Hail mary stack unwwind for ARM

On ARM, if we have no CFI, and no other option, we can just look at the
link register, and replace the instruction pointer with it. Unlike on
32- and 64-bit intel platforms, it appears that the linux aarch64
toolchain will not generate FDE data for PLT entries - this means we
need this for the case that a process happens to be in a PLT entry when
we take a backtrace.

Author: peadar

libpstack/proc.h

@@ -66,6 +66,9 @@ enum class UnwindMechanism {
     // The stack frame was built up by scanning a log file.
     LOGFILE,
 
+    // For ARM - last-ditch effort - link register
+    LINKREG,
+
     INVALID,
 };
 

process.cc

@@ -674,6 +674,7 @@ std::ostream &operator << (std::ostream &os, UnwindMechanism mech) {
       case UnwindMechanism::BAD_IP_RECOVERY: return os << "popped faulting IP";
       case UnwindMechanism::TRAMPOLINE: return os << "signal trampoline";
       case UnwindMechanism::LOGFILE: return os << "log file";
+      case UnwindMechanism::LINKREG: return os << "link register";
       case UnwindMechanism::INVALID: return os << "invalid";
    }
    abort();
@@ -1067,6 +1068,15 @@ ThreadStack::unwind(Process &p, Elf::CoreRegisters &regs)
                     stack.emplace_back(UnwindMechanism::TRAMPOLINE, newRegs);
                     continue;
                 }
+                // last ditch effort for ARM is to just replace the PC with the
+                // LR - this is useful for PLT entries, for example.
+                if (prev.regs.regs[30] != prev.regs.pc) {
+                   Elf::CoreRegisters newRegs = prev.regs;
+                   newRegs.pc = newRegs.regs[30];
+                   stack.emplace_back(UnwindMechanism::LINKREG, newRegs);
+                   continue;
+                }
+
 #endif
 #if defined(__i386__)
                 // Deal with signal trampolines for i386