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

[Guan-Chun Wu <409411716@gms.tku.edu.tw>]

On Thu, Sep 11, 2025 at 11:27:42AM -0700, Eric Biggers wrote:
> 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.
>

Got it, I'll update the commit message in the next version.

> > 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.
> 

Got it, I'll update the commit message in the next version.

> > 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.
> 

You're right, the decoder already rejected invalid inputs.
What has been strengthened in the new version is the padding handling 
(length must be a multiple of 4, and = only allowed in the last two positions).

> > 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?
> 

The stricter correctness here refers to the decoder, specifically the padding
checks (length must be a multiple of 4, and = only allowed in the last two positions).

> > 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.
> 

Got it, thanks for the feedback. I'll simplify this in the next version.

> >   * This implementation hasn't been optimized for performance.
> 
> But the commit message claims performance improvements.
> 

That was my mistake. I forgot to update this part of the comment.
I’ll fix it in the next version.

> >   *
> >   * 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

Ok, I'll update the comments to use Base64.

Best regards,
Guan-chun