libunwind: Don't attempt to authenticate a null return address.

[pcc]

Summary

Null return addresses can appear at the bottom of the stack (i.e. the
frame corresponding to the entry point). Authenticating these addresses
will set the error code in the address, which will lead to a segfault
in the sigreturn trampoline detection code. Fix this problem by not
authenticating null addresses.

[rprichard]

It seems OK to me. Does it make sense to also skip (pc == 0) in UnwindCursor<A, R>::setInfoForSigReturn?

[pcc]

In D96560#2558596, @rprichard wrote:

It seems OK to me. Does it make sense to also skip (pc == 0) in UnwindCursor<A, R>::setInfoForSigReturn?

Hmm, that seems redundant with the check that I'm adding here. Let's start with one and we can reconsider if we find another bug here.

[rprichard]

In D96560#2558737, @pcc wrote:

In D96560#2558596, @rprichard wrote:

It seems OK to me. Does it make sense to also skip (pc == 0) in UnwindCursor<A, R>::setInfoForSigReturn?

Hmm, that seems redundant with the check that I'm adding here. Let's start with one and we can reconsider if we find another bug here.

Oh, yeah, it would be redundant with this check earlier in setInfoBasedOnIPRegister:

// Exit early if at the top of the stack.
if (pc == 0) {
  _unwindInfoMissing = true;
  return;
}

[danielkiss]

LGTM, Thanks.

[mstorsjo]

LGTM too

libunwind/src/DwarfInstructions.hpp

 
 #if defined(_LIBUNWIND_TARGET_AARCH64)
       // If the target is aarch64 then the return address may have been signed
       // using the v8.3 pointer authentication extensions. The original
       // return address needs to be authenticated before the return address is
       // restored. autia1716 is used instead of autia as autia1716 assembles
       // to a NOP on pre-v8.3a architectures.
       if ((R::getArch() == REGISTERS_ARM64) &&
-          prolog.savedRegisters[UNW_ARM64_RA_SIGN_STATE].value) {
+          prolog.savedRegisters[UNW_ARM64_RA_SIGN_STATE].value &&
+          returnAddress != 0) {
 #if !defined(_LIBUNWIND_IS_NATIVE_ONLY)
         return UNW_ECROSSRASIGNING;
 #else
         register unsigned long long x17 __asm("x17") = returnAddress;
         register unsigned long long x16 __asm("x16") = cfa;
 
         // These are the autia1716/autib1716 instructions. The hint instructions
         // are used here as gcc does not assemble autia1716/autib1716 for pre