================
@@ -1922,12 +1922,20 @@ class _LIBUNWIND_HIDDEN Registers_arm64 {
void
loadAndAuthenticateLinkRegister(reg_t inplaceAuthedLinkRegister,
link_reg_t *referenceAuthedLinkRegister) {
- // If we are in an arm64/arm64e frame, then the PC should have been signed
- // with the SP
- *referenceAuthedLinkRegister =
- (uint64_t)ptrauth_auth_data((void *)inplaceAuthedLinkRegister,
- ptrauth_key_return_address,
- _registers.__sp);
+ // In arm64e or pauthtest ABI frame, the PC should have been signed with
+ // the SP - resign it with a schema that can be represented with __ptrauth.
+ //
+ // Call the resign builtin explicitly instead of relying on implicit
signing
+ // of the authenticated value on assignment to
*referenceAuthedLinkRegister,
+ // as the latter results in comparison against 0 between auth and sign
+ // operations.
----------------
atrosinenko wrote:
> Anyway, could you file a bug on me for this, I don't believe that we should
> need to null check after the auth, but there are all sorts of weird semantics
> to worry about, but should be doable.
Is the existing behavior of Clang incorrect? This looks rather natural with
these two assumptions:
* `ptrauth_auth_data` is directly translated to a target-specific authentication
* extra care is taken by Clang to keep `nullptr`s represented with all zero
bits when dealing with `__ptrauth`-qualified variables
This way, the existing implementation of `loadAndAuthenticateLinkRegister` is
lowered as expected. First, `inplaceAuthedLinkRegister` is explicitly
authenticated as requested by the programmer, producing _a value_. Then, an
_arbitrary value_ is assigned to a `__ptrauth`-qualified variable, which
according to Clang has the semantic "sign if non-zero, assign literal zero
otherwise".
PS: `__ptrauth(..., "authenticates-null-values")` looks great for such use
cases!
https://github.com/llvm/llvm-project/pull/183603
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