ptrace: Report error in PT_PEEK via errno

The syscall wrapper for ptrace needs to return the peeked value when
using  PT_PEEK.
Because of this, the user has to check errno to detect an error in
PT_PEEK.

This commit changes the actual syscall's interface (only for PT_PEEK) to
allow the syscall wrapper to detect an error and change errno.

Kernel/Ptrace.cpp

@@ -92,8 +92,10 @@ KResultOr<u32> handle_syscall(const Kernel::Syscall::SC_ptrace_params& params, P
 
         auto& peer_saved_registers = peer->get_register_dump_from_stack();
         // Verify that the saved registers are in usermode context
-        if ((peer_saved_registers.cs & 0x03) != 3)
+        if ((peer_saved_registers.cs & 0x03) != 3) {
             return -EFAULT;
+        }
+
         {
             SmapDisabler disabler;
             PtraceRegisters* regs = reinterpret_cast<PtraceRegisters*>(params.addr);
@@ -104,12 +106,24 @@ KResultOr<u32> handle_syscall(const Kernel::Syscall::SC_ptrace_params& params, P
     }
 
     case PT_PEEK: {
-        u32* addr = reinterpret_cast<u32*>(params.addr);
+        Kernel::Syscall::SC_ptrace_peek_params peek_params;
-        return peer->process().peek_user_data(addr);
+        if (!caller.validate_read_and_copy_typed(&peek_params, reinterpret_cast<Kernel::Syscall::SC_ptrace_peek_params*>(params.addr)))
+            return -EFAULT;
+        // read validation is done inside 'peek_user_data'
+        auto result = peer->process().peek_user_data(peek_params.address);
+        if (result.is_error())
+            return -EFAULT;
+        peer->process().validate_write(peek_params.out_data, sizeof(u32));
+        {
+            SmapDisabler disabler;
+            *(peek_params.out_data) = result.value();
+        }
+        break;
     }
 
     case PT_POKE: {
         u32* addr = reinterpret_cast<u32*>(params.addr);
+        // write validation is done inside 'poke_user_data'
         return peer->process().poke_user_data(addr, params.data);
     }
 

Kernel/Syscall.h

@@ -432,6 +432,11 @@ struct SC_ptrace_params {
     int data;
 };
 
+struct SC_ptrace_peek_params {
+    u32* address;
+    u32* out_data;
+};
+
 void initialize();
 int sync();
 

Libraries/LibC/sys/ptrace.cpp

@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/LogStream.h>
 #include <Kernel/Syscall.h>
 #include <errno.h>
 #include <sys/ptrace.h>
@@ -32,6 +33,20 @@ extern "C" {
 
 int ptrace(int request, pid_t pid, void* addr, int data)
 {
+
+    // PT_PEEK needs special handling since the syscall wrapper
+    // returns the peeked value as an int, which can be negative because of the cast.
+    // When using PT_PEEK, the user can check if an error occured
+    // by looking at errno rather than the return value.
+
+    u32 out_data;
+    Syscall::SC_ptrace_peek_params peek_params;
+    if (request == PT_PEEK) {
+        peek_params.address = reinterpret_cast<u32*>(addr);
+        peek_params.out_data = &out_data;
+        addr = &peek_params;
+    }
+
     Syscall::SC_ptrace_params params {
         request,
         pid,
@@ -39,6 +54,16 @@ int ptrace(int request, pid_t pid, void* addr, int data)
         data
     };
     int rc = syscall(SC_ptrace, &params);
+
+    if (request == PT_PEEK) {
+        if (rc < 0) {
+            errno = -rc;
+            return -1;
+        }
+        errno = 0;
+        return static_cast<int>(out_data);
+    }
+
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 }