Re: [PATCH v2 2/5] lib/base64: rework encoder/decoder with customizable support and update nvme-auth

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Sep 11, 2025 at 03:41:59PM +0800, Guan-Chun Wu wrote:
> Rework base64_encode() and base64_decode() with extended interfaces
> that support custom 64-character tables and optional '=' padding.
> This makes them flexible enough to cover both standard RFC4648 Base64
> and non-standard variants such as base64url.

RFC4648 specifies both base64 and base64url.

> The encoder is redesigned to process input in 3-byte blocks, each
> mapped directly into 4 output symbols. Base64 naturally encodes
> 24 bits of input as four 6-bit values, so operating on aligned
> 3-byte chunks matches the algorithm's structure. This block-based
> approach eliminates the need for bit-by-bit streaming, reduces shifts,
> masks, and loop iterations, and removes data-dependent branches from
> the main loop.

There already weren't any data-dependent branches in the encoder.

> The decoder replaces strchr()-based lookups with direct table-indexed
> mapping. It processes input in 4-character groups and supports both
> padded and non-padded forms. Validation has been strengthened: illegal
> characters and misplaced '=' padding now cause errors, preventing
> silent data corruption.

The decoder already detected invalid inputs.

> While this is a mechanical update following the lib/base64 rework,
> nvme-auth also benefits from the performance improvements in the new
> encoder/decoder, achieving faster encode/decode without altering the
> output format.
> 
> The reworked encoder and decoder unify Base64 handling across the kernel
> with higher performance, stricter correctness, and flexibility to support
> subsystem-specific variants.

Which part is more strictly correct?

> diff --git a/lib/base64.c b/lib/base64.c
> index 9416bded2..b2bd5dab5 100644
> --- a/lib/base64.c
> +++ b/lib/base64.c
> @@ -15,104 +15,236 @@
>  #include <linux/string.h>
>  #include <linux/base64.h>
>  
> -static const char base64_table[65] =
> -	"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
> +#define BASE64_6BIT_MASK      0x3f /* Mask to extract lowest 6 bits */
> +#define BASE64_BITS_PER_BYTE  8
> +#define BASE64_CHUNK_BITS     6
> +
> +/* Output-char-indexed shifts: for output chars 0,1,2,3 respectively */
> +#define BASE64_SHIFT_OUT0	(BASE64_CHUNK_BITS * 3) /* 18 */
> +#define BASE64_SHIFT_OUT1	(BASE64_CHUNK_BITS * 2) /* 12 */
> +#define BASE64_SHIFT_OUT2	(BASE64_CHUNK_BITS * 1) /* 6  */
> +/* OUT3 uses 0 shift and just masks with BASE64_6BIT_MASK */
> +
> +/* For extracting bytes from the 24-bit value (decode main loop) */
> +#define BASE64_SHIFT_BYTE0        (BASE64_BITS_PER_BYTE * 2) /* 16 */
> +#define BASE64_SHIFT_BYTE1        (BASE64_BITS_PER_BYTE * 1) /* 8  */
> +
> +/* Tail (no padding) shifts to extract bytes */
> +#define BASE64_TAIL2_BYTE0_SHIFT  ((BASE64_CHUNK_BITS * 2) - BASE64_BITS_PER_BYTE)       /* 4  */
> +#define BASE64_TAIL3_BYTE0_SHIFT  ((BASE64_CHUNK_BITS * 3) - BASE64_BITS_PER_BYTE)       /* 10 */
> +#define BASE64_TAIL3_BYTE1_SHIFT  ((BASE64_CHUNK_BITS * 3) - (BASE64_BITS_PER_BYTE * 2)) /* 2  */
> +
> +/* Extra: masks for leftover validation (no padding) */
> +#define BASE64_MASK(n) ({        \
> +	unsigned int __n = (n);  \
> +	__n ? ((1U << __n) - 1U) : 0U; \
> +})
> +#define BASE64_TAIL2_UNUSED_BITS  (BASE64_CHUNK_BITS * 2 - BASE64_BITS_PER_BYTE)     /* 4 */
> +#define BASE64_TAIL3_UNUSED_BITS  (BASE64_CHUNK_BITS * 3 - BASE64_BITS_PER_BYTE * 2) /* 2 */

These #defines make the code unnecessarily hard to read.  Most of them
should just be replaced with the integer literals.

>   * This implementation hasn't been optimized for performance.

But the commit message claims performance improvements.

>   *
>   * Return: the length of the resulting decoded binary data in bytes,
>   *	   or -1 if the string isn't a valid base64 string.

base64 => Base64, since multiple variants are supported now.  Refer to
the terminology used by RFC4686.  Base64 is the general term, and
"base64" and "base64url" specific variants of Base64.

- Eric