Re: [PATCH 01/17] kallsyms: support big kernel symbols (2-byte lengths)

Gary Guo
2021-07-04T21:20:49Z

On Sun, 4 Jul 2021 22:04:49 +0100
Matthew Wilcox <willy@infradead.org> wrote:

> On Sun, Jul 04, 2021 at 10:27:40PM +0200, ojeda@kernel.org wrote:
> > From: Miguel Ojeda <ojeda@kernel.org>
> >
> > Rust symbols can become quite long due to namespacing introduced
> > by modules, types, traits, generics, etc.
> >
> > Increasing to 255 is not enough in some cases, and therefore
> > we need to introduce 2-byte lengths to the symbol table. We call
> > these "big" symbols.
> >
> > In order to avoid increasing all lengths to 2 bytes (since most
> > of them only require 1 byte, including many Rust ones), we use
> > length zero to mark "big" symbols in the table.
>
> What happened to my suggestion from last time of encoding symbols <
> 128 as 0-127 and symbols larger than that as (data[0] - 128) * 256 +
> data[1]) ?

Yeah, I agree ULEB128 or similar encoding scheme would be better than
using 0 as an escape byte. If ULEB128 is used and we restrict number of
bytes to 2, it will encode numbers up to 2**14 instead of 2**16 like the
current scheme, but that should be sufficient anyway.

- Gary