[clang][PAC] Add ptrauth.h helpers for computing type discriminators

[ojhunt]

Adds an additional helper that had not been upstreamed, and adds documentation for both ptrauth_type_discriminator and ptrauth_function_pointer_type_discriminator

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-backend-x86

Author: Oliver Hunt (ojhunt)

Changes

Adds an additional helper that had not been upstreamed, and adds documentation for both ptrauth_type_discriminator and ptrauth_function_pointer_type_discriminator

---

Full diff: https://github.com/llvm/llvm-project/pull/163456.diff

2 Files Affected:

• (modified) clang/docs/PointerAuthentication.rst (+30)
• (modified) clang/lib/Headers/ptrauth.h (+12)

diff --git a/clang/docs/PointerAuthentication.rst b/clang/docs/PointerAuthentication.rst
index 96eb498bc48b6..7e65f4b1b4915 100644
--- a/clang/docs/PointerAuthentication.rst
+++ b/clang/docs/PointerAuthentication.rst
@@ -592,6 +592,36 @@ The result value is never zero and always within range for both the
 
 This can be used in constant expressions.
 
+``ptrauth_type_discriminator``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: c
+
+  ptrauth_type_discriminator(type)
+
+Compute the constant discriminator derived from the given type, as is computed
+for automatically type diversified schemas.
+
+``type`` must be a type. The result has the type ``ptrauth_extra_data_t``.
+
+This can be used in constant expressions.
+
+``ptrauth_function_pointer_type_discriminator``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: c
+
+  ptrauth_function_pointer_type_discriminator(function_type)
+
+Compute the constant discriminator derived from the provided function type, for
+use in contexts where the default function authentication schema. If function
+pointer type diversity is enabled, this is equivalent to
+`ptrauth_type_discriminator(function_type)`, if it is not enabled this is `0`.
+
+``function_type`` must be a function type. The result has the type ``ptrauth_extra_data_t``.
+
+This can be used in constant expressions.
+
 ``ptrauth_strip``
 ^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/Headers/ptrauth.h b/clang/lib/Headers/ptrauth.h
index f902ca1e3bbd3..1de8324e1ec6f 100644
--- a/clang/lib/Headers/ptrauth.h
+++ b/clang/lib/Headers/ptrauth.h
@@ -241,6 +241,17 @@ typedef __UINTPTR_TYPE__ ptrauth_generic_signature_t;
 #define ptrauth_type_discriminator(__type)                                     \
   __builtin_ptrauth_type_discriminator(__type)
 
+/* Compute the constant discriminator used by Clang to sign pointers with the
+   given C function pointer type.
+
+   A call to this function is an integer constant expression. */
+#if __has_feature(ptrauth_function_pointer_type_discrimination)
+#define ptrauth_function_pointer_type_discriminator(__type) \
+  __builtin_ptrauth_type_discriminator(__type)
+#else
+#define ptrauth_function_pointer_type_discriminator(__type) ((ptrauth_extra_data_t)0)
+#endif
+
 /* Compute a signature for the given pair of pointer-sized values.
    The order of the arguments is significant.
 
@@ -372,6 +383,7 @@ typedef __UINTPTR_TYPE__ ptrauth_generic_signature_t;
   })
 
 #define ptrauth_type_discriminator(__type) ((ptrauth_extra_data_t)0)
+#define ptrauth_function_pointer_type_discriminator(__type) ((ptrauth_extra_data_t)0)
 
 #define ptrauth_sign_generic_data(__value, __data)                             \
   ({                                                                           \

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[kovdan01]

LGTM, thanks!

[kovdan01]

Nit: maybe ptrauth_function_pointer_discriminator (w/o type) would suite better here? We either have a type-based discriminator if the feature is requested or just have zero, so it might be better to call the macro just ptrauth_function_pointer_discriminator w/o making the user think that it's a type discriminator.

Totally optional and feel free to ignore.

[kovdan01]

@ojhunt Thanks for introducing this! It's probably worth applying the auto-formatting suggestion before merging the PR

[ojhunt]

@ojhunt Thanks for introducing this! It's probably worth applying the auto-formatting suggestion before merging the PR

Oh yeah, durrrr, copying code while forgetting that we have reformatted a lot over time :D