naked functions #1689

folkertdev · 2024-12-01T15:56:02Z

tracking issue: rust-lang/rust#90957
earlier version: #1153

Pending on rust-lang/rust#128004. Once merged, we'll move for stabliziation.

I'm working off of the original PR to the reference, written 2+ years ago when the reference was less strict. I suspect some refinement and editing will be needed here to satisfy the quality requirements of the spec, and update the text to accurately reflect the current state and the exact guarantees/requirements.

Stabilization PR:

Stabilize naked_functions rust#134213

cc @bstrie @traviscross @Amanieu @bjorn3

src/attributes/codegen.md

src/inline-assembly.md

Amanieu · 2024-12-01T17:40:33Z

src/inline-assembly.md

+- The compiler cannot assume that the instructions in the asm are the ones that will actually end up executed.
+  - This effectively means that the compiler must treat the `naked_asm!` as a black box and only take the interface specification into account, not the instructions themselves.
+  - Runtime code patching is allowed, via target-specific mechanisms.
+  - However there is no guarantee that each `naked_asm!` directly corresponds to a single instance of instructions in the object file: the compiler is free to duplicate or deduplicate `naked_asm!` blocks.


This is not true, naked functions cannot be duplicated/merged, unlike asm!.

and reading this over, in this case it seems important that we do actually guarantee (and hence the compiler can assume) that the instructions in the naked_asm! invocation are exactly the ones that will be executed. or is that too strict?

I think that poses problems on SPIR-V or NVPTX targets. I seem to recall that the loader just inlines the functions at the Shader Assembly level.

@chorman0773 just to be sure, you're referring to Amanieu's comment right?

What we do in rustc (or well, we will once that PR makes it through the queue) is that a naked function

#[naked] extern "C" fn foo() {}

is emitted as something similar to

core::arch::global_asm!( "foo:", "ret" ); extern "C" { fn foo(); }

and we're relying on the compiler (rustc, llvm) not duplicating that bit of global assembly. I suspect we already rely on that in general, because global assembly can define symbols today.

If you still think this is problematic, how could we verify the behavior of this new codegen strategy for the targets you list?

and reading this over, in this case it seems important that we do actually guarantee (and hence the compiler can assume) that the instructions in the naked_asm! invocation are exactly the ones that will be executed. or is that too strict?

No, they may not be the same instructions that are executed since you can still do runtime patching. We only guarantee that assembly code only appears once in the object file for the purposes of symbols.

src/inline-assembly.md

folkertdev · 2024-12-12T13:21:04Z

src/attributes/codegen.md

+r[attributes.codegen.naked.no-duplication]
+The asm code may not be duplicated by the compiler.
+This property is important for naked functions that define symbols in the assembly code.


so, something I just thought of: we do duplicate the assembly when monomorphizing generic naked functions. So this phrasing should be a bit more specific I think.

src/inline-assembly.md

folkertdev · 2025-03-07T19:46:19Z

src/inline-assembly.md

+# #[cfg(target_arch = "x86_64")] {
+#[naked]
+extern "C-unwind" fn naked_function() {
+    unsafe {
+        core::arch::naked_asm!(
+            ".cfi_startproc",
+            "push rbp",
+            ".cfi_def_cfa_offset 16",
+            ".cfi_offset rbp, -16",
+            "mov rbp, rsp",
+            ".cfi_def_cfa_register rbp",
+            "",
+            "call {function}",
+            "",
+            "pop rbp",
+            ".cfi_def_cfa rsp, 8",
+            "ret",
+            ".cfi_endproc",
+            function = sym function_that_panics,
+        )
+    }
+}
+
+extern "C-unwind" fn function_that_panics() {
+    panic!("unwind!");
+}
+# }


I don't think I can currently run this because #[naked] is still unstable. So reminder to self: update this before merging.

cc @traviscross

We won't merge this anyway until the stabilization lands, so you can just update it and leave the CI red until then (add the feature flag and test it locally though). That's what we normally do.

