Re: [PATCH v3 3/6] lib/base64: rework encode/decode for speed and stricter validation

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

On Tue, Sep 30, 2025 at 05:11:12PM -0700, Caleb Sander Mateos wrote:
> On Fri, Sep 26, 2025 at 12:01 AM 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  -> ~64ns   (~23.9x)
> >   1KB ~27726ns  -> ~982ns  (~28.3x)
> >
> > 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>
> > ---
> >  lib/base64.c | 150 +++++++++++++++++++++++++++++++++++++--------------
> >  1 file changed, 110 insertions(+), 40 deletions(-)
> >
> > diff --git a/lib/base64.c b/lib/base64.c
> > index b20fdf168..fd1db4611 100644
> > --- a/lib/base64.c
> > +++ b/lib/base64.c
> > @@ -93,26 +93,43 @@ static const s8 base64_rev_tables[][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];
> > +
> > +               *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;
> >         }
> > -       while (bits < 0) {
> > -               *cp++ = '=';
> > -               bits += 2;
> > +
> > +       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++ = '=';
> > +               }
> > +               break;
> >         }
> >         return cp - dst;
> >  }
> > @@ -128,39 +145,92 @@ 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;
> > -
> > -       for (i = 0; i < srclen; i++) {
> > -               if (src[i] == '=') {
> > -                       ac = (ac << 6);
> > -                       bits += 6;
> > -                       if (bits >= 8)
> > -                               bits -= 8;
> > -                       continue;
> > -               }
> > -               ch = base64_rev_tables[variant][(u8)src[i]];
> > -               if (ch == -1)
> > +       s8 input1, input2, input3, input4;
> > +       u32 val;
> > +
> > +       if (srclen == 0)
> > +               return 0;
> 
> Doesn't look like this special case is necessary; all the if and while
> conditions below are false if srclen == 0, so the function will just
> end up returning 0 in that case anyways. It would be nice to avoid
> this branch, especially as it seems like an uncommon case.
>

You're right. I'll remove it. Thanks.

> > +
> > +       /* Validate the input length for padding */
> > +       if (unlikely(padding && (srclen & 0x03) != 0))
> > +               return -1;
> > +
> > +       while (srclen >= 4) {
> > +               /* Decode the next 4 characters */
> > +               input1 = base64_rev_tables[variant][(u8)src[0]];
> > +               input2 = base64_rev_tables[variant][(u8)src[1]];
> > +               input3 = base64_rev_tables[variant][(u8)src[2]];
> > +               input4 = base64_rev_tables[variant][(u8)src[3]];
> > +
> > +               /* Return error if any Base64 character is invalid */
> > +               if (unlikely(input1 < 0 || input2 < 0 || (!padding && (input3 < 0 || input4 < 0))))
> > +                       return -1;
> > +
> > +               /* Handle padding */
> > +               if (unlikely(padding && ((input3 < 0 && input4 >= 0) ||
> > +                                        (input3 < 0 && src[2] != '=') ||
> > +                                        (input4 < 0 && src[3] != '=') ||
> > +                                        (srclen > 4 && (input3 < 0 || input4 < 0)))))
> 
> Would be preferable to check and strip the padding (i.e. decrease
> srclen) before this main loop. That way we could avoid several
> branches in this hot loop that are only necessary to handle the
> padding chars.
> 

You're right. As long as we check and strip the padding first, the
behavior with or without padding can be the same, and it could also
reduce some unnecessary branches. I'll make the change.

Best regards,
Guan-Chun

> > +                       return -1;
> > +               val = ((u32)input1 << 18) |
> > +                     ((u32)input2 << 12) |
> > +                     ((u32)((input3 < 0) ? 0 : input3) << 6) |
> > +                     (u32)((input4 < 0) ? 0 : input4);
> > +
> > +               *bp++ = (u8)(val >> 16);
> > +
> > +               if (input3 >= 0)
> > +                       *bp++ = (u8)(val >> 8);
> > +               if (input4 >= 0)
> > +                       *bp++ = (u8)val;
> > +
> > +               src += 4;
> > +               srclen -= 4;
> > +       }
> > +
> > +       /* Handle leftover characters when padding is not used */
> > +       if (!padding && srclen > 0) {
> > +               switch (srclen) {
> > +               case 2:
> > +                       input1 = base64_rev_tables[variant][(u8)src[0]];
> > +                       input2 = base64_rev_tables[variant][(u8)src[1]];
> > +                       if (unlikely(input1 < 0 || input2 < 0))
> > +                               return -1;
> > +
> > +                       val = ((u32)input1 << 6) | (u32)input2; /* 12 bits */
> > +                       if (unlikely(val & 0x0F))
> > +                               return -1; /* low 4 bits must be zero */
> > +
> > +                       *bp++ = (u8)(val >> 4);
> > +                       break;
> > +               case 3:
> > +                       input1 = base64_rev_tables[variant][(u8)src[0]];
> > +                       input2 = base64_rev_tables[variant][(u8)src[1]];
> > +                       input3 = base64_rev_tables[variant][(u8)src[2]];
> > +                       if (unlikely(input1 < 0 || input2 < 0 || input3 < 0))
> > +                               return -1;
> > +
> > +                       val = ((u32)input1 << 12) |
> > +                             ((u32)input2 << 6) |
> > +                             (u32)input3; /* 18 bits */
> > +
> > +                       if (unlikely(val & 0x03))
> > +                               return -1; /* low 2 bits must be zero */
> > +
> > +                       *bp++ = (u8)(val >> 10);
> > +                       *bp++ = (u8)((val >> 2) & 0xFF);
> 
> "& 0xFF" is redundant with the cast to u8.
> 
> Best,
> Caleb
> 
> > +                       break;
> > +               default:
> >                         return -1;
> > -               ac = (ac << 6) | ch;
> > -               bits += 6;
> > -               if (bits >= 8) {
> > -                       bits -= 8;
> > -                       *bp++ = (u8)(ac >> bits);
> >                 }
> >         }
> > -       if (ac & ((1 << bits) - 1))
> > -               return -1;
> > +
> >         return bp - dst;
> >  }
> >  EXPORT_SYMBOL_GPL(base64_decode);
> > --
> > 2.34.1
> >
> >