The Windows OS tries to conserve physical memory usage by allowing DLLs loaded in their base (desired) address in different processes to use the same physical pages with copy-on-write semantics; this article, for example, describes the mechanism.

Now, the Windows Virtual Memory API allows you to change the protection mode of memory pages via the VirtualProtect function. You'll notice that two of the available protection modes include copy-on-write semantics.

Now that we have the prelude out of the way, the question is: if you take a copy-on-write memory page from a just-loaded DLL and change its protection mode to, say, read-write, how does it affect the other processes? Before the change all the processes use the same physical memory page, so does that single page suddenly lose its copy-on-write voodoo and becomes a weird kind of shared memory?

For sanity's sake I have to assume that if you change the protection mode of a copy-on-write page to read-write (or any non-COW protection mode), the kernel will spawn your own private page with the identical content and the other processes will live happily with their original COW page. The trouble is, I couldn't find any reference to support this, so if anyone could point me to anything that says explicitly that this is indeed the case I'd appreciate that.

EDIT: An even more interesting question would be what happens if you change the COW page to read-write and then write in it - does the change affect the other processes (since the page is no longer COW, and mapped in other processes too, it sounds like it should, but that would be clearly unsafe)?

  • I can write a general answer, or are you looking for a pointer to a reference on the specific behavior of Windows? – Blrfl Feb 24 '16 at 21:47
  • A general answer might help me get on the trail of the right reference material, so either one is good. – alkove Feb 25 '16 at 5:09
  • I ask because questions looking for off-site resources are off-topic. – Blrfl Feb 25 '16 at 11:13
  • What I need is a Windows-specific answer backed by official sources. – alkove Feb 25 '16 at 11:17

Typically when something is "copy on write"; if a process writes to their instance of it, then that page is converted to "read/write" by creating a copy of the page and replacing the original with the copy for that process. Other processes keep using the original (until/unless they write to the page too and get their own copy).

If a process uses Windows Virtual Memory API to convert their instance of a page from "copy on write" to "read/write"; then (I assume) it'd have exactly the same effect as writing to the page - Windows would create a copy it, and that copy would be "read/write" for that process; and it wouldn't effect the original and wouldn't effect any other process' instance of it.

  • That's what I'm thinking too, but I really want to find some kind of official reference that says this is indeed the case. – alkove Feb 25 '16 at 9:49

There are multiple process involved which access the page having different rights. One process can change its own rights, but cannot change the rights of other processes. And the operating system maintains the page as well.

If you change your access from "copy on write" to "read write" then nobody else must be affected. Other processes may have "read only" or "copy on write" access. They don't expect the page to change spontaneously. If you change your access to read write then the page must be copied, or the OS is broken. Basically the same thing must happen as if you just wrote to the page.


No there is no COW will occur if you just change the page protection, it doesn't matter what you turn it on whether it was read-write or execute, the change only affects the current process not the entire system.

As far as I know there is no official sources by Microsoft that confirms this, but I worked on reverse engineering of the windows operating system for a very long time and I almost RE the entire windows so I can assure you that the memory page will not ever get instanceed and you will not get a private copy of that page unless you write into it. this is the only way to force the OS to give your process a private copy of the specified memory page.

If you succeed to writ to memory page that was shared by the OS and many other processes and this page contains critical data and no COW was implemented by the SO This may cause instability for OS or even can crash.

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