i#4318 xarch memtrace: Make alt lib dir take precedence (#4360)

Moves the -alt_module_dir for finding binaries during trace
post-processing to take precedence over the recorded path, to allow
avoiding problems when an identical-seeming path exists on the
processing machine (e.g., system libraries) on UNIX where we have no
checksum or other library distinguisher.

Tested manually on a case where a system library in the same path as
on the recorded machine caused failure.

Issue: #4318

clients/drcachesim/common/options.cpp

@@ -98,7 +98,7 @@ droption_t<std::string> op_alt_module_dir(
     DROPTION_SCOPE_FRONTEND, "alt_module_dir", "", "Alternate module search directory",
     "Specifies a directory containing libraries referenced in -module_file for "
     "analysis tools, or in the raw modules file for post-prcoessing of offline "
-    "raw trace files.");
+    "raw trace files.  This directory takes precedence over the recorded path.");
 
 droption_t<std::string> op_funclist_file(
     DROPTION_SCOPE_ALL, "funclist_file", "",

clients/drcachesim/tools/opcode_mix_launcher.cpp

@@ -64,7 +64,8 @@ static droption_t<std::string> op_module_file(
 
 static droption_t<std::string> op_alt_module_dir(
     DROPTION_SCOPE_FRONTEND, "alt_module_dir", "", "Alternate module search directory",
-    "Specifies a directory containing libraries referenced in -module_file.");
+    "Specifies a directory containing libraries referenced in -module_file.  "
+    "This directory takes precedence over the recorded path.");
 
 droption_t<unsigned int> op_verbose(DROPTION_SCOPE_ALL, "verbose", 0, 0, 64,
                                     "Verbosity level",

clients/drcachesim/tracer/raw2trace.cpp

@@ -311,6 +311,14 @@ raw2trace_t::read_and_map_modules()
     return module_mapper_->get_last_error();
 }
 
+// Maps each module into the address space.
+// There are several types of mapping entries in the module list:
+// 1) Raw bits directly stored.  It is simply pointed at.
+// 2) Extra segments for a module.  A single mapping is used for all
+//    segments, so extras are ignored.
+// 3) A main segment.  The module's file is located by first looking in
+//    the alt_module_dir_; if not found, the path present during tracing
+//    is searched.
 void
 module_mapper_t::read_and_map_modules()
 {
@@ -345,19 +353,29 @@ module_mapper_t::read_and_map_modules()
                                        // 0 size indicates this is a secondary segment.
                                        modvec_[info.containing_index].map_base, 0));
         } else {
-            size_t map_size;
-            byte *base_pc =
-                dr_map_executable_file(info.path, DR_MAPEXE_SKIP_WRITABLE, &map_size);
-            if (base_pc == NULL && !alt_module_dir_.empty()) {
+            size_t map_size = 0;
+            byte *base_pc = NULL;
+            if (!alt_module_dir_.empty()) {
+                // First try the specified module dir.  It takes precedence to allow
+                // overriding the recorded path even when an identical-seeming path
+                // exists on the processing machine (e.g., system libraries).
+                // XXX: We should add a checksum on UNIX to match Windows and have
+                // a sanity check on the library version.
                 std::string basename(info.path);
                 size_t sep_index = basename.find_last_of(DIRSEP ALT_DIRSEP);
                 if (sep_index != std::string::npos)
                     basename = std::string(basename, sep_index + 1, std::string::npos);
                 std::string new_path = alt_module_dir_ + DIRSEP + basename;
-                VPRINT(1, "Failed to find %s; trying %s\n", info.path, new_path.c_str());
+                VPRINT(2, "Trying to map %s\n", new_path.c_str());
                 base_pc = dr_map_executable_file(new_path.c_str(),
                                                  DR_MAPEXE_SKIP_WRITABLE, &map_size);
             }
+            if (base_pc == NULL) {
+                // Try the recorded path.
+                VPRINT(2, "Trying to map %s\n", info.path);
+                base_pc =
+                    dr_map_executable_file(info.path, DR_MAPEXE_SKIP_WRITABLE, &map_size);
+            }
             if (base_pc == NULL) {
                 // We expect to fail to map dynamorio.dll for x64 Windows as it
                 // is built /fixed.  (We could try to have the map succeed w/o relocs,

clients/drcachesim/tracer/raw2trace_launcher.cpp

@@ -57,7 +57,7 @@ static droption_t<std::string>
 static droption_t<std::string> op_alt_module_dir(
     DROPTION_SCOPE_FRONTEND, "alt_module_dir", "", "Alternate module search directory",
     "Specifies a directory to look for binaries needed to post-process "
-    "the trace that are not found in the path recorded during tracing.");
+    "the trace.  This directory takes precedence over the recorded path.");
 
 static droption_t<unsigned int> op_verbose(DROPTION_SCOPE_FRONTEND, "verbose", 0,
                                            "Verbosity level for diagnostic output",