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

Masahiro Yamada
2024-06-18T16:28:59Z

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On Tue, Jun 18, 2024 at 2:58 AM Sami Tolvanen <samitolvanen@google.com> 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).
>
> Another compatibility issue is fitting the extremely long mangled
> Rust symbol names into struct modversion_info, which can only hold
> 55-character names (on 64-bit systems). Previous proposals suggested
> changing the structure to support longer names, but the conclusion was
> that we cannot break userspace tools that parse the version table.
>
> The next two patches of the series implement support for hashed
> symbol names in struct modversion_info, where names longer than 55
> characters are hashed, and the hash is stored in the name field. To
> avoid breaking userspace tools, the binary hash is prefixed with a
> null-terminated string containing the name of the hash function. While
> userspace tools can later be updated to potentially produce more
> useful information about the long symbols, this allows them to
> continue working in the meantime.
>
> The final patch allows CONFIG_MODVERSIONS to be selected with Rust,
> provided that debugging information is also available.

I am surprised at someone who attempts to add another variant of genksyms.

I am also surprised at the tool being added under the tools/ directory.

At least, we can avoid the latter misfortune, though.

A patch attached (on top of your patch set).

Such a tool can be compiled in scripts/, or even better
in rust/Makefile if it is only used in rust/Makefile.

--
Best Regards
Masahiro Yamada

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