[libunwind,EHABI,ARM] Fix get/set of RA_AUTH_CODE.

[simon_tatham]

Summary

According to EHABI32 §8.5.2, the PAC for the return address of a
function described in an exception table is supposed to be addressed
in the _Unwind_VRS_{Get,Set} API by setting regclass=_UVRSC_PSEUDO and
regno=0. (The space of 'regno' values is independent for each
regclass, and for _UVRSC_PSEUDO, there is only one valid regno so far.)

That is indeed what libunwind's _Unwind_VRS_{Get,Set} functions expect
to receive. But at two call sites, the wrong values are passed in:
regno is being set to UNW_ARM_RA_AUTH_CODE (0x8F) instead of 0, and in
one case, regclass is _UVRSC_CORE instead of _UVRSC_PSEUDO.

As a result, those calls to _Unwind_VRS_{Get,Set} return
_UVRSR_FAILED, which their callers ignore. So if you compile in the
AUTG instruction that actually validates the PAC, it will try to
validate what's effectively an uninitialised register as an
authentication code, and trigger a CPU fault even on correct exception
unwinding.

[danielkiss]

LGTM.

[simon_tatham]

Hmm, Phabricator tells me that some other review should have been done before I landed this. That wasn't obvious, sorry – I saw an email saying someone had accepted the change, and assumed that was enough to push it.

If I violated a procedure, sorry about that! But I hope this change will be considered harmless enough to not revert.

libunwind/src/Unwind-EHABI.cpp

#endif
   if (!wrotePC) {
     uint32_t lr;
     _Unwind_VRS_Get(context, _UVRSC_CORE, UNW_ARM_LR, _UVRSD_UINT32, &lr);
 #ifdef __ARM_FEATURE_PAUTH
     if (hasReturnAddrAuthCode) {
       uint32_t sp;
       uint32_t pac;
       _Unwind_VRS_Get(context, _UVRSC_CORE, UNW_ARM_SP, _UVRSD_UINT32, &sp);
-      _Unwind_VRS_Get(context, _UVRSC_PSEUDO, UNW_ARM_RA_AUTH_CODE,
-                      _UVRSD_UINT32, &pac);
+      _Unwind_VRS_Get(context, _UVRSC_PSEUDO, 0, _UVRSD_UINT32, &pac);
       __asm__ __volatile__("autg %0, %1, %2" : : "r"(pac), "r"(lr), "r"(sp) :);
     }
 #else
     (void)hasReturnAddrAuthCode;
 #endif
     _Unwind_VRS_Set(context, _UVRSC_CORE, UNW_ARM_IP, _UVRSD_UINT32, &lr);
   }
   return _URC_CONTINUE_UNWIND;
case _UVRSC_PSEUDO: {
       // Return Address Authentication code (PAC) - discriminator 0
       uint32_t *sp;
       if (_Unwind_VRS_Get(context, _UVRSC_CORE, UNW_ARM_SP, _UVRSD_UINT32,
                           &sp) != _UVRSR_OK) {
         return _UVRSR_FAILED;
       }
       uint32_t pac = *sp++;
       _Unwind_VRS_Set(context, _UVRSC_CORE, UNW_ARM_SP, _UVRSD_UINT32, &sp);
-      return _Unwind_VRS_Set(context, _UVRSC_CORE, UNW_ARM_RA_AUTH_CODE,
-                             _UVRSD_UINT32, &pac);
+      return _Unwind_VRS_Set(context, _UVRSC_PSEUDO, 0, _UVRSD_UINT32, &pac);
     }
   }
   _LIBUNWIND_ABORT("unsupported register class");
 }
 
 /// Not used by C++.
 /// Unwinds stack, calling "stop" function at each frame.
 /// Could be used to implement longjmp().