[lldb] [Process/NetBSD] Fix handling piod_len from PT_IO calls

Fix bugs in piod_len return value processing in ReadMemory()
and WriteMemory() methods.  In particular, add support for piod_len == 0
indicating EOF, and fix summing bytes_read/bytes_written when PT_IO does
partial reads/writes.

The EOF condition could happen if LLDB attempts to read past
vm.maxaddress, e.g. as a result of RBP containing large (invalid) value.
Previously, the 0 return caused the function to retry reading via PT_IO
indefinitely, effectively deadlooping lldb-server.

Partial reads probably did not occur in practice, yet they would cause
ReadMemory() to return incorrect bytes_read and/or overwrite previously
read data.

WriteMemory() suffered from analoguous problems.

Differential Revision: https://reviews.llvm.org/D61310

llvm-svn: 359572

lldb/source/Plugins/Process/NetBSD/NativeProcessNetBSD.cpp

@@ -695,10 +695,10 @@ Status NativeProcessNetBSD::ReadMemory(lldb::addr_t addr, void *buf,
     io.piod_addr = dst + bytes_read;
 
     Status error = NativeProcessNetBSD::PtraceWrapper(PT_IO, GetID(), &io);
-    if (error.Fail())
+    if (error.Fail() || io.piod_len == 0)
       return error;
 
-    bytes_read = io.piod_len;
+    bytes_read += io.piod_len;
     io.piod_len = size - bytes_read;
   } while (bytes_read < size);
 
@@ -723,10 +723,10 @@ Status NativeProcessNetBSD::WriteMemory(lldb::addr_t addr, const void *buf,
     io.piod_offs = (void *)(addr + bytes_written);
 
     Status error = NativeProcessNetBSD::PtraceWrapper(PT_IO, GetID(), &io);
-    if (error.Fail())
+    if (error.Fail() || io.piod_len == 0)
       return error;
 
-    bytes_written = io.piod_len;
+    bytes_written += io.piod_len;
     io.piod_len = size - bytes_written;
   } while (bytes_written < size);