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The no_mangle attribute is unsafe since Rust 2024

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Philipp Oppermann
2025-03-27 15:39:15 +01:00
parent 9345886d44
commit 9753695744
47 changed files with 149 additions and 149 deletions
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8
blog/content/edition-2/posts/02-minimal-rust-kernel/index.md
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@@ -236,7 +236,7 @@ fn panic(_info: &PanicInfo) -> ! {
loop {}
}
#[no_mangle] // don't mangle the name of this function
#[unsafe(no_mangle)] // don't mangle the name of this function
pub extern "C" fn _start() -> ! {
// this function is the entry point, since the linker looks for a function
// named `_start` by default
@@ -301,7 +301,7 @@ We see that `cargo build` now recompiles the `core`, `rustc-std-workspace-core`
The Rust compiler assumes that a certain set of built-in functions is available for all systems. Most of these functions are provided by the `compiler_builtins` crate that we just recompiled. However, there are some memory-related functions in that crate that are not enabled by default because they are normally provided by the C library on the system. These functions include `memset`, which sets all bytes in a memory block to a given value, `memcpy`, which copies one memory block to another, and `memcmp`, which compares two memory blocks. While we didn't need any of these functions to compile our kernel right now, they will be required as soon as we add some more code to it (e.g. when copying structs around).
Since we can't link to the C library of the operating system, we need an alternative way to provide these functions to the compiler. One possible approach for this could be to implement our own `memset` etc. functions and apply the `#[no_mangle]` attribute to them (to avoid the automatic renaming during compilation). However, this is dangerous since the slightest mistake in the implementation of these functions could lead to undefined behavior. For example, implementing `memcpy` with a `for` loop may result in an infinite recursion because `for` loops implicitly call the [`IntoIterator::into_iter`] trait method, which may call `memcpy` again. So it's a good idea to reuse existing, well-tested implementations instead.
Since we can't link to the C library of the operating system, we need an alternative way to provide these functions to the compiler. One possible approach for this could be to implement our own `memset` etc. functions and apply the `#[unsafe(no_mangle)]` attribute to them (to avoid the automatic renaming during compilation). However, this is dangerous since the slightest mistake in the implementation of these functions could lead to undefined behavior. For example, implementing `memcpy` with a `for` loop may result in an infinite recursion because `for` loops implicitly call the [`IntoIterator::into_iter`] trait method, which may call `memcpy` again. So it's a good idea to reuse existing, well-tested implementations instead.
[`IntoIterator::into_iter`]: https://doc.rust-lang.org/stable/core/iter/trait.IntoIterator.html#tymethod.into_iter
@@ -319,7 +319,7 @@ build-std = ["core", "compiler_builtins"]
(Support for the `compiler-builtins-mem` feature was only [added very recently](https://github.com/rust-lang/rust/pull/77284), so you need at least Rust nightly `2020-09-30` for it.)
Behind the scenes, this flag enables the [`mem` feature] of the `compiler_builtins` crate. The effect of this is that the `#[no_mangle]` attribute is applied to the [`memcpy` etc. implementations] of the crate, which makes them available to the linker.
Behind the scenes, this flag enables the [`mem` feature] of the `compiler_builtins` crate. The effect of this is that the `#[unsafe(no_mangle)]` attribute is applied to the [`memcpy` etc. implementations] of the crate, which makes them available to the linker.
[`mem` feature]: https://github.com/rust-lang/compiler-builtins/blob/eff506cd49b637f1ab5931625a33cef7e91fbbf6/Cargo.toml#L54-L55
[`memcpy` etc. implementations]: https://github.com/rust-lang/compiler-builtins/blob/eff506cd49b637f1ab5931625a33cef7e91fbbf6/src/mem.rs#L12-L69
@@ -357,7 +357,7 @@ The implementation looks like this:
```rust
static HELLO: &[u8] = b"Hello World!";
#[no_mangle]
#[unsafe(no_mangle)]
pub extern "C" fn _start() -> ! {
let vga_buffer = 0xb8000 as *mut u8;