Userland: Add "functrace" utility

functrace traces the function calls a program makes.
It's like strace, but for userspace.

It works by using Debugging functionality to insert breakpoints
at call&ret instructions.

Libraries/LibDebug/DebugSession.h

@@ -0,0 +1,186 @@
+/*
+ * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/Demangle.h>
+#include <AK/HashMap.h>
+#include <AK/MappedFile.h>
+#include <AK/Optional.h>
+#include <AK/OwnPtr.h>
+#include <AK/String.h>
+#include <LibC/sys/arch/i386/regs.h>
+#include <LibELF/Loader.h>
+#include <signal.h>
+#include <stdio.h>
+#include <sys/ptrace.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+class DebugSession {
+public:
+    static OwnPtr<DebugSession> exec_and_attach(const String& command);
+
+    // Has to be public for OwnPtr::make
+    DebugSession(int pid);
+    ~DebugSession();
+
+    int pid() const { return m_debugee_pid; }
+
+    bool poke(u32* address, u32 data);
+    Optional<u32> peek(u32* address) const;
+
+    enum class BreakPointState {
+        Enabled,
+        Disabled,
+    };
+
+    struct BreakPoint {
+        void* address;
+        u32 original_first_word;
+        BreakPointState state;
+    };
+
+    bool insert_breakpoint(void* address);
+    bool disable_breakpoint(const BreakPoint&);
+    bool enable_breakpoint(const BreakPoint&);
+
+    PtraceRegisters get_registers() const;
+    void set_registers(const PtraceRegisters&);
+
+    void continue_debugee();
+    void* single_step();
+
+    template<typename Callback>
+    void run(Callback callback);
+
+    const ELF::Loader& elf() const { return m_elf; }
+    const MappedFile& executable() const { return m_executable; }
+
+    enum DebugDecision {
+        Continue,
+        SingleStep,
+        Detach,
+        Kill,
+    };
+
+    enum DebugBreakReason {
+        Breakpoint,
+        Exited,
+    };
+
+private:
+    // x86 breakpoint instruction "int3"
+    static constexpr u8 BREAKPOINT_INSTRUCTION
+        = 0xcc;
+
+    int m_debugee_pid { -1 };
+    bool m_is_debugee_dead { false };
+
+    MappedFile m_executable;
+    ELF::Loader m_elf;
+
+    HashMap<void*, BreakPoint> m_breakpoints;
+};
+
+template<typename Callback>
+void DebugSession::run(Callback callback)
+{
+
+    enum class State {
+        FreeRun,
+        ConsecutiveBreakpoint,
+        SingleStep,
+    };
+
+    State state { State::FreeRun };
+
+    for (;;) {
+        if (state == State::FreeRun) {
+            continue_debugee();
+
+            int wstatus = 0;
+            if (waitpid(m_debugee_pid, &wstatus, WSTOPPED | WEXITED) != m_debugee_pid) {
+                perror("waitpid");
+                ASSERT_NOT_REACHED();
+            }
+
+            // FIXME: This check actually only checks whether the debugee
+            // stopped because it hit a breakpoint/is in single stepping mode or not
+            if (WSTOPSIG(wstatus) != SIGTRAP) {
+                callback(DebugBreakReason::Exited, Optional<PtraceRegisters>());
+                m_is_debugee_dead = true;
+                break;
+            }
+        }
+
+        auto regs = get_registers();
+        Optional<BreakPoint> current_breakpoint;
+
+        if (state == State::FreeRun) {
+            current_breakpoint = m_breakpoints.get((void*)((u32)regs.eip - 1));
+        } else {
+            current_breakpoint = m_breakpoints.get((void*)regs.eip);
+        }
+
+        if (current_breakpoint.has_value()) {
+            // We want to make the breakpoint transparrent to the user of the debugger
+            regs.eip = reinterpret_cast<u32>(current_breakpoint.value().address);
+            set_registers(regs);
+            disable_breakpoint(current_breakpoint.value());
+        }
+
+        DebugDecision decision = callback(DebugBreakReason::Breakpoint, regs);
+
+        if (decision == DebugDecision::Continue) {
+            state = State::FreeRun;
+        }
+
+        if (current_breakpoint.has_value()) {
+            // Re-enable the breakpoint
+            auto stopped_address = single_step();
+            enable_breakpoint(current_breakpoint.value());
+            // If there is another breakpoint after the current one,
+            // Then we are already on it (because of single_step)
+            auto breakpoint_at_next_instruction = m_breakpoints.get(stopped_address);
+            if (breakpoint_at_next_instruction.has_value()
+                && breakpoint_at_next_instruction.value().state == BreakPointState::Enabled) {
+                state = State::ConsecutiveBreakpoint;
+            }
+        }
+
+        if (decision == DebugDecision::SingleStep) {
+            state = State::SingleStep;
+        } else {
+            // TODO: implement DebugDecision:: Kill, Detach
+            ASSERT(decision == DebugDecision::Continue);
+        }
+
+        if (state == State::SingleStep) {
+            single_step();
+        }
+    }
+}