Commit 9c7d3cf9 authored by Guan-Chun Wu's avatar Guan-Chun Wu Committed by Andrew Morton
Browse files

lib/base64: rework encode/decode for speed and stricter validation 

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)

[akpm@linux-foundation.org: remove u32 casts, per David and Guan-Chun]
Link: https://lkml.kernel.org/r/20251114060132.89279-1-409411716@gms.tku.edu.tw


Co-developed-by: default avatarKuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: default avatarKuan-Wei Chiu <visitorckw@gmail.com>
Co-developed-by: default avatarYu-Sheng Huang <home7438072@gmail.com>
Signed-off-by: default avatarYu-Sheng Huang <home7438072@gmail.com>
Signed-off-by: default avatarGuan-Chun Wu <409411716@gms.tku.edu.tw>
Reviewed-by: default avatarDavid Laight <david.laight.linux@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Cc: Xiubo Li <xiubli@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent c4eb7ad3

lib/base64.c

+68 −41
Original line number Diff line number Diff line
@@ -80,28 +80,38 @@ 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 = src[0] << 16 | src[1] << 8 | src[2];

		*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;

	}



	switch (srclen) {

	case 2:

		ac = src[0] << 16 | 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 = src[0] << 16;

		*cp++ = base64_table[ac >> 18];

		*cp++ = base64_table[(ac >> 12) & 0x3f];

		if (padding) {

		while (bits < 0) {

			*cp++ = '=';

			bits += 2;

			*cp++ = '=';

		}

		break;

	}

	return cp - dst;

}
@@ -117,41 +127,58 @@ 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)

	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;

		ac = (ac << 6) | ch;

		bits += 6;

		if (bits >= 8) {

			bits -= 8;

			*bp++ = (u8)(ac >> bits);

			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);