Emit function declarations for functions with `#[linkage="extern_weak"]` #138349

1c3t3a · 2025-03-11T11:21:06Z

Currently, when declaring an extern weak function in Rust, we use the following syntax:

unsafe extern "C" {
   #[linkage = "extern_weak"]
   static FOO: Option<unsafe extern "C" fn() -> ()>;
}

This allows runtime-checking the extern weak symbol through the Option.

When emitting LLVM-IR, the Rust compiler currently emits this static as an i8, and a pointer that is initialized with the value of the global i8 and represents the nullabilty e.g.

@FOO = extern_weak global i8
@_rust_extern_with_linkage_FOO = internal global ptr @FOO

This approach does not work well with CFI, where we need to attach CFI metadata to a concrete function declaration, which was pointed out in #115199.

This change switches to emitting a proper function declaration instead of a global i8. This allows CFI to work for extern_weak functions. Example:

@_rust_extern_with_linkage_FOO = internal global ptr @FOO
...
declare !type !61 !type !62 !type !63 !type !64 extern_weak void @FOO(double) unnamed_addr #6

We keep initializing the Rust internal symbol with the function declaration, which preserves the correct behavior for runtime checking the Option.

r? @rcvalle

cc @jakos-sec

try-job: test-various

rustbot · 2025-03-11T11:21:12Z

Some changes occurred in tests/codegen/sanitizer

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

compiler/rustc_codegen_llvm/src/consts.rs

compiler-errors · 2025-03-11T13:37:40Z

Squash the last two commits into the first, since there's no reason for the commit history to record all of the fixing lol

1c3t3a · 2025-03-11T13:39:04Z

Ah, I was told that this is the workflow and that there is an avoidance of force-pushes. I am happy to squash anything.

compiler/rustc_codegen_llvm/src/consts.rs

rustbot · 2025-03-11T13:40:08Z

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

tests/codegen/sanitizer/cfi/external_weak_symbols.rs

rcvalle · 2025-03-14T18:20:58Z

Thank you for your time and work on this, @1c3t3a and @jakos-sec, and for reviewing it, @bjorn3 and @compiler-errors! Much appreciated. Hopefully, with this we can soon remove all the previously added no_sanitize to core and stdlib such as in #115200 and #138002.

rcvalle · 2025-03-14T18:21:07Z

@bors r+

bors · 2025-03-14T18:21:10Z

📌 Commit 86492fa has been approved by rcvalle

It is now in the queue for this repository.

Emit function declarations for functions with `#[linkage="extern_weak"]` Currently, when declaring an extern weak function in Rust, we use the following syntax: ```rust unsafe extern "C" { #[linkage = "extern_weak"] static FOO: Option<unsafe extern "C" fn() -> ()>; } ``` This allows runtime-checking the extern weak symbol through the Option. When emitting LLVM-IR, the Rust compiler currently emits this static as an i8, and a pointer that is initialized with the value of the global i8 and represents the nullabilty e.g. ``` `@FOO` = extern_weak global i8 `@_rust_extern_with_linkage_FOO` = internal global ptr `@FOO` ``` This approach does not work well with CFI, where we need to attach CFI metadata to a concrete function declaration, which was pointed out in rust-lang#115199. This change switches to emitting a proper function declaration instead of a global i8. This allows CFI to work for extern_weak functions. Example: ``` `@_rust_extern_with_linkage_FOO` = internal global ptr `@FOO` ... declare !type !61 !type !62 !type !63 !type !64 extern_weak void `@FOO(double)` unnamed_addr #6 ``` We keep initializing the Rust internal symbol with the function declaration, which preserves the correct behavior for runtime checking the Option. r? `@rcvalle` cc `@jakos-sec`

Rollup of 8 pull requests Successful merges: - rust-lang#138056 (rustc_target: Add target features for LoongArch v1.1) - rust-lang#138349 (Emit function declarations for functions with `#[linkage="extern_weak"]`) - rust-lang#138451 (Build GCC on CI with GCC, not Clang) - rust-lang#138454 (Improve post-merge workflow) - rust-lang#138460 (Pass struct field HirId when check_expr_struct_fields) - rust-lang#138482 (Fix HIR printing of parameters) - rust-lang#138507 (Mirror NetBSD sources) - rust-lang#138511 (Make `Parser::parse_expr_cond` public) r? `@ghost` `@rustbot` modify labels: rollup

fmease · 2025-03-14T23:17:30Z

Failed in rollup: #138516 (comment)
@bors r-

@foo

Currently, when declaring an extern weak function in Rust, we use the following syntax: ```rust unsafe extern "C" { #[linkage = "extern_weak"] static FOO: Option<unsafe extern "C" fn() -> ()>; } ``` This allows runtime-checking the extern weak symbol through the Option. When emitting LLVM-IR, the Rust compiler currently emits this static as an i8, and a pointer that is initialized with the value of the global i8 and represents the nullabilty e.g. ``` @foo = extern_weak global i8 @_rust_extern_with_linkage_FOO = internal global ptr @foo ``` This approach does not work well with CFI, where we need to attach CFI metadata to a concrete function declaration, which was pointed out in rust-lang#115199. This change switches to emitting a proper function declaration instead of a global i8. This allows CFI to work for extern_weak functions. We keep initializing the Rust internal symbol with the function declaration, which preserves the correct behavior for runtime checking the Option. Co-authored-by: Jakob Koschel <jakobkoschel@google.com>

1c3t3a · 2025-03-17T08:40:45Z

I added the mentioned flag to fix the issue (and a bunch of other tests in this directory have it). Is there any way to trigger this test again? Or does it only happen during rollup?

fmease · 2025-03-17T08:46:40Z

I'll do a try build.

