linux-kernel - Re: [PATCH] modpost: support arbitrary symbol length in modversion

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Date:   Mon, 13 Mar 2023 22:48:53 +0100
From:   Michal Suchánek <msuchanek@...e.de>
To:     Andrea Righi <andrea.righi@...onical.com>
Cc:     Gary Guo <gary@...yguo.net>, Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        linux-kbuild@...r.kernel.org,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        Joel Stanley <joel@....id.au>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Nicolas Schier <nicolas@...sle.eu>,
        rust-for-linux@...r.kernel.org,
        Guo Zhengkui <guozhengkui@...o.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, Julia Lawall <Julia.Lawall@...ia.fr>,
        Luis Chamberlain <mcgrof@...nel.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-modules@...r.kernel.org
Subject: Re: [PATCH] modpost: support arbitrary symbol length in modversion

Hello,

On Mon, Mar 13, 2023 at 09:32:16PM +0100, Andrea Righi wrote:
> On Wed, Jan 11, 2023 at 04:11:51PM +0000, Gary Guo wrote:
> > Currently modversion uses a fixed size array of size (64 - sizeof(long))
> > to store symbol names, thus placing a hard limit on length of symbols.
> > Rust symbols (which encodes crate and module names) can be quite a bit
> > longer. The length limit in kallsyms is increased to 512 for this reason.
> > 
> > It's a waste of space to simply expand the fixed array size to 512 in
> > modversion info entries. I therefore make it variably sized, with offset
> > to the next entry indicated by the initial "next" field.
> > 
> > In addition to supporting longer-than-56/60 byte symbols, this patch also
> > reduce the size for short symbols by getting rid of excessive 0 paddings.
> > There are still some zero paddings to ensure "next" and "crc" fields are
> > properly aligned.
> > 
> > This patch does have a tiny drawback that it makes ".mod.c" files generated
> > a bit less easy to read, as code like
> > 
> > 	"\x08\x00\x00\x00\x78\x56\x34\x12"
> > 	"symbol\0\0"
> > 
> > is generated as opposed to
> > 
> > 	{ 0x12345678, "symbol" },
> > 
> > because the structure is now variable-length. But hopefully nobody reads
> > the generated file :)
> > 
> > Link: b8a94bfb3395 ("kallsyms: increase maximum kernel symbol length to 512")
> > Link: https://github.com/Rust-for-Linux/linux/pull/379
> > 
> > Signed-off-by: Gary Guo <gary@...yguo.net>
> 
> Is there any newer version of this patch?
> 
> I'm doing some tests with it, but I'm getting boot failures on ppc64
> with this applied (at boot kernel is spitting out lots of oops'es and
> unfortunately it's really hard to copy paste or just read them from the
> console).

Are you using the ELF ABI v1 or v2?

v1 may have some additional issues when it comes to these symbol tables.

Thanks

Michal