Changeset 19433

Timestamp:

Jul 18, 2002, 1:52:56 PM (23 years ago)

Author:

achimha

Message:

documented PE DLL memory layout

File:

• tags/trunk/src/kernel32/windllpeldr.cpp (modified) (2 diffs)

tags/trunk/src/kernel32/windllpeldr.cpp ¶

@@ -1 @@
-/* $Id: windllpeldr.cpp,v 1.9 2001-06-15 09:42:48 bird Exp $ */
+/* $Id: windllpeldr.cpp,v 1.10 2002-07-18 11:52:56 achimha Exp $ */
 
 /*
@@ -33 +33 @@
 #define DBG_LOCALLOG    DBG_windllpeldr
 #include "dbglocal.h"
+
+//******************************************************************************
+// Design information on PE DLL memory layout - AH 2002-07-18
+//
+// We are currently using (high) private memory for all PE objects, including the
+// read/execute code segments, constant data segments and global data segments.
+// Global data segments might not be implemented correctly at all as we've never
+// encountered any applictions making use of them. Therefore we are actually
+// wasting memory when running multiple processes using the same PE DLLs.
+//
+// There are several reasons for this design decisions. Both OS/2 (LX) and
+// Windows NT put all DLL segments into the shared arena. What they do for
+// instance data is map it for each process to read-only pages initially. When
+// a write attempt is made by a process, an exception will be triggered. This
+// makes the operating system to copy the data to a new page that is read/write
+// and change the page table of the process to map the linear process in the
+// shared arena to private memory (this is called "copy-on-write").
+// Even though an application is not guaranteed any virtual address for instance
+// data segments, they always end up in the shared region and the virtual addreses
+// are contiguous. An application could therefore make nasty assumptions.
+// Unfortunately, it is not possible for us from ring 3 to replicate the behavior
+// for our PE loader. While we can make the page read only and catch the
+// exception, we have no method to remap the pages to private memory.
+//
+// One solution would be to create another reagion with the private region,
+// i.e. define some address space range as reserved in Odin (configurable to
+// workaround issues with certain PE images requiring those addresses). We
+// could then load the instance data segments of PE DLLs into this private
+// memory arena and still guarantee identical virtual addresses for each
+// process.
+//
+// While the above method should work fine (assuming an application does not
+// make any nasty assumptions), there is one major problem. If we enable the
+// PE on-demand loader (i.e. the mmap loads each page from the PE file when
+// it is accesses for the first time - very much like NT), then we would have
+// nasty concurrency issues. A process could access a page for the first time
+// and the exception is triggered. We commit the page read the data in using
+// a call to DosRead. If the very same page is accessed from a different
+// process after we have committed it but before we have finished the DosRead,
+// we would run into problems. Unfortunately, there does not seem to be any
+// solution for this.
+//
+// The bottomline is that we put everything into private memory and accept the
+// drawback of wasting memory.
+//******************************************************************************