Add and implement --enable-deref-arg feature

generate_eh_elf.py

@@ -148,6 +148,11 @@ def process_args():
                         help=("Force re-generation of the output files, even "
                               "when those files are newer than the target "
                               "ELF."))
+    parser.add_argument('--enable-deref-arg', action='store_true',
+                        help=("Pass the `--enable-deref-arg` to "
+                              "dwarf-assembly, enabling an extra `deref` "
+                              "argument for each lookup function, allowing "
+                              "to work on remote address spaces."))
     # c_opt_level
     opt_level_grp = parser.add_mutually_exclusive_group()
     opt_level_grp.add_argument('-O0', action='store_const', const='-O0',
@@ -197,6 +202,8 @@ def main():
     for opt in DW_ASSEMBLY_OPTS:
         if opt in args and args_dict[opt] is not None:
             dwarf_assembly_opts.append(DW_ASSEMBLY_OPTS[opt])
+    if args.enable_deref_arg:
+        dwarf_assembly_opts.append('--enable-deref-arg')
 
     for obj in args.object:
         args.gen_func(obj, args, dwarf_assembly_opts)

shared/context_struct.h

@@ -5,3 +5,5 @@ typedef struct {
 } unwind_context_t;
 
 typedef unwind_context_t (*_fde_func_t)(unwind_context_t, uintptr_t);
+
+typedef uintptr_t (*deref_func_t)(uintptr_t);

src/CodeGenerator.cpp

@@ -65,9 +65,13 @@ void CodeGenerator::gen_of_dwarf() {
 }
 
 void CodeGenerator::gen_unwind_func_header(const std::string& name) {
+    string deref_arg;
+    if(settings::enable_deref_arg)
+        deref_arg = ", deref_func_t deref";
+
     os << "unwind_context_t "
        << name
-       << "(unwind_context_t ctx, uintptr_t pc) {\n"
+       << "(unwind_context_t ctx, uintptr_t pc" << deref_arg << ") {\n"
        << "\tunwind_context_t out_ctx;\n"
        << "\tswitch(pc) {" << endl;
 }
@@ -168,11 +172,19 @@ void CodeGenerator::gen_of_reg(const SimpleDwarf::DwRegister& reg) {
             os << ctx_of_dw_name(reg.reg)
                 << " + (" << reg.offset << ")";
             break;
-        case SimpleDwarf::DwRegister::REG_CFA_OFFSET:
+        case SimpleDwarf::DwRegister::REG_CFA_OFFSET: {
+            if(settings::enable_deref_arg) {
+                os << "deref(out_ctx.rsp + ("
+                   << reg.offset
+                   << "))";
+            }
+            else {
                 os << "*((uintptr_t*)(out_ctx.rsp + ("
                    << reg.offset
                    << ")))";
+            }
             break;
+        }
         case SimpleDwarf::DwRegister::REG_NOT_IMPLEMENTED:
             os << "0; assert(0)";
             break;

src/README.md

@@ -23,3 +23,20 @@ a list of all PCs in the ELF. The file contains one 8-bytes chunk per PC,
 which is the PC in little endian.
 
 `--pc-list PC_LIST_FILE_PATH`
+
+### Dereferencing function
+
+The lookup functions can also take an additional argument, a pointer to a
+function of prototype
+
+```C
+  uintptr_t deref(uintptr_t address)
+```
+
+that will, in spirit, contain a `return *((uintptr_t*)address);`.
+
+This argument can be used to work on remote address spaces instead of local
+address spaces, eg. to work with `libunwind`.
+
+To enable the presence of this argument, you must pass the option
+`--enable-deref-arg`

src/main.cpp

@@ -56,6 +56,10 @@ MainOptions options_parse(int argc, char** argv) {
                 settings::pc_list = argv[option_pos];
             }
         }
+
+        else if(option == "--enable-deref-arg") {
+            settings::enable_deref_arg = true;
+        }
     }
 
     if(!seen_switch_gen_policy) {

src/settings.cpp

@@ -3,4 +3,5 @@
 namespace settings {
     SwitchGenerationPolicy switch_generation_policy = SGP_SwitchPerFunc;
     std::string pc_list = "";
+    bool enable_deref_arg = false;
 }

src/settings.hpp

@@ -15,4 +15,5 @@ namespace settings {
 
     extern SwitchGenerationPolicy switch_generation_policy;
     extern std::string pc_list;
+    extern bool enable_deref_arg;
 }