[hwasan] Add __hwasan_untag_pointer

[hctim]

Summary

Now that we have architecture-specific tagging, it's useful to have a
helper function exposed that allows you to untag pointers.

This is useful for testing D100783, as we don't have to implement a tagging
helper function in userspace. As I suspect that HWASan-x64 will move to a
dynamic bit depth at runtime, exposing this helper is probably going to be
necessary in time.

[morehouse]

compiler-rt/include/sanitizer/hwasan_interface.h

47

Isn't this equivalent to __hwasan_tag_pointer(p, 0)?

[hctim]

compiler-rt/include/sanitizer/hwasan_interface.h

47

That's a very good point.

[eugenis]

compiler-rt/include/sanitizer/hwasan_interface.h

47

it is, in *userspace*
(which is all this library needs to worry about)

compiler-rt/include/sanitizer/hwasan_interface.h

#endif
   void __hwasan_enable_allocator_tagging(void);
   void __hwasan_disable_allocator_tagging(void);
 
   // Mark region of memory with the given tag. Both address and size need to be
   // 16-byte aligned.
   void __hwasan_tag_memory(const volatile void *p, unsigned char tag,
                            size_t size);
 
+  // Return an untagged version of the provided pointer. Provided to enable
+  // architecture- and runtime-independent untagging. Note that a tagged and an
+  // untagged pointer may be pointers to different memory, as is the case on x64
+  // with page aliasing.
+  void *__hwasan_untag_pointer(const volatile void *p);
+
   /// Set pointer tag. Previous tag is lost.
   void *__hwasan_tag_pointer(const volatile void *p, unsigned char tag);
 
   // Set memory tag from the current SP address to the given address to zero.
   // This is meant to annotate longjmp and other non-local jumps.
   // This function needs to know the (almost) exact destination frame address;
   // clearing shadow for the entire thread stack like __asan_handle_no_return
   // does would cause false reports.

compiler-rt/lib/hwasan/hwasan.cpp

 void __hwasan_store16_noabort(uptr p) {
   CheckAddress<ErrorAction::Recover, AccessType::Store, 4>(p);
 }
 
 void __hwasan_tag_memory(uptr p, u8 tag, uptr sz) {
   TagMemoryAligned(p, sz, tag);
 }
 
+uptr __hwasan_untag_pointer(uptr p) { return UntagAddr(p); }
+
 uptr __hwasan_tag_pointer(uptr p, u8 tag) {
   return AddTagToPointer(p, tag);
 }
 
 void __hwasan_handle_longjmp(const void *sp_dst) {
   uptr dst = (uptr)sp_dst;
   // HWASan does not support tagged SP.
   CHECK(GetTagFromPointer(dst) == 0);

compiler-rt/lib/hwasan/hwasan_interface_internal.h

 void __hwasan_store8_noabort(uptr);
 SANITIZER_INTERFACE_ATTRIBUTE
 void __hwasan_store16_noabort(uptr);
 
 SANITIZER_INTERFACE_ATTRIBUTE
 void __hwasan_tag_memory(uptr p, u8 tag, uptr sz);
 
 SANITIZER_INTERFACE_ATTRIBUTE
+uptr __hwasan_untag_pointer(uptr p);
+
+SANITIZER_INTERFACE_ATTRIBUTE
 uptr __hwasan_tag_pointer(uptr p, u8 tag);
 
 SANITIZER_INTERFACE_ATTRIBUTE
 void __hwasan_tag_mismatch(uptr addr, u8 ts);
 
 SANITIZER_INTERFACE_ATTRIBUTE
 void __hwasan_tag_mismatch4(uptr addr, uptr access_info, uptr *registers_frame,
                             size_t outsize);