@gms.tku.edu.tw>

Reviewed-by: David Laight <david.laight.linux@gmail.com>

But see minor nit below.
> ---
> lib/base64.c | 109 ++++++++++++++++++++++++++++++++-------------------
> 1 file changed, 68 insertions(+), 41 deletions(-)
>
> diff --git a/lib/base64.c b/lib/base64.c
> index 9d1074bb821c..1a6d8fe37eda 100644
> --- a/lib/base64.c
> +++ b/lib/base64.c
> @@ -79,28 +79,38 @@ static const s8 base64_rev_maps[][256] = {
> int base64_encode(const u8 *src, int srclen, char *dst, bool padding, enum base64_variant variant)
> {
> u32 ac = 0;
> - int bits = 0;
> - int i;
> char *cp = dst;
> const char *base64_table = base64_tables[variant];
>
> - for (i = 0; i < srclen; i++) {
> - ac = (ac << 8) | src[i];
> - bits += 8;
> - do {
> - bits -= 6;
> - *cp++ = base64_table[(ac >> bits) & 0x3f];
> - } while (bits >= 6);
> - }
> - if (bits) {
> - *cp++ = base64_table[(ac << (6 - bits)) & 0x3f];
> - bits -= 6;
> + while (srclen >= 3) {
> + ac = (u32)src[0] << 16 | (u32)src[1] << 8 | (u32)src[2];

There is no need for the (u32) casts.
All char/short values are promoted to 'int' prior to any maths.

> + *cp++ = base64_table[ac >> 18];
> + *cp++ = base64_table[(ac >> 12) & 0x3f];
> + *cp++ = base64_table[(ac >> 6) & 0x3f];
> + *cp++ = base64_table[ac & 0x3f];
> +
> + src += 3;
> + srclen -= 3;
> }
> - if (padding) {
> - while (bits < 0) {
> +
> + switch (srclen) {
> + case 2:
> + ac = (u32)src[0] << 16 | (u32)src[1] << 8;
> + *cp++ = base64_table[ac >> 18];
> + *cp++ = base64_table[(ac >> 12) & 0x3f];
> + *cp++ = base64_table[(ac >> 6) & 0x3f];
> + if (padding)
> + *cp++ = '=';
> + break;
> + case 1:
> + ac = (u32)src[0] << 16;
> + *cp++ = base64_table[ac >> 18];
> + *cp++ = base64_table[(ac >> 12) & 0x3f];
> + if (padding) {
> + *cp++ = '=';
> *cp++ = '=';
> - bits += 2;
> }
> + break;
> }
> return cp - dst;
> }
> @@ -116,41 +126,58 @@ EXPORT_SYMBOL_GPL(base64_encode);
> *
> * Decodes a string using the selected Base64 variant.
> *
> - * This implementation hasn't been optimized for performance.
> - *
> * Return: the length of the resulting decoded binary data in bytes,
> * or -1 if the string isn't a valid Base64 string.
> */
> int base64_decode(const char *src, int srclen, u8 *dst, bool padding, enum base64_variant variant)
> {
> - u32 ac = 0;
> - int bits = 0;
> - int i;
> u8 *bp = dst;
> - s8 ch;
> + s8 input[4];
> + s32 val;
> + const u8 *s = (const u8 *)src;
> + const s8 *base64_rev_tables = base64_rev_maps[variant];
>
> - for (i = 0; i < srclen; i++) {
> - if (padding) {
> - if (src[i] == '=') {
> - ac = (ac << 6);
> - bits += 6;
> - if (bits >= 8)
> - bits -= 8;
> - continue;
> - }
> - }
> - ch = base64_rev_maps[variant][(u8)src[i]];
> - if (ch == -1)
> - return -1;
> - ac = (ac << 6) | ch;
> - bits += 6;
> - if (bits >= 8) {
> - bits -= 8;
> - *bp++ = (u8)(ac >> bits);
> + while (srclen >= 4) {
> + input[0] = base64_rev_tables[s[0]];
> + input[1] = base64_rev_tables[s[1]];
> + input[2] = base64_rev_tables[s[2]];
> + input[3] = base64_rev_tables[s[3]];
> +
> + val = input[0] << 18 | input[1] << 12 | input[2] << 6 | input[3];
> +
> + if (unlikely(val < 0)) {
> + if (!padding || srclen != 4 || s[3] != '=')
> + return -1;
> + padding = 0;
> + srclen = s[2] == '=' ? 2 : 3;
> + break;
> }
> +
> + *bp++ = val >> 16;
> + *bp++ = val >> 8;
> + *bp++ = val;
> +
> + s += 4;
> + srclen -= 4;
> }
> - if (ac & ((1 << bits) - 1))
> +
> + if (likely(!srclen))
> + return bp - dst;
> + if (padding || srclen == 1)
> return -1;
> +
> + val = (base64_rev_tables[s[0]] << 12) | (base64_rev_tables[s[1]] << 6);
> + *bp++ = val >> 10;
> +
> + if (srclen == 2) {
> + if (val & 0x800003ff)
> + return -1;
> + } else {
> + val |= base64_rev_tables[s[2]];
> + if (val & 0x80000003)
> + return -1;
> + *bp++ = val >> 2;
> + }
> return bp - dst;
> }
> EXPORT_SYMBOL_GPL(base64_decode);