Stabilize naked_functions

[folkertdev]

tracking issue: #90957
request for stabilization on tracking issue: #90957 (comment)
reference PR: rust-lang/reference#1689

Request for Stabilization

Two years later, we're ready to try this again. Even though this issue is already marked as having passed FCP, given the amount of time that has passed and the changes in implementation strategy, we should follow the process again.

Summary

The naked_functions feature has two main parts: the #[naked] function attribute, and the naked_asm! macro.

An example of a naked function:

const THREE: usize = 3;

#[naked]
pub extern "sysv64" fn add_n(number: usize) -> usize {
    // SAFETY: the validity of the used registers 
    // is guaranteed according to the "sysv64" ABI
    unsafe {
        core::arch::naked_asm!(
            "add rdi, {}",
            "mov rax, rdi",
            "ret",
            const THREE,
        )
    }
}

When the #[naked] attribute is applied to a function, the compiler won't emit a function prologue or epilogue when generating code for this function. This attribute is analogous to __attribute__((naked)) in C. The use of this feature allows the programmer to have precise control over the assembly that is generated for a given function.

The body of a naked function must consist of a single naked_asm! invocation, a heavily restricted variant of the asm! macro: the only legal operands are const and sym, and the only legal options are raw and att_syntax. In lieu of specifying operands, the naked_asm! within a naked function relies on the function's calling convention to determine the validity of registers.

Documentation

The Rust Reference: rust-lang/reference#1153

Tests

• tests/run-make/naked-symbol-visiblity verifies that pub, #[no_mangle] and #[linkage = "..."] work correctly for naked functions
• tests/codegen/naked-fn has tests for function alignment, use of generics, and validates the exact assembly output on linux, macos, windows and thumb
• tests/ui/asm/naked-* tests for incompatible attributes, generating errors around incorrect use of naked_asm!, etc

History

This feature was originally proposed in RFC 1201, filed on 2015-07-10 and accepted on 2016-03-21. Support for this feature was added in #32410, landing on 2016-03-23. Development languished for several years as it was realized that the semantics given in RFC 1201 were insufficiently specific. To address this, a minimal subset of naked functions was specified by RFC 2972, filed on 2020-08-07 and accepted on 2021-11-16. Prior to the acceptance of RFC 2972, all of the stricter behavior specified by RFC 2972 was implemented as a series of warn-by-default lints that would trigger on existing uses of the naked attribute; these lints became hard errors in #93153 on 2022-01-22. As a result, today RFC 2972 has completely superseded RFC 1201 in describing the semantics of the naked attribute.

More recently, the naked_asm! macro was added to replace the earlier use of a heavily restricted asm! invocation. The naked_asm! name is clearer in error messages, and provides a place for documenting the specific requirements of inline assembly in naked functions.

The implementation strategy was changed to emitting a global assembly block. In effect, an extern function

extern "C" fn foo() {
    core::arch::naked_asm!("ret")
}

is emitted as something similar to

core::arch::global_asm!( 
    "foo:",
    "ret"
);

extern "C" {
    fn foo();
}

The codegen approach was chosen over the llvm naked function attribute because:

• the rust compiler can guarantee the behavior (no sneaky additional instructions, no inlining, etc.)
• behavior is the same on all backends (llvm, cranelift, gcc, etc)

Finally, there is now an allow list of compatible attributes on naked functions, so that e.g. #[inline] is rejected with an error.

relevant PRs for these recent changes

• #[naked]: report incompatible attributes #127853
• add naked_asm! macro for use in #[naked] functions #128651
• codegen #[naked] functions using global asm #128004

unresolved questions

None

r? @Amanieu

I've validated the tests on x86_64 and aarch64

[rustbot]

Some changes occurred in src/doc/unstable-book/src/compiler-flags/sanitizer.md

cc @rust-lang/project-exploit-mitigations, @rcvalle

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

[bors]

☔ The latest upstream changes (presumably #134395) made this pull request unmergeable. Please resolve the merge conflicts.

[Amanieu]

r? lang

[Amanieu]

This probably needs a new lang FCP since the old one is probably outdated (the implementation of naked function has changed signficantly).

[tmandry]

Thanks for the thorough report @folkertdev!

@rfcbot fcp merge

[rfcbot]

Team member @tmandry has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[tmandry]

Actually, @rust-lang/libs-api does this need your FCP? I think the path of core::arch::naked_asm! is the only part that might.