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

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Date:   Sun, 4 Jul 2021 22:20:43 +0100
From:   Gary Guo <gary@...yguo.net>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     ojeda@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alex Gaynor <alex.gaynor@...il.com>,
        Geoffrey Thomas <geofft@...reload.com>,
        Finn Behrens <me@...enk.de>,
        Adam Bratschi-Kaye <ark.email@...il.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>
Subject: Re: [PATCH 01/17] kallsyms: support big kernel symbols (2-byte
 lengths)

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

> On Sun, Jul 04, 2021 at 10:27:40PM +0200, ojeda@...nel.org wrote:
> > From: Miguel Ojeda <ojeda@...nel.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