linux-kernel - Re: [PATCH 00/15] Implement MODVERSIONS for Rust

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Date: Tue, 18 Jun 2024 19:18:39 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Matthew Maurer <mmaurer@...gle.com>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Gary Guo <gary@...yguo.net>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-modules@...r.kernel.org,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 00/15] Implement MODVERSIONS for Rust

On Wed, Jun 19, 2024 at 01:50:36AM +0900, Masahiro Yamada wrote:
> On Wed, Jun 19, 2024 at 1:44 AM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> >
> > On Mon, Jun 17, 2024 at 05:58:19PM +0000, Sami Tolvanen wrote:
> > > Hi folks,
> > >
> > > This series implements CONFIG_MODVERSIONS for Rust, an important
> > > feature for distributions like Android that want to ship Rust
> > > kernel modules, and depend on modversions to help ensure module ABI
> > > compatibility.
> > >
> > > There have been earlier proposals [1][2] that would allow Rust
> > > modules to coexist with modversions, but none that actually implement
> > > symbol versioning. Unlike C, Rust source code doesn't have sufficient
> > > information about the final ABI, as the compiler has considerable
> > > freedom in adjusting structure layout for improved performance [3],
> > > for example, which makes using a source code parser like genksyms
> > > a non-starter. Based on Matt's suggestion and previous feedback
> > > from maintainers, this series uses DWARF debugging information for
> > > computing versions. DWARF is an established and relatively stable
> > > format, which includes all the necessary ABI details, and adding a
> > > CONFIG_DEBUG_INFO dependency for Rust symbol versioning seems like a
> > > reasonable trade-off.
> > >
> > > The first 12 patches of this series add a small tool for computing
> > > symbol versions from DWARF, called gendwarfksyms. When passed a list
> > > of exported symbols, the tool generates an expanded type string
> > > for each symbol, and computes symbol CRCs similarly to genksyms.
> > > gendwarfksyms is written in C and uses libdw to process DWARF, mainly
> > > because of the existing support for C host tools that use elfutils
> > > (e.g., objtool).
> >
> > That's cool, can the C code be switched to also use this?  That way we
> > only have one path/code for all of this?
> 
> 
> As the description says, it requires CONFIG_DEBUG_INFO.
> We can strip the debug info from the final vmlinux, but
> I guess the build speed will be even slower than the current genksyms.

For people who want genksyms (i.e. distros), don't they normally already
enable DEBUG_INFO as well?  The problems of genksyms are well known and
a pain (I speak from experience), so replacing it with info based on
DWARF would be great, I'll gladly trade off the DEBUG_INFO issue for
stablilty!

thanks,

greg k-h