Re: [PATCH v3 5/6] objtool/rust: list `noreturn` Rust functions

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Jul 25, 2024 at 8:35 PM Miguel Ojeda <ojeda@kernel.org> wrote:
>
> Rust functions may be `noreturn` (i.e. diverging) by returning the
> "never" type, `!`, e.g.
>
>     fn f() -> ! {
>         loop {}
>     }
>
> Thus list the known `noreturn` functions to avoid such warnings.
>
> Without this, `objtool` would complain if enabled for Rust, e.g.:
>
>     rust/core.o: warning: objtool:
>     _R...9panic_fmt() falls through to next function _R...18panic_nounwind_fmt()
>
>     rust/alloc.o: warning: objtool:
>     .text: unexpected end of section
>
> In order to do so, we cannot match symbols' names exactly, for two
> reasons:
>
>   - Rust mangling scheme [1] contains disambiguators [2] which we
>     cannot predict (e.g. they may vary depending on the compiler version).
>
>     One possibility to solve this would be to parse v0 and ignore/zero
>     those before comparison.
>
>   - Some of the diverging functions come from `core`, i.e. the Rust
>     standard library, which may change with each compiler version
>     since they are implementation details (e.g. `panic_internals`).
>
> Thus, to workaround both issues, only part of the symbols are matched,
> instead of using the `NORETURN` macro in `noreturns.h`.
>
> Ideally, just like for the C side, we should have a better solution. For
> instance, the compiler could give us the list via something like:
>
>     $ rustc --emit=noreturns ...
>
> Link: https://rust-lang.github.io/rfcs/2603-rust-symbol-name-mangling-v0.html [1]
> Link: https://doc.rust-lang.org/rustc/symbol-mangling/v0.html#disambiguator [2]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Tested-by: Alice Ryhl <aliceryhl@google.com>