Resolve container native library lookup

- Retry failed libdwfl module opens through /proc/<pid>/root for container-private absolute paths.
- Store target PID in DWFL module userdata so find_elf can build the process-root path.
- Preserve build-id and normal host path lookup before falling back to the target process root.

Signed-off-by: Pramod Kumbhar <prkumbhar@nvidia.com>

Author: pramodk

src/pystack/_pystack/elf_common.cpp

@@ -14,6 +14,45 @@ namespace pystack {
 
 using file_unique_ptr = std::unique_ptr<FILE, std::function<int(FILE*)>>;
 
+static int
+set_module_process_pid_userdata(
+        Dwfl_Module* mod __attribute__((unused)),
+        void** userdata,
+        const char* name __attribute__((unused)),
+        Dwarf_Addr start __attribute__((unused)),
+        void* arg)
+{
+    *userdata = arg;
+    return DWARF_CB_OK;
+}
+
+// Retry an absolute module path relative to /proc/<pid>/root for container targets.
+static int
+find_elf_through_proc_pid_root(void** userdata, const char* modname, char** file_name)
+{
+    if (userdata == nullptr || *userdata == nullptr || modname == nullptr || modname[0] != '/') {
+        return -1;
+    }
+
+    const auto pid = *static_cast<const int*>(*userdata);
+    const std::string rooted_path = "/proc/" + std::to_string(pid) + "/root" + modname;
+    int fd = open(rooted_path.c_str(), O_RDONLY);
+    if (fd < 0) {
+        return -1;
+    }
+    if (file_name == nullptr) {
+        return fd;
+    }
+
+    *file_name = strdup(rooted_path.c_str());
+    if (*file_name == nullptr) {
+        close(fd);
+        return -1;
+    }
+
+    return fd;
+}
+
 int
 pystack_find_elf(
         Dwfl_Module* mod,
@@ -31,10 +70,14 @@ pystack_find_elf(
         return ret;
     }
     ret = dwfl_linux_proc_find_elf(mod, userdata, modname, base, file_name, elfp);
-    if (file_name == nullptr) {
+    if (ret < 0) {
+        ret = find_elf_through_proc_pid_root(userdata, modname, file_name);
+    }
+    if (ret < 0) {
         LOG(DEBUG) << "Could not locate debug info for " << the_modname;
     } else {
-        LOG(DEBUG) << "Located debug info for " << the_modname << " by path in " << *file_name;
+        const char* the_filename = (file_name == nullptr || *file_name == nullptr) ? "???" : *file_name;
+        LOG(DEBUG) << "Located debug info for " << the_modname << " by path in " << the_filename;
     }
     return ret;
 }
@@ -285,6 +328,11 @@ ProcessAnalyzer::ProcessAnalyzer(pid_t pid)
         throw ElfAnalyzerError("Failed to analyze DWARF information for the remote process");
     }
 
+    // The find_elf callback needs the PID to retry module paths through /proc/<pid>/root.
+    if (dwfl_getmodules(d_dwfl.get(), set_module_process_pid_userdata, &d_pid, 0) == -1) {
+        throw ElfAnalyzerError("Failed to associate DWARF modules with the remote process");
+    }
+
     if (dwfl_linux_proc_attach(d_dwfl.get(), pid, true) != 0) {
         throw ElfAnalyzerError("Could not attach the DWARF process analyzer");
     }