breakpad patch to avoid the pseudo-register .ra to be self-assigned

0001-avoid-cyclic-assigment-of-.ra.patch

From 13a5ab8b7f1a427182027ad6e3a2f64246fe27a7 Mon Sep 17 00:00:00 2001
From: Gawen Arab <[email protected]>
Date: Thu, 24 Oct 2019 12:54:16 +0200
Subject: [PATCH] fix: DW_CFA_same_value of the return address for .ra

A DWARF CFI rule `DW_CFA_same_value` of the return address register
assigned to `.ra` should reload the return address register and assign
it to `.ra`. The recorded rule should be `.ra: <address register>`.

But `DwarfCFIToModule::RegisterName` always replaced the return address
register by `.ra` (dwarf_cfi_to_module.cc:179), and the rule becomes
`.ra: .ra`. The assignment is cyclic and makes Breakpad reject the CFI
when unwinding a stacktrace.

This fix forces to assign the physical return address register when a
same value rule is emitted for `.ra`.
---
 src/common/dwarf_cfi_to_module.cc |  6 +++---
 src/common/dwarf_cfi_to_module.h  | 14 +++++++++++++-
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/src/common/dwarf_cfi_to_module.cc b/src/common/dwarf_cfi_to_module.cc
index bd298a2f..ae3550a5 100644
--- a/src/common/dwarf_cfi_to_module.cc
+++ b/src/common/dwarf_cfi_to_module.cc
@@ -169,14 +169,14 @@ bool DwarfCFIToModule::Entry(size_t offset, uint64 address, uint64 length,
   return true;
 }
 
-string DwarfCFIToModule::RegisterName(int i) {
+string DwarfCFIToModule::RegisterName(int i, enum RegisterNameOption opt) {
   assert(entry_);
   if (i < 0) {
     assert(i == kCFARegister);
     return cfa_name_;
   }
   unsigned reg = i;
-  if (reg == return_address_)
+  if (opt != REGISTER_NAME_KEEP_RETURN_ADDRESS && reg == return_address_)
     return ra_name_;
 
   // Ensure that a non-empty name exists for this register value.
@@ -217,7 +217,7 @@ bool DwarfCFIToModule::UndefinedRule(uint64 address, int reg) {
 
 bool DwarfCFIToModule::SameValueRule(uint64 address, int reg) {
   ostringstream s;
-  s << RegisterName(reg);
+  s << RegisterName(reg, REGISTER_NAME_KEEP_RETURN_ADDRESS);
   Record(address, reg, s.str());
   return true;
 }
diff --git a/src/common/dwarf_cfi_to_module.h b/src/common/dwarf_cfi_to_module.h
index a5302e15..5ce01c52 100644
--- a/src/common/dwarf_cfi_to_module.h
+++ b/src/common/dwarf_cfi_to_module.h
@@ -154,8 +154,20 @@ class DwarfCFIToModule: public CallFrameInfo::Handler {
   virtual bool End();
 
  private:
+  enum RegisterNameOption {
+      REGISTER_NAME_NORMAL = 0,
+
+      // do not replace register return_address_ by '.ra'
+      REGISTER_NAME_KEEP_RETURN_ADDRESS,
+  };
+
   // Return the name to use for register REG.
-  string RegisterName(int i);
+  string RegisterName(int i, enum RegisterNameOption opt);
+  inline string RegisterName(int i) {
+    return RegisterName(i, REGISTER_NAME_NORMAL);
+  }
+
+  // Return the name to use for register REG
 
   // Record RULE for register REG at ADDRESS.
   void Record(Module::Address address, int reg, const string &rule);
-- 
2.23.0