@bors try

bors · 2025-03-17T08:47:53Z

⌛ Trying commit b30cf11 with merge c29cad1...

Emit function declarations for functions with `#[linkage="extern_weak"]` Currently, when declaring an extern weak function in Rust, we use the following syntax: ```rust unsafe extern "C" { #[linkage = "extern_weak"] static FOO: Option<unsafe extern "C" fn() -> ()>; } ``` This allows runtime-checking the extern weak symbol through the Option. When emitting LLVM-IR, the Rust compiler currently emits this static as an i8, and a pointer that is initialized with the value of the global i8 and represents the nullabilty e.g. ``` `@FOO` = extern_weak global i8 `@_rust_extern_with_linkage_FOO` = internal global ptr `@FOO` ``` This approach does not work well with CFI, where we need to attach CFI metadata to a concrete function declaration, which was pointed out in rust-lang#115199. This change switches to emitting a proper function declaration instead of a global i8. This allows CFI to work for extern_weak functions. Example: ``` `@_rust_extern_with_linkage_FOO` = internal global ptr `@FOO` ... declare !type !61 !type !62 !type !63 !type !64 extern_weak void `@FOO(double)` unnamed_addr rust-lang#6 ``` We keep initializing the Rust internal symbol with the function declaration, which preserves the correct behavior for runtime checking the Option. r? `@rcvalle` cc `@jakos-sec` try-job: test-various

1c3t3a · 2025-03-17T09:34:12Z

Not sure I can do that but it'd be helpful to not rollup this patch.

@bors rollup=never

bors · 2025-03-17T09:34:15Z

@1c3t3a: 🔑 Insufficient privileges: not in try users

bors · 2025-03-17T10:09:37Z

☀️ Try build successful - checks-actions
Build commit: c29cad1 (c29cad1f56daa7fd1a315c02c28ac415a132c381)

1c3t3a · 2025-03-17T11:32:36Z

Nice! Should we try to submit it again? :)

fmease · 2025-03-17T12:40:05Z

@bors r=rcvalle

bors · 2025-03-17T12:40:08Z

📌 Commit b30cf11 has been approved by rcvalle

It is now in the queue for this repository.

…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#133870 (Stabilize `asm_goto` feature gate) - rust-lang#137449 (Denote `ControlFlow` as `#[must_use]`) - rust-lang#137465 (mir_build: Avoid some useless work when visiting "primary" bindings) - rust-lang#138349 (Emit function declarations for functions with `#[linkage="extern_weak"]`) - rust-lang#138412 (Install licenses into `share/doc/rust/licenses`) - rust-lang#138577 (rustdoc-json: Don't also include `#[deprecated]` in `Item::attrs`) - rust-lang#138588 (Avoid double lowering of idents) Failed merges: - rust-lang#138321 ([bootstrap] Distribute split debuginfo if present) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#138349 - 1c3t3a:external-weak-cfi, r=rcvalle Emit function declarations for functions with `#[linkage="extern_weak"]` Currently, when declaring an extern weak function in Rust, we use the following syntax: ```rust unsafe extern "C" { #[linkage = "extern_weak"] static FOO: Option<unsafe extern "C" fn() -> ()>; } ``` This allows runtime-checking the extern weak symbol through the Option. When emitting LLVM-IR, the Rust compiler currently emits this static as an i8, and a pointer that is initialized with the value of the global i8 and represents the nullabilty e.g. ``` `@FOO` = extern_weak global i8 `@_rust_extern_with_linkage_FOO` = internal global ptr `@FOO` ``` This approach does not work well with CFI, where we need to attach CFI metadata to a concrete function declaration, which was pointed out in rust-lang#115199. This change switches to emitting a proper function declaration instead of a global i8. This allows CFI to work for extern_weak functions. Example: ``` `@_rust_extern_with_linkage_FOO` = internal global ptr `@FOO` ... declare !type !61 !type !62 !type !63 !type !64 extern_weak void `@FOO(double)` unnamed_addr rust-lang#6 ``` We keep initializing the Rust internal symbol with the function declaration, which preserves the correct behavior for runtime checking the Option. r? `@rcvalle` cc `@jakos-sec` try-job: test-various

rustbot assigned rcvalle Mar 11, 2025

rustbot added PG-exploit-mitigations S-waiting-on-review T-compiler labels Mar 11, 2025

bjorn3 reviewed Mar 11, 2025

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1c3t3a force-pushed the external-weak-cfi branch from 4ee7222 to fb32abe Compare March 11, 2025 13:40

1c3t3a force-pushed the external-weak-cfi branch 2 times, most recently from d630c28 to 55ed3be Compare March 11, 2025 13:49

rcvalle reviewed Mar 13, 2025

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1c3t3a force-pushed the external-weak-cfi branch from 55ed3be to 86492fa Compare March 13, 2025 10:22

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fmease mentioned this pull request Mar 14, 2025

Rollup of 8 pull requests #138516

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1c3t3a force-pushed the external-weak-cfi branch from 86492fa to b30cf11 Compare March 17, 2025 08:28

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matthiaskrgr mentioned this pull request Mar 17, 2025

Rollup of 7 pull requests #138611

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bors merged commit 8f5c09b into rust-lang:master Mar 17, 2025
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Emit function declarations for functions with `#[linkage="extern_weak"]` #138349

Emit function declarations for functions with `#[linkage="extern_weak"]` #138349

1c3t3a commented Mar 11, 2025 •

edited by fmease

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Emit function declarations for functions with #[linkage="extern_weak"] #138349

Emit function declarations for functions with #[linkage="extern_weak"] #138349

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Emit function declarations for functions with `#[linkage="extern_weak"]` #138349

Emit function declarations for functions with `#[linkage="extern_weak"]` #138349

1c3t3a commented Mar 11, 2025 •

edited by fmease

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