Make sure we can run with no locally available objects.

Add an env var that lets us tell pstack to never open libraries locally.
this makes it easier to test debuginfod, and check for correctness for
some upstream apps.

Author: peadar

context.cc

@@ -129,7 +129,6 @@ Context::flush(std::shared_ptr<Elf::Object> o)
     dwarfCache.erase(o);
 }
 
-
 // pretty-printer for key types.
 template <typename T> struct ContainerKeyDescr{};
 std::ostream &operator << (std::ostream &os, const ContainerKeyDescr<std::filesystem::path> &) { return os << "path"; }
@@ -201,6 +200,8 @@ Context::getImageInPath(const std::vector<std::filesystem::path> &paths, NameMap
  */
 std::shared_ptr<Elf::Object>
 Context::getImage(const std::filesystem::path &name) {
+    if (options.noLocalFiles)
+        return nullptr;
     return getImageInPath(exePrefixes, imageByName, name, false, false);
 }
 

libpstack/context.h

@@ -21,6 +21,7 @@ struct Options {
     bool noExtDebug = false; // don't look for exernal ELF info, i.e., using debuglink, or buildid.
     bool withDebuginfod = false; // use debuginfod client library.
     bool noBuildIds = false;
+    bool noLocalFiles = false;
     int maxdepth = std::numeric_limits<int>::max();
     int maxframes = 30;
 };

libpstack/proc.h

@@ -241,9 +241,10 @@ struct MappedObject {
 };
 
 class Process : public ps_prochandle {
-    Elf::Addr entry;
-    Elf::Addr dt_debug;
-    Elf::Addr interpBase;
+    Elf::Addr entry{};
+    Elf::Addr dt_debug{};
+    Elf::Addr interpBase{};
+    Elf::Addr execBase{};
     void loadSharedObjects(Elf::Addr);
     Elf::Addr extractDtDebugFromDynamicSegment(const Elf::Phdr &phdr, Elf::Addr loadAddr, const char *);
 public:

process.cc

@@ -172,19 +172,16 @@ Process::load()
     auto auxv = getAUXV();
     if (auxv)
         processAUXV(*auxv);
-
-    // by now, we should know what executable was loaded for the process.
-    if (!execImage)
-        throw (Exception() << "no executable image located for process");
-
     try {
-        Elf::Addr r_debug_addr = findRDebugAddr();
-        bool isStatic = r_debug_addr == 0 || r_debug_addr == Elf::Addr(-1);
+        findRDebugAddr();
+        bool isStatic = dt_debug == 0 || dt_debug == Elf::Addr(-1);
 
-        if (isStatic)
-            addElfObject("", execImage, 0);
-        else
-            loadSharedObjects(r_debug_addr);
+        if (isStatic) {
+            if (execImage)
+                addElfObject("", execImage, 0);
+        } else {
+            loadSharedObjects(dt_debug);
+        }
     }
     catch (const Exception &) {
         // We were unable to read the link map.
@@ -297,13 +294,14 @@ Process::processAUXV(const Reader &auxio)
 
                 break;
             }
+
 #endif
             case AT_PHDR:
-               phOff = hdr;
-               break;
+                phOff = hdr;
+                break;
             case AT_PHNUM:
-               phNum = hdr;
-               break;
+                phNum = hdr;
+                break;
             default:
                break;
         }
@@ -820,13 +818,17 @@ Process::loadSharedObjects(Elf::Addr rdebugAddr)
         // If we see the executable, just add it in and avoid going through the path
         // replacement work
         if (mapAddr == Elf::Addr(rDebug.r_map)) {
-            auto loadAddr = entry - execImage->getHeader().e_entry;
-            if (loadAddr != map.l_addr) {
-                *context.debug << "calculated load address for executable from process entrypoint ("
-                << std::hex << loadAddr << ") does not match link map (" << map.l_addr
-                << "). Trusting link-map\n" << std::dec;
+            if (execImage) {
+                if (execBase == 0) {
+                    execBase = entry - execImage->getHeader().e_entry;
+                }
+                if (execBase != map.l_addr) {
+                    *context.debug << "calculated load address for executable from process entrypoint ("
+                    << std::hex << execBase << ") does not match link map (" << map.l_addr
+                    << "). Trusting link-map\n" << std::dec;
+                }
+                addElfObject("(exe)", execImage, map.l_addr);
             }
-            addElfObject("(exe)", execImage, map.l_addr);
             continue;
         }
         // If we've loaded the VDSO, and we see it in the link map, just skip it.
@@ -869,7 +871,7 @@ Process::findRDebugAddr()
      * the difference is the load address.
      */
 
-    if (dt_debug == 0) {
+    if (dt_debug == 0 && execImage) {
        // Iterate over the PT_DYNAMIC segment of the loaded executable. (We
        // should not get here, as we should have found the program headers in
        // the AT_PHDR auxv entry, and done this already
@@ -884,7 +886,7 @@ Process::findRDebugAddr()
      * library, which doesn't have an _r_debug symbol. Use the address of
      * _r_debug in the interpreter
      */
-    if (dt_debug == 0 && interpBase && execImage->getInterpreter() != "") {
+    if (dt_debug == 0 && execImage && interpBase && execImage->getInterpreter() != "") {
         try {
             addElfObject(execImage->getInterpreter(), nullptr, interpBase);
             dt_debug = resolveSymbol("_r_debug", false,
@@ -909,7 +911,7 @@ Process::findSegment(Elf::Addr addr)
     if (it != objects.begin()) {
        --it;
        auto obj = it->second.object(context);
-       if (it->first + obj->endVA() >= addr) {
+       if (obj && it->first + obj->endVA() >= addr) {
            auto segment = obj->getSegmentForAddress(addr - it->first);
            if (segment)
                return std::make_tuple(it->first, obj, segment);
@@ -926,6 +928,8 @@ Process::resolveSymbolDetail(const char *name, bool includeDebug,
         if (!match(loaded.second.name()))
            continue;
         auto obj = loaded.second.object(context);
+        if (!obj)
+            continue;
         auto [sym,idx] = obj->findDynamicSymbol(name);
         if (sym.st_shndx != SHN_UNDEF)
            return std::make_tuple(obj, loaded.first, sym);

pstack.cc

@@ -376,6 +376,9 @@ emain(int argc, char **argv, Context &context)
 #ifdef DEBUGINFOD
     .add("debuginfod", 'R', "use debuginfod client", Flags::setf( context.options.withDebuginfod ) )
 #endif
+    .add("no-local-files", Flags::LONGONLY,
+          "don't assume local files match the process's view, and don't open them",
+          Flags::setf( context.options.noLocalFiles ) )
 
     .parse(argc, argv);