Locally I still even get errors like this

Couldn't compile the test. ---- inline-assembly.md - Inline_assembly::Options (line 1360) stdout ---- error[E0658]: label operands for inline assembly are unstable --> inline-assembly.md:1364:32 | 6 | core::arch::asm!("jmp {}", label { | ________________________________^ 7 | | println!(); 8 | | }, options(noreturn)); | |_____^ | = note: see issue #119364 <https://github.com/rust-lang/rust/issues/119364> for more information

with these versions in the reference folder (so I don't think there is a rustup override active)

> cargo --version cargo 1.88.0-nightly (d811228b1 2025-04-15) > rustc --version rustc 1.88.0-nightly (191df20fc 2025-04-18)

That feature has been stable on nightly for a while, so I must be missing something. Anyway, I believe the current code to be correct, and CI will catch it before merge if there are minor issues.

I've had trouble with overrides too on the Reference for reasons I haven't yet worked out. Perhaps if you rustup default nightly (or rustup default stage1 or whatnot) it will work for you.

traviscross · 2025-03-08T01:30:50Z

src/attributes/codegen.md

+The *`naked` [attribute]* prevents the compiler from emitting a function prologue and
+epilogue for the attributed function.


We're unwrapping lines on new things added, so go ahead and unwrap these all if you would.

src/inline-assembly.md

also maybe make links work?

Co-authored-by: bjorn3 <[email protected]>

Co-authored-by: Laine Taffin Altman <[email protected]>

this only applies to `asm!` (even though the rule has been removed there since copying it for naked asm

…mbly

traviscross · 2025-04-20T23:48:14Z

src/attributes/codegen.md

+No function prologue or epilogue are generated for the attributed function: the contents of the `naked_asm!` invocation make up the full body of a naked function.
+
+r[attributes.codegen.naked.unsafe-attribute]
+The `naked` attribute is an [unsafe attribute]. Annotating a function with `#[unsafe(naked)]` comes with the safety obligation that the body respects the function's calling convention, and that the body either returns or diverges.


...and that the body either returns or diverges

Is it correct that this should be removed since we've determined that unwinding is OK?

Amanieu reviewed Dec 1, 2024

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ehuss added S-waiting-on-stabilization Waiting for a stabilization PR to be merged in the main Rust repository S-waiting-on-review Status: The marked PR is awaiting review from a maintainer labels Dec 5, 2024

folkertdev commented Dec 12, 2024

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folkertdev mentioned this pull request Dec 18, 2024

Stabilize naked_functions rust-lang/rust#134213

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Add documentation for the naked function attribute #1153

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bstrie and others added 4 commits April 19, 2025 12:39

Add documentation for the naked function attribute

4ee56d4

Add naked to the attribute index

69d5534

document naked_asm!

e42068d

mention incompatible attributes

04d5b88

folkertdev and others added 11 commits April 19, 2025 12:39

remove unused links

f64c8ca

changes after code review

2401d21

also maybe make links work?

fix link

961ef79

add rule on black box/code patching

6f74661

fix rule name

b17fcf3

Co-authored-by: bjorn3 <[email protected]>

add note about monomorphization

5e43776

Update src/inline-assembly.md

189004e

Co-authored-by: Laine Taffin Altman <[email protected]>

remove the rule on unwinding.

5928c4b

this only applies to `asm!` (even though the rule has been removed there since copying it for naked asm

use naked_asm! block to clarify we're talking just about naked asse…

1c3e520

…mbly

reintroduce a section on unwinding out of naked_asm!

0f837d8

formatting + update to unsafe(naked)

c4d68c9

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naked functions #1689

naked functions #1689

folkertdev commented Dec 1, 2024 •

edited by traviscross

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		The `naked` [attribute] prevents the compiler from emitting a function prologue and
		epilogue for the attributed function.

naked functions #1689

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naked functions #1689

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folkertdev commented Dec 1, 2024 •

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