Kernel+LibELF+LibCoreDump+CrashReporter: Use JSON for ProcessInfo

This is in preparation of adding (much) more process information to
coredumps. As we can only have one null-terminated char[] of arbitrary
length in each struct it's now a single JSON blob, which is a great fit:
easily extensible in the future and allows for key/value pairs and even
nested objects, which will be used e.g. for the process environment, for
example.

Kernel/CoreDump.cpp

@@ -219,15 +219,15 @@ ByteBuffer CoreDump::create_notes_process_data() const
 
     ELF::Core::ProcessInfo info {};
     info.header.type = ELF::Core::NotesEntryHeader::Type::ProcessInfo;
-    info.pid = m_process->pid().value();
-    info.termination_signal = m_process->termination_signal();
-
     process_data.append((void*)&info, sizeof(info));
 
-    auto executable_path = String::empty();
+    JsonObject process_obj;
-    if (auto executable = m_process->executable())
+    process_obj.set("pid", m_process->pid().value());
-        executable_path = executable->absolute_path();
+    process_obj.set("termination_signal", m_process->termination_signal());
-    process_data.append(executable_path.characters(), executable_path.length() + 1);
+    process_obj.set("executable_path", m_process->executable() ? m_process->executable()->absolute_path() : String::empty());
+
+    auto json_data = process_obj.to_string();
+    process_data.append(json_data.characters(), json_data.length() + 1);
 
     return process_data;
 }

Userland/Applications/CrashReporter/main.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, Linus Groh <mail@linusgroh.de>
+ * Copyright (c) 2020-2021, Linus Groh <mail@linusgroh.de>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -96,11 +96,10 @@ int main(int argc, char** argv)
             warnln("Could not open coredump '{}'", coredump_path);
             return 1;
         }
-        auto& process_info = coredump->process_info();
         backtrace = build_backtrace(*coredump);
-        executable_path = String(process_info.executable_path);
+        executable_path = coredump->process_executable_path();
-        pid = process_info.pid;
+        pid = coredump->process_pid();
-        termination_signal = process_info.termination_signal;
+        termination_signal = coredump->process_termination_signal();
     }
 
     auto app = GUI::Application::construct(argc, argv);

Userland/Libraries/LibCoreDump/Reader.cpp

@@ -28,6 +28,7 @@
 #include <AK/JsonValue.h>
 #include <LibCoreDump/Backtrace.h>
 #include <LibCoreDump/Reader.h>
+#include <signal_numbers.h>
 #include <string.h>
 #include <sys/stat.h>
 
@@ -88,7 +89,7 @@ void Reader::NotesEntryIterator::next()
     switch (type()) {
     case ELF::Core::NotesEntryHeader::Type::ProcessInfo: {
         const auto* current = reinterpret_cast<const ELF::Core::ProcessInfo*>(m_current);
-        m_current = reinterpret_cast<const ELF::Core::NotesEntry*>(current->executable_path + strlen(current->executable_path) + 1);
+        m_current = reinterpret_cast<const ELF::Core::NotesEntry*>(current->json_data + strlen(current->json_data) + 1);
         break;
     }
     case ELF::Core::NotesEntryHeader::Type::ThreadInfo: {
@@ -127,14 +128,24 @@ Optional<uint32_t> Reader::peek_memory(FlatPtr address) const
     return *(const uint32_t*)(&region_data[offset_in_region]);
 }
 
