[PAC] Encoder and hash (1/8) by jchlanda · Pull Request #159071 · rust-lang/rust

jchlanda · 2026-07-10T11:16:08Z

This patch implements Rust's equivalent of Clang's function pointer type discriminator computation used for pointer authentication. Compatibility with Clang is a primary design goal. For a given extern "C" function type, the discriminator produced by Rust must match the value computed by Clang so that function pointers can be exchanged safely between Rust and C code while preserving pointer authentication semantics.

The implementation mirrors Clang's behavior in ASTContext::encodeTypeForFunctionPointerAuth, ensuring that identical C-compatible function types produce identical discriminators. See: https://clang.llvm.org/doxygen/ASTContext_8cpp.html#abb1375e068e807917527842d05cadea3.

This is part 1 of a sequence of 8 PRs that together implement support for function pointer type discrimination:

Useful links:

Previous PAC work:
- pauthtest introduction: Introduce aarch64-unknown-linux-pauthtest target #155722
- Library support follow up: Library support for aarch64-unknown-linux-pauthtest target #156548
- Config follow up: Pointer authentication config and user facing options #156712
Project goal and tracking issue
Clang's implementation of ASTContext::encodeTypeForFunctionPointerAuth
LLVM's SpiHash

kovdan01

Would it be possible to add unit tests for hash computation like the ones present in llvm source tree at llvm/unittests/Support/SipHashTest.cpp?

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jchlanda · 2026-07-20T11:56:47Z

Would it be possible to add unit tests for hash computation like the ones present in llvm source tree at llvm/unittests/Support/SipHashTest.cpp?

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Done in: 5e05159

rustbot · 2026-07-24T18:07:24Z

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

rustbot · 2026-07-24T18:07:27Z

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jchlanda · 2026-07-24T18:07:27Z

r? compiler

This patch implements Rust's equivalent of Clang's function pointer type discriminator computation used in pointer authentication. Compatibility with Clang is a primary goal. The discriminator produced for a given external "C" function type must match the value computed by Clang so that function pointers can be exchanged safely between Rust and C code while preserving pointer authentication semantics. The implementation mirrors Clang's behavior in `ASTContext::encodeTypeForFunctionPointerAuth`, ensuring that identical C-compatible function types produce identical discriminators. See: <https://clang.llvm.org/doxygen/ASTContext_8cpp.html#abb1375e068e807917527842d05cadea3>.

rustbot · 2026-07-27T08:12:18Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

davidtwco · 2026-07-27T10:14:28Z

r? @davidtwco

davidtwco · 2026-07-27T10:16:43Z

@@ -0,0 +1,493 @@
+/*!


Use //! so that this is a documentation comment

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davidtwco · 2026-07-27T10:21:34Z

@@ -0,0 +1,142 @@
+// LLVM SipHash-2-4


Why can't we just call LLVM's definitions of these over FFI from rustc_codegen_llvm? That let us avoid the need for this to remain identical to LLVM's definition

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davidtwco · 2026-07-27T10:22:20Z

+/// This is not a full semantic translation of Rust types. It is a lossy mapping
+/// that intentionally matches Clang's function pointer authentication encoding
+/// rules where Rust has a direct language-level equivalent.


These lines are duplicated

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davidtwco · 2026-07-27T10:24:24Z

+    let ty = canonicalize_c_type(tcx, ty);
+    match ty.kind() {
+        // C void / Rust ()
+        ty::Tuple(list) if list.is_empty() => ClangDiscTy::Void,


_ if ty.is_unit() is clearer

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davidtwco · 2026-07-27T10:29:34Z

+
+    loop {
+        match ty.kind() {
+            ty::Adt(def, args) if tcx.is_diagnostic_item(sym::Option, def.did()) => {


You should use the LangItem::Option, rather than the diagnostic item, given this isn't for a diagnostic.

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davidtwco · 2026-07-27T10:30:44Z

+
+// Clang disc type.
+#[derive(Debug)]
+enum ClangDiscTy<'tcx> {


I'm not sure what we're gaining from having this intermediate representation - the mapping seems straightforward enough that you could just go direct from a Ty to a encoding

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davidtwco · 2026-07-27T10:40:04Z

 All tests from `assembly-llvm`, `codegen-llvm`, `codegen-units`, `coverage`,
-`crashes`, `incremental`, `library`, `mir-opt`, `run-make`, `ui` and
-`ui-fulldeps` subsets are expected to pass.
+`crashes`, `incremental`, `library`, `mir-opt`, `run-make`, `rust_middle` `ui`


Suggested change

`crashes`, `incremental`, `library`, `mir-opt`, `run-make`, `rust_middle` `ui`

`crashes`, `incremental`, `library`, `mir-opt`, `run-make`, `rust_middle`, `ui`

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rustbot · 2026-07-27T10:51:14Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

jchlanda force-pushed the jakub/pac_ty_disc_PR_1 branch 2 times, most recently from bca55be to 8cf67b9 Compare July 14, 2026 07:26

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jchlanda force-pushed the jakub/pac_ty_disc_PR_1 branch from 7ee3f2c to fdd1189 Compare July 17, 2026 07:01

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jchlanda force-pushed the jakub/pac_ty_disc_PR_1 branch from d798449 to 4c1b0ac Compare July 17, 2026 09:46

kovdan01 reviewed Jul 20, 2026

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jchlanda force-pushed the jakub/pac_ty_disc_PR_1 branch 2 times, most recently from 03337eb to 8b0e0ca Compare July 24, 2026 15:29

jchlanda changed the title ~~[WIP] 1 - Encoder and hash~~ [PAC] Encoder and hash (1/8) Jul 24, 2026

jchlanda marked this pull request as ready for review July 24, 2026 18:07

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 24, 2026

rustbot assigned wesleywiser Jul 24, 2026

rustbot assigned tiif and unassigned wesleywiser Jul 24, 2026

jchlanda added 2 commits July 27, 2026 08:11

[PAC] Add unit tests for sip hash

67fd305

jchlanda force-pushed the jakub/pac_ty_disc_PR_1 branch from 8b0e0ca to 67fd305 Compare July 27, 2026 08:12

rustbot assigned davidtwco and unassigned tiif Jul 27, 2026

davidtwco requested changes Jul 27, 2026

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rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 27, 2026

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jul 27, 2026

	`crashes`, `incremental`, `library`, `mir-opt`, `run-make`, `rust_middle` `ui`
	`crashes`, `incremental`, `library`, `mir-opt`, `run-make`, `rust_middle`, `ui`

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