Kernel: Dump backtrace to debugger for DefaultSignalAction::DumpCore.

This makes assertion failures generate backtraces again. Sorry to everyone
who suffered from the lack of backtraces lately. :^)

We share code with the /proc/PID/stack implementation. You can now get the
current backtrace for a Thread via Thread::backtrace(), and all the traces
for a Process via Process::backtrace().

Kernel/FileSystem/ProcFS.cpp

@@ -314,37 +314,7 @@ ByteBuffer procfs$pid_stack(InodeIdentifier identifier)
     if (!handle)
         return {};
     auto& process = handle->process();
-    ProcessPagingScope paging_scope(process);
-    struct RecognizedSymbol {
-        u32 address;
-        const KSym* ksym;
-    };
-    StringBuilder builder;
-    process.for_each_thread([&](Thread& thread) {
-        builder.appendf("Thread %d:\n", thread.tid());
-        Vector<RecognizedSymbol, 64> recognized_symbols;
-        recognized_symbols.append({ thread.tss().eip, ksymbolicate(thread.tss().eip) });
-        for (u32* stack_ptr = (u32*)thread.frame_ptr(); process.validate_read_from_kernel(VirtualAddress((u32)stack_ptr)); stack_ptr = (u32*)*stack_ptr) {
-            u32 retaddr = stack_ptr[1];
-            recognized_symbols.append({ retaddr, ksymbolicate(retaddr) });
-        }
-
-        for (auto& symbol : recognized_symbols) {
-            if (!symbol.address)
-                break;
-            if (!symbol.ksym) {
-                builder.appendf("%p\n", symbol.address);
-                continue;
-            }
-            unsigned offset = symbol.address - symbol.ksym->address;
-            if (symbol.ksym->address == ksym_highest_address && offset > 4096)
-                builder.appendf("%p\n", symbol.address);
-            else
-                builder.appendf("%p  %s +%u\n", symbol.address, symbol.ksym->name, offset);
-        }
-        return IterationDecision::Continue;
-    });
-    return builder.to_byte_buffer();
+    return process.backtrace(*handle).to_byte_buffer();
 }
 
 ByteBuffer procfs$pid_regs(InodeIdentifier identifier)

Kernel/Process.cpp

@@ -1279,6 +1279,7 @@ int Process::sys$kill(pid_t pid, int signal)
         ASSERT(pid != -1);
     }
     if (pid == m_pid) {
+        // FIXME: If we ignore this signal anyway, we don't need to block here, right?
         current->send_signal(signal, this);
         (void)current->block<Thread::SemiPermanentBlocker>(Thread::SemiPermanentBlocker::Reason::Signal);
         return 0;
@@ -2752,3 +2753,14 @@ int Process::sys$dbgputstr(const u8* characters, int length)
         IO::out8(0xe9, characters[i]);
     return 0;
 }
+
+String Process::backtrace(ProcessInspectionHandle& handle) const
+{
+    StringBuilder builder;
+    for_each_thread([&](Thread& thread) {
+        builder.appendf("Thread %d:\n", thread.tid());
+        builder.append(thread.backtrace(handle));
+        return IterationDecision::Continue;
+    });
+    return builder.to_string();
+}

Kernel/Process.h

@@ -56,6 +56,8 @@ public:
         Ring3 = 3,
     };
 
+    String backtrace(ProcessInspectionHandle&) const;
+
     bool is_dead() const { return m_dead; }
 
     Thread::State state() const { return main_thread().state(); }

Kernel/Thread.cpp

@@ -1,3 +1,4 @@
+#include <AK/StringBuilder.h>
 #include <Kernel/FileSystem/FileDescription.h>
 #include <Kernel/Process.h>
 #include <Kernel/Scheduler.h>
@@ -319,7 +320,12 @@ ShouldUnblockThread Thread::dispatch_signal(u8 signal)
         case DefaultSignalAction::Stop:
             set_state(Stopped);
             return ShouldUnblockThread::No;
-        case DefaultSignalAction::DumpCore:
+        case DefaultSignalAction::DumpCore: {
+            ProcessInspectionHandle handle(process());
+            dbg() << "Dumping \"Core\" for " << process();
+            dbg() << process().backtrace(handle);
+        }
+            [[fallthrough]];
         case DefaultSignalAction::Terminate:
             m_process.terminate_due_to_signal(signal);
             return ShouldUnblockThread::No;
@@ -543,3 +549,35 @@ void Thread::set_state(State new_state)
         Scheduler::update_state_for_thread(*this);
     }
 }
+
+String Thread::backtrace(ProcessInspectionHandle&) const
+{
+    auto& process = const_cast<Process&>(this->process());
+    ProcessPagingScope paging_scope(process);
+    struct RecognizedSymbol {
+        u32 address;
+        const KSym* ksym;
+    };
+    StringBuilder builder;
+    Vector<RecognizedSymbol, 64> recognized_symbols;
+    recognized_symbols.append({ tss().eip, ksymbolicate(tss().eip) });
+    for (u32* stack_ptr = (u32*)frame_ptr(); process.validate_read_from_kernel(VirtualAddress((u32)stack_ptr)); stack_ptr = (u32*)*stack_ptr) {
+        u32 retaddr = stack_ptr[1];
+        recognized_symbols.append({ retaddr, ksymbolicate(retaddr) });
+    }
+
+    for (auto& symbol : recognized_symbols) {
+        if (!symbol.address)
+            break;
+        if (!symbol.ksym) {
+            builder.appendf("%p\n", symbol.address);
+            continue;
+        }
+        unsigned offset = symbol.address - symbol.ksym->address;
+        if (symbol.ksym->address == ksym_highest_address && offset > 4096)
+            builder.appendf("%p\n", symbol.address);
+        else
+            builder.appendf("%p  %s +%u\n", symbol.address, symbol.ksym->name, offset);
+    }
+    return builder.to_string();
+}

Kernel/Thread.h

@@ -16,6 +16,7 @@
 class Alarm;
 class FileDescription;
 class Process;
+class ProcessInspectionHandle;
 class Region;
 
 enum class ShouldUnblockThread {
@@ -49,6 +50,8 @@ public:
     Process& process() { return m_process; }
     const Process& process() const { return m_process; }
 
+    String backtrace(ProcessInspectionHandle&) const;
+
     void finalize();
 
     enum State : u8 {
@@ -205,6 +208,7 @@ public:
 
     u16 selector() const { return m_far_ptr.selector; }
     TSS32& tss() { return m_tss; }
+    const TSS32& tss() const { return m_tss; }
     State state() const { return m_state; }
     const char* state_string() const;
     u32 ticks() const { return m_ticks; }
@@ -371,3 +375,7 @@ inline IterationDecision Thread::for_each_in_state(State state, Callback callbac
     return Scheduler::for_each_nonrunnable(new_callback);
 }
 
+inline const LogStream& operator<<(const LogStream& stream, const Thread& value)
+{
+    return stream << "Thread{" << &value << "}(" << value.pid() << ":" << value.tid() << ")";
+}