objtool/rust: list `noreturn` Rust functions

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 ...

[ Kees agrees this should be automated and Peter says:

    So it would be fairly simple to make objtool consume a magic section
    emitted by the compiler.. I think we've asked the compiler folks
    for that at some point even, but I don't have clear recollections.

  We will ask upstream Rust about it. And if they agree, then perhaps
  we can get Clang/GCC to implement something similar too -- for this
  sort of thing we can take advantage of the shorter cycles of `rustc`
  as well as their unstable features concept to experiment.

  Gary proposed using DWARF (though it would need to be available), and
  wrote a proof of concept script using the `object` and `gimli` crates:
  https://gist.github.com/nbdd0121/449692570622c2f46a29ad9f47c3379a

    - Miguel ]

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]
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Tested-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20240725183325.122827-6-ojeda@kernel.org
[ Added `len_mismatch_fail` symbol for new `kernel` crate code merged
  since then as well as 3 more `core::panicking` symbols that appear
  in `RUST_DEBUG_ASSERTIONS=y` builds.  - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

0 files changed, 0 insertions, 0 deletions