-const ELF::Core::ProcessInfo& Reader::process_info() const
+const JsonObject Reader::process_info() const
 {
+    const ELF::Core::ProcessInfo* process_info_notes_entry = nullptr;
     for (NotesEntryIterator it((const u8*)m_coredump_image.program_header(m_notes_segment_index).raw_data()); !it.at_end(); it.next()) {
         if (it.type() != ELF::Core::NotesEntryHeader::Type::ProcessInfo)
             continue;
-        return reinterpret_cast<const ELF::Core::ProcessInfo&>(*it.current());
+        process_info_notes_entry = reinterpret_cast<const ELF::Core::ProcessInfo*>(it.current());
+        break;
     }
-    ASSERT_NOT_REACHED();
+    if (!process_info_notes_entry)
+        return {};
+    auto process_info_json_value = JsonValue::from_string(process_info_notes_entry->json_data);
+    if (!process_info_json_value.has_value())
+        return {};
+    if (!process_info_json_value.value().is_object())
+        return {};
+    return process_info_json_value.value().as_object();
+    // FIXME: Maybe just cache this on the Reader instance after first access.
 }
 
 const ELF::Core::MemoryRegionInfo* Reader::region_containing(FlatPtr address) const
@@ -155,6 +166,30 @@ const Backtrace Reader::backtrace() const
     return Backtrace(*this);
 }
 
+int Reader::process_pid() const
+{
+    auto process_info = this->process_info();
+    auto pid = process_info.get("pid");
+    return pid.to_number<int>();
+}
+
+u8 Reader::process_termination_signal() const
+{
+    auto process_info = this->process_info();
+    auto termination_signal = process_info.get("termination_signal");
+    auto signal_number = termination_signal.to_number<int>();
+    if (signal_number <= SIGINVAL || signal_number >= NSIG)
+        return SIGINVAL;
+    return (u8)signal_number;
+}
+
+String Reader::process_executable_path() const
+{
+    auto process_info = this->process_info();
+    auto executable_path = process_info.get("executable_path");
+    return executable_path.as_string_or({});
+}
+
 const HashMap<String, String> Reader::metadata() const
 {
     const ELF::Core::Metadata* metadata_notes_entry = nullptr;

Userland/Libraries/LibCoreDump/Reader.h

@@ -44,8 +44,6 @@ public:
     static OwnPtr<Reader> create(const String&);
     ~Reader();
 
-    const ELF::Core::ProcessInfo& process_info() const;
-
     template<typename Func>
     void for_each_memory_region_info(Func func) const;
 
@@ -66,6 +64,10 @@ public:
     const LibraryData* library_containing(FlatPtr address) const;
 
     const Backtrace backtrace() const;
+
+    int process_pid() const;
+    u8 process_termination_signal() const;
+    String process_executable_path() const;
     const HashMap<String, String> metadata() const;
 
 private:
@@ -86,6 +88,11 @@ private:
         const u8* start { nullptr };
     };
 
+    // Private as we don't need anyone poking around in this JsonObject
+    // manually - we know very well what should be included and expose that
+    // as getters with the appropriate (non-JsonValue) types.
+    const JsonObject process_info() const;
+
     NonnullRefPtr<MappedFile> m_coredump_file;
     ELF::Image m_coredump_image;
     ssize_t m_notes_segment_index { -1 };

Userland/Libraries/LibELF/CoreDump.h

@@ -50,9 +50,14 @@ struct [[gnu::packed]] NotesEntry {
 
 struct [[gnu::packed]] ProcessInfo {
     NotesEntryHeader header;
-    int pid;
+    // Information is stored as JSON blob to allow arbitrary
-    u8 termination_signal;
+    // number and length of strings/objects/arrays.
-    char executable_path[]; // Null terminated
+    //
+    // Keys:
+    // - "pid" (int)
+    // - "termination_signal" (u8)
+    // - "executable_path" (String)
+    char json_data[]; // Null terminated
 };
 
 struct [[gnu::packed]] ThreadInfo {
@@ -81,6 +86,11 @@ struct [[gnu::packed]] MemoryRegionInfo {
 
 struct [[gnu::packed]] Metadata {
     NotesEntryHeader header;
+    // Arbitrary metadata, set via SC_set_coredump_metadata.
+    // Limited to 16 entries and 16 KiB keys/values by the kernel.
+    //
+    // Well-known keys:
+    // - "assertion": Used by LibC's __assertion_failed() to store assertion info
     char json_data[]; // Null terminated
 };