ptrace: Add PT_PEEK

PT_PEEK reads a single word from the tracee's address space and returns
it to the tracer.

Applications/Debugger/main.cpp

@@ -108,9 +108,17 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    printf("hit breakpoint\n");
+    PtraceRegisters regs;
+    if (ptrace(PT_GETREGS, g_pid, &regs, 0) == -1) {
+        perror("getregs");
+        return 1;
+    }
 
-    sleep(1);
+    printf("hit breakpoint\n");
+    printf("eip:0x%x\n", regs.eip);
+
+    uint32_t data = ptrace(PT_PEEK, g_pid, (void*)regs.eip, 0);
+    printf("data: 0x%x\n", data);
 
     if (ptrace(PT_CONTINUE, g_pid, 0, 0) == -1) {
         perror("continue");

Kernel/Process.cpp

@@ -69,6 +69,7 @@
 #include <Kernel/Time/TimeManagement.h>
 #include <Kernel/VM/PageDirectory.h>
 #include <Kernel/VM/PrivateInodeVMObject.h>
+#include <Kernel/VM/ProcessPagingScope.h>
 #include <Kernel/VM/PurgeableVMObject.h>
 #include <Kernel/VM/SharedInodeVMObject.h>
 #include <LibBareMetal/IO.h>
@@ -4973,6 +4974,21 @@ int Process::sys$ptrace(const Syscall::SC_ptrace_params* user_params)
         }
         break;
     }
+    case PT_PEEK: {
+        uint32_t* addr = reinterpret_cast<uint32_t*>(params.addr);
+        if (!MM.validate_user_read(peer->process(), VirtualAddress(addr), sizeof(uint32_t))) {
+            return -EFAULT;
+        }
+
+        uint32_t result;
+
+        SmapDisabler dis;
+        ProcessPagingScope scope(peer->process());
+        result = *addr;
+
+        return result;
+        break;
+    }
 
     default:
         return -EINVAL;

Kernel/UnixTypes.h

@@ -554,3 +554,4 @@ struct rtentry {
 #define PT_SYSCALL 4
 #define PT_GETREGS 5
 #define PT_DETACH 6
+#define PT_PEEK 7

Libraries/LibC/sys/ptrace.h

@@ -36,6 +36,7 @@ __BEGIN_DECLS
 #define PT_SYSCALL 4
 #define PT_GETREGS 5
 #define PT_DETACH 6
+#define PT_PEEK 7
 
 int ptrace(int request, pid_t pid, void* addr, int data);