fails to build with `error: use of undeclared identifier 'ptrauth_key_cxx_vtable_pointer'`

[sylvestre]

on linux ubuntu noble

FAILED: compiler-rt/lib/ubsan/CMakeFiles/RTUbsan_cxx.aarch64.dir/ubsan_type_hash_itanium.cpp.o 
/build/source/build-llvm/./bin/clang++ --target=aarch64-unknown-linux-gnu -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/build/source/compiler-rt/lib/ubsan/.. -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -mbranch-protection=standard -Wno-unused-command-line-argument -Wdate-time -D_FORTIFY_SOURCE=3 -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -fdebug-prefix-map=/build/source/build-llvm/runtimes/runtimes-bins=../../../ -fdebug-prefix-map=/build/source/= -no-canonical-prefixes -ffile-prefix-map=/build/source/build-llvm/runtimes/runtimes-bins=../../../ -ffile-prefix-map=/build/source/= -no-canonical-prefixes -Wall -Wno-unused-parameter -O3 -DNDEBUG -std=c++17 -march=armv8-a -fPIC -fno-builtin -fno-exceptions -fomit-frame-pointer -funwind-tables -fno-stack-protector -fno-sanitize=safe-stack -fvisibility=hidden -fno-lto -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta -O3 -gline-tables-only -Wno-gnu -Wno-variadic-macros -Wno-c99-extensions -ftrivial-auto-var-init=pattern -nostdinc++ -frtti -DUBSAN_CAN_USE_CXXABI -MD -MT compiler-rt/lib/ubsan/CMakeFiles/RTUbsan_cxx.aarch64.dir/ubsan_type_hash_itanium.cpp.o -MF compiler-rt/lib/ubsan/CMakeFiles/RTUbsan_cxx.aarch64.dir/ubsan_type_hash_itanium.cpp.o.d -o compiler-rt/lib/ubsan/CMakeFiles/RTUbsan_cxx.aarch64.dir/ubsan_type_hash_itanium.cpp.o -c /build/source/compiler-rt/lib/ubsan/ubsan_type_hash_itanium.cpp
/build/source/compiler-rt/lib/ubsan/ubsan_type_hash_itanium.cpp:210:34: error: use of undeclared identifier 'ptrauth_key_cxx_vtable_pointer'
  210 |   Vtable = ptrauth_strip(Vtable, ptrauth_key_cxx_vtable_pointer);
      |                                  ^
1 error generated.

[asl]

@sylvestre Is this a release branch? Or main?

[sylvestre]

@asl sorry, it is main

[asl]

@ahatanak Will you please take a look? I think the ptrauth_strip should be guarded by #if __has_feature(ptrauth_calls)? Or something like this?

@sylvestre The constant is defined in ptrauth.h, somehow it was not included

[asl]

@sylvestre What is the configuration you're building? Are you having pac-ret enabled by default?

[sylvestre]

dunno what pac-ret is, sorry :)

Full log is here:
https://llvm-jenkins.debian.net/job/llvm-toolchain-noble-binaries/architecture=arm64,distribution=noble,label=arm64/253/consoleFull

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[asl]

So, I think I know what is going on. The code in https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/sanitizer_common/sanitizer_ptrauth.h#L15 uses inline function for ptrauth_strip. As a result, if the key argument is not defined (it is defined in ptrauth.h header that is included only if pauth feature is enabled), then the compilation will fail.

Interesting enough, the change was there since 4220531 (LLVM 13). Tagging @DanielKristofKiss

[asl]

@sylvestre You are using -mbranch-protection=standard, so PAC-RET is enabled, yes

[asl]

Actually, this:

inline unsigned long ptrauth_strip(void* __value, unsigned int __key) {
  // On the stack the link register is protected with Pointer
  // Authentication Code when compiled with -mbranch-protection.
  // Let's stripping the PAC unconditionally because xpaclri is in
  // the NOP space so will do nothing when it is not enabled or not available.
  unsigned long ret;
  asm volatile(
      "mov x30, %1\n\t"
      "hint #7\n\t"  // xpaclri
      "mov %0, x30\n\t"
      : "=r"(ret)
      : "r"(__value)
      : "x30");
  return ret;
}

Is just terribly wrong... for a generic-purpose pauth_strip... we are clobbering LR and leaving unauthenticated value here.