[PAC] Fn ptr type discrimination tests (8/8) by jchlanda · Pull Request #159087 · rust-lang/rust

jchlanda · 2026-07-10T14:02:37Z

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Adds tests for function pointer type discrimination.

Also updates the main pauthtest document to:

list the new tests;
remove the requirement to patch libc, since those changes have already landed; and
document that cc-rs still requires patching because compiler/rustc_llvm/Cargo.toml pins cc to the older version =1.2.16.

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

4.5 ABI-compatibility

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

cezarbbb

On the aarch64 Linux platform, there is no float128, only long double is 128-bit, so it will only send e, not g.
Windows and Apple targets should be excluded, see #163193

View changes since this review

cezarbbb · 2026-09-23T04:46:50Z

+            16 => enc.push_str("DF16_"),
            32 => enc.push(b'f'),
            64 => enc.push(b'd'),
            128 => enc.push(b'g'),


Suggested change

128 => enc.push(b'g'),

128 => match tcx.sess.target.arch {

Arch::AArch64 if !is_like_darwin && !is_like_windows => enc.push(b'e'),

_ => enc.push(b'g'),

},

Done, thank you!

cezarbbb · 2026-09-23T04:50:32Z

+// NO_DISC: @{{.*}}T_2D = constant ptr ptrauth (ptr @f_2d, i32 0), align 8
+#[used]
+static T_2D: fn_2d = f_2d;
+// discriminator: 51179 (0xC7EB), encoding: FggE


Suggested change

// discriminator: 51179 (0xC7EB), encoding: FggE

// discriminator: 25877 (0x6515), encoding: FeeE

Done, thank you.

cezarbbb · 2026-09-23T04:51:18Z

+#[used]
+static T_2D: fn_2d = f_2d;
+// discriminator: 51179 (0xC7EB), encoding: FggE
+// DISC: @{{.*}}T_LD = constant ptr ptrauth (ptr @f_ld, i32 0, i64 51179), align 8


Same above.

cezarbbb · 2026-09-23T04:51:48Z

+        // DISC: %{{.*}} = call double ptrauth (ptr @f_2d, i32 0, i64 38695)(double {{.*}}, double {{.*}}) {{.*}} [ "ptrauth"(i32 0, i64 38695) ]
+        // NO_DISC: %{{.*}} = call double ptrauth (ptr @f_2d, i32 0)(double {{.*}}, double {{.*}}) {{.*}} [ "ptrauth"(i32 0, i64 0) ]
+        let _ = T_2D(1.0, 2.0);
+        // DISC: %{{.*}} = call fp128 ptrauth (ptr @f_ld, i32 0, i64 51179)(fp128 {{.*}}) {{.*}} [ "ptrauth"(i32 0, i64 51179) ]


Same above.

This patch introduces the following: * Extends `FnAbi` (`callconv`) with a `ptrauth_type_discriminator` field. This field is only used when emitting pointer authentication call bundles. It is stored in `FnAbi` because the call site is not guaranteed to have access to an `Instance`, so the discriminator cannot always be computed on demand. * Adds support for `llvm.ptrauth.resign`. This intrinsic will be used when support for semantic transmute is added. * Performs a minor API redesign as groundwork for allowing call sites to modify schemas in place.

Also tighten the handling of enums.

rustbot · 2026-10-08T13:36:14Z

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.

Also remove error messages/tests that used to guarded it.

The codegen now walks the layout of static initializer types to find extern "C" function pointer fields, computes their type discriminators, and applies those discriminators when emitting authenticated function pointer relocations. Also make sure that type discrimination is never applied to init/fini entries.

This covers standalone function pointer constants, promoted temporaries, immutable and mutable statics, arrays of function pointers, and mixed structs containing function pointers. Consult pauth-fn-ptr-type-discrimination-static-allocs.rs test for example uses. Revolves around threading PAC information through: * static_addr_of (StaticCodegenMethods) * from_const and from_const_alloc (both on rustc_codegen_ssa::mir::operand / OperandRef)

Also a fix for non function (closure, coroutines, etc) in discriminator_input. Fix in v-table assert.

Meaning if two types are ABI compatible they must have the same encoding and hash value. Provide a ui test which groups the function pointers by the ABI compatibility rules and enforces the rule.

