Re: [PATCH v2 1/5] lib/base64: Replace strchr() for better performance

[Kuan-Wei Chiu <visitorckw@gmail.com>]

On Sun, Sep 14, 2025 at 09:12:43PM +0100, David Laight wrote:
> On Fri, 12 Sep 2025 00:38:20 +0800
> Kuan-Wei Chiu <visitorckw@gmail.com> wrote:
> 
> ... 
> > Or I just realized that since different base64 tables only differ in the
> > last two characters, we could allocate a 256 entry reverse table inside
> > the base64 function and set the mapping for those two characters. That
> > way, users wouldn't need to pass in a reverse table. The downside is that
> > this would significantly increase the function's stack size.
> 
> How many different variants are there?

Currently there are 3 variants:
RFC 4648 (standard), RFC 4648 (base64url), and RFC 3501.
They use "+/", "-_", and "+," respectively for the last two characters.

> IIRC there are only are two common ones.
> (and it might not matter is the decoder accepted both sets since I'm
> pretty sure the issue is that '/' can't be used because it has already
> been treated as a separator.)
> 
> Since the code only has to handle in-kernel users - which presumably
> use a fixed table for each call site, they only need to pass in
> an identifier for the table.
> That would mean they can use the same identifier for encode and decode,
> and the tables themselves wouldn't be replicated and would be part of
> the implementation.
> 
So maybe we can define an enum in the header like this:

enum base64_variant {
    BASE64_STD,       /* RFC 4648 (standard) */ 
    BASE64_URLSAFE,   /* RFC 4648 (base64url) */ 
    BASE64_IMAP,      /* RFC 3501 */ 
};

Then the enum value can be passed as a parameter to base64_encode/decode,
and in base64.c we can define the tables and reverse tables like this:

static const char base64_tables[][64] = {
    [BASE64_STD] = "ABC...+/",
    [BASE64_URLSAFE] = "ABC...-_",
    [BASE64_IMAP] = "ABC...+,",
};

What do you think about this approach?

Regards,
Kuan-Wei