[libcxx-commits] [PATCH] D102912: [libunwind] AARCH64 use inline assembly for pointer authentication

[Maksim Kita via Phabricator via libcxx-commits]

kitaisreal added a comment.

In D102912#2775036 <https://reviews.llvm.org/D102912#2775036>, @pcc wrote:

> Which version of Clang are you using? I'm reasonably sure that we're building libunwind on Android with Clang, and the pointer authentication code works there.

I tried to make native AARCH64 build with clang-11.
I am using clang-11, native build on AARCH64, there was a crash during O0 build. Let me show example:

  #include <stdint.h>
  
  uintptr_t __attribute__ ((noinline)) authenticatePtrKeyB(uintptr_t pointer, uintptr_t modifier)
  {
      register unsigned long long x17 __asm("x17") = pointer;
      register unsigned long long x16 __asm("x16") = modifier;
  
      asm volatile("hint 0xe" : "+r"(x17) : "r"(x16)); // autib1716
      pointer = x17;
  
      return pointer;
  }
  
  int main(int argc, char **argv)
  {
      uintptr_t value_to_take = 5;
      uintptr_t ptr_to_value = reinterpret_cast<uintptr_t>(&value_to_take);
      authenticatePtrKeyB(ptr_to_value, 3);
  
      return 0;
  }

  /usr/bin/clang-11 -O0 register_example.cpp -o register_example
  
  Stack dump:
  0.	Program arguments: /usr/lib/llvm-11/bin/clang -cc1 -triple aarch64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name register_example.cpp -mrelocation-model static -mframe-pointer=non-leaf -fmath-errno -fno-rounding-math -mconstructor-aliases -target-cpu generic -target-feature +neon -target-abi aapcs -fallow-half-arguments-and-returns -fno-split-dwarf-inlining -debugger-tuning=gdb -resource-dir /usr/lib/llvm-11/lib/clang/11.0.0 -internal-isystem /usr/bin/../lib/gcc/aarch64-linux-gnu/10/../../../../include/c++/10 -internal-isystem /usr/bin/../lib/gcc/aarch64-linux-gnu/10/../../../../include/aarch64-linux-gnu/c++/10 -internal-isystem /usr/bin/../lib/gcc/aarch64-linux-gnu/10/../../../../include/aarch64-linux-gnu/c++/10 -internal-isystem /usr/bin/../lib/gcc/aarch64-linux-gnu/10/../../../../include/c++/10/backward -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm-11/lib/clang/11.0.0/include -internal-externc-isystem /usr/include/aarch64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -O0 -fdeprecated-macro -fdebug-compilation-dir /home/ubuntu/test -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fcxx-exceptions -fexceptions -fcolor-diagnostics -faddrsig -o /tmp/register_example-0ba515.o -x c++ register_example.cpp
  1.	<eof> parser at end of file
  2.	Code generation
  3.	Running pass 'Function Pass Manager' on module 'register_example.cpp'.
  4.	Running pass 'RegBankSelect' on function '@_Z19authenticatePtrKeyBmm'
  #0 0x0000ffff8f9809fc llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/lib/aarch64-linux-gnu/libLLVM-11.so.1+0xa5c9fc)

If I compile this file using O2 <https://reviews.llvm.org/owners/package/2/> it will work and in objdump code for function is correctly generated.

  0000000000400544 <_Z19authenticatePtrKeyBmm>:
    400544:       aa0003f1        mov     x17, x0
    400548:       aa0103f0        mov     x16, x1
    40054c:       d50321df        autib1716
    400550:       aa1103e0        mov     x0, x17
    400554:       d65f03c0        ret

  /usr/bin/clang-11 --version
  Ubuntu clang version 11.0.0-2~ubuntu20.04.1
  Target: aarch64-unknown-linux-gnu
  Thread model: posix
  InstalledDir: /usr/bin

I checked following links:

1. https://clang.llvm.org/docs/UsersManual.html#gcc-extensions-not-implemented-yet.
2. It is for other libunwind implementation https://github.com/libunwind/libunwind/issues/215

Seems like it is better to not use partially supported gcc extensions.



================
Comment at: libunwind/src/DwarfInstructions.hpp:229
+                       "mov x16, %x1;\n\t"
+                       "hint %[key];\n\t"
+                       "mov %0, x17\n\t"
----------------
MaskRay wrote:
> Just use `hint 0xe`?
Discussed in previous comments that it make sense to be more specific and use constant.


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