linux-kernel - Re: [PATCH 01/13] kallsyms: Support "big" kernel symbols (2-byte lengths)

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Message-ID: <20210414194454.GV2531743@casper.infradead.org>
Date:   Wed, 14 Apr 2021 20:44:54 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     ojeda@...nel.org
Cc:     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 01/13] kallsyms: Support "big" kernel symbols (2-byte
 lengths)

On Wed, Apr 14, 2021 at 08:45:52PM +0200, ojeda@...nel.org wrote:
> 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.

How about doing something a bit more utf-8-like?

	len = data[0];
	if (len == 0)
		error
	else if (len < 128)
		return len;
	else if (len < 192)
		return 128 + (len - 128) * 256 + data[1];
... that takes you all the way out to 16511 bytes.  You probably don't
even need the third byte option.  But if you do ...
	else if (len < 223)
		return 16512 + (len - 192) * 256 * 256 +
			data[1] * 256 + data[2];
which takes you all the way out to 2,113,663 bytes and leaves 224-255 unused.

Alternatively, if the symbols are really this long, perhaps we should not
do string matches.  A sha-1 (... or whatever ...) hash of the function
name is 160 bits.  Expressed as hex digits, that's 40 characters.
Expressed in base-64, it's 27 characters.  We'd also want a "pretty"
name to go along with the hash, but that seems preferable to printing
out a mangled-with-types-and-who-knows-what name.

> Co-developed-by: Alex Gaynor <alex.gaynor@...il.com>
> Signed-off-by: Alex Gaynor <alex.gaynor@...il.com>

If you have C-d-b, you don't also need S-o-b.