And corresponding doc_example.

Implement pointer authentication resigning for function pointer transmutes that differ in their discriminators. Resigning only happens for function pointers (their transparent wrappers and Option<T>). Aggregates (even those containing function pointer members) are deliberately kept as opaque values with no resigning.

Recognize raw pointers as 0-discriminated. Also, when canonicalizing, move away from hard coded Option<T> check to a generic variant with 2 fields and correct niche.

…addr` call sites Fill in function pointer type discriminators logic across remaining `get_fn_addr` call sites and explicitly avoid applying it where discrimination is not meaningful. Some uses of `get_fn_addr` are intentionally left unsigned, including the EH personality function, entry wrappers, and compiler-generated Rust ABI shims.

And update to the main pauthtest document: * list new tests * remove the need for patching `libc` as the changes to it already went in. Unfortunately `cc-rs` is held back by `compiler/rustc_llvm/Cargo.toml` which pins to an old version: `cc = "=1.2.16"`

And small fixes.

The [RFC](rust-lang/rfcs#3453) specifies that it should be represented according to IEEE 754 `binary16`, so it should land on `DF16_` (corresponding to C's `_Float16`, C++'s `std::float16_t` and LLVM's `half`). At the same time add f128 test, that was missing.

See rust-lang#163193 for context.

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jchlanda force-pushed the jakub/pac_ty_disc_PR_8 branch 2 times, most recently from 7527456 to 979ff1f Compare July 14, 2026 07:36

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jchlanda force-pushed the jakub/pac_ty_disc_PR_8 branch from 979ff1f to 0e9fa4d Compare July 14, 2026 08:55

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jchlanda force-pushed the jakub/pac_ty_disc_PR_8 branch from 0e9fa4d to 753b459 Compare July 17, 2026 07:08

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jchlanda force-pushed the jakub/pac_ty_disc_PR_8 branch 8 times, most recently from e07a163 to fa14e50 Compare July 23, 2026 07:48

cezarbbb reviewed Sep 23, 2026

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jchlanda added 3 commits October 8, 2026 12:31

[PAC] Use Option<u64> in FnAbi's discriminator field

2da42a7

[PAC] repr(transparent) and correct handling of Option<T> in encoder

0b9d8ba

Also tighten the handling of enums.

jchlanda added 6 commits October 8, 2026 14:11

[PAC] Enable support for FPTR_TYPE_DISCR in ABI Version

afff16c

Also remove error messages/tests that used to guarded it.

[PAC] Use scalar_to_backend_with_pac for const globals

b266f9d

Also a fix for non function (closure, coroutines, etc) in discriminator_input. Fix in v-table assert.

[PAC] Make sure that the encoder honors ABI compatibility rules for fn

e4e958a

Meaning if two types are ABI compatible they must have the same encoding and hash value. Provide a ui test which groups the function pointers by the ABI compatibility rules and enforces the rule.

[PAC] Add rustc_dump_ptrauth_discriminator attribute doc

051269b

And corresponding doc_example.

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jchlanda added 10 commits October 8, 2026 14:21

[PAC] Correctly handle raw pointers in memory-based path

8e46662

Recognize raw pointers as 0-discriminated. Also, when canonicalizing, move away from hard coded Option<T> check to a generic variant with 2 fields and correct niche.

[PAC] Minicore updates to support fn ptr type discriminator tests

3801fca

[PAC] Tests for repr(transparent), const globals, enums, Option<T>

ae4a85a

And small fixes.

[PAC] Remove unnecessary only-pauthtest from LLVM codegen tests

9a2d1d4

[PAC] Extend raw pointer tests

992b350

[PAC] Encode f128 as e, not g for platforms without _Float128

0e8c557

See rust-lang#163193 for context.

-=> enc.push(b'g'),
+=> match tcx.sess.target.arch {
+                   Arch::AArch64 if !is_like_darwin && !is_like_windows => enc.push(b'e'),
+                   _ => enc.push(b'g'),
+            },

	// discriminator: 51179 (0xC7EB), encoding: FggE
	// discriminator: 25877 (0x6515), encoding: FeeE

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jchlanda commented Jul 10, 2026 •

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