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Re: NtCreateProcess redux

On 25/04/2011 6:05 PM, Daniel Colascione wrote:
On 4/25/2011 12:33 PM, Ryan Johnson wrote:
I know that folks have looked before into NtCreateProcess as a way of
doing a real fork() in cygwin, but it's very unclear from the various
list archives why it's still a bad idea today, other than its being

It's a bad idea because it doesn't work. You can certainly create a forked child with NtCreateProcess, but without being able to connect it to csrss and the rest of the win32 subsystem, this new process is useless. NtCreateProcess-fork works for Interix because it has its own NT subsystem, but Cygwin has to live within win32, and I don't think creating a new subsystem is feasible for anyone without access to the NT source.
That would definitely go in the Bad Things category... I didn't realize you had to manually deal with subsystems. Looking at the NT internals book, I see now that their fork example is a thoroughly scary hack (casting arbitrary hex numbers to function pointers and trying to call them... shudder).

As far as the address space issue goes: when NT creates a new process, the loader, in ntdll, gains control before the entry point is ever called, and this loader is what's responsible for the initial VM layout. Because ntdll is a "known dll", you can't replace it with a friendlier implementation. After the loader completes its work, the kernel does some black magic and resets the initial thread's stack so that it begins executing in the ntdll thread startup routine, so you never actually _see_ the loader executing.
Yes, I've noticed that. Windbg can actually trace the load process, thoughI don't have any debug symbols to know what's going on. There are even several nameless dlls which get loaded and unloaded before WOW64 hands over control.

The only thing that might have a chance of working is to unload everything except user32, kernel32, and a few other components, then start fresh with a more constrained module loading strategy.
Unfortunately, AFAICT it's impossible to unload statically-linked DLLs: you can call FreeModule() on their handle, and it returns success, but the image remains loaded in memory.

However, the main crazy idea I've been toying with uses the same basic premise: make the .exe a minimal stub (maybe not even linking cygwin1.dll directly) which dynamically loads a .dll containing all the application's code and link-time dependencies. Doing so would minimize the number of address space changes the NT loader could impose during process startup. Most fork failures I see right now are due to statically-linked dlls moving around, which we can't really do anything to avoid or fix, other than calling rebaseall with crossed fingers. At least with dynamically-loaded dlls we have a semblance of control.

Not necessarily what you want to do all the time, but for these problematic dll-heavy apps which also like to fork... I'll send a separate email soon with more details.

[1] If process A has section S, the contents of which we'd like to duplicate in child-process B as S', and B inherits a handle to S, it's slower to remap S in B and memcpy it to S' than it is to just initialize S' from A's address space with NtCopyVirtualMemory. But that's the single-threaded case. It turns out that if we have the child map S somewhere and have one thread touch S[0], S'[0], S[4096], S'[4096], etc. while another thread does a mempcy from S to S', we handily beat the NtCopyVirtualMemory approach.
A trade-off between the cost of traps to fault in pages vs. the cost of syscalls to do inter-process memory transfers? It seems like the latter would win if you copied enough pages at a time (the actual memcpy cost should be about the same either way). What happens if both threads just call NtCopyVirtualMemory in parallel?


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