[ros-dev] ReactOS versioning
timo.kreuzer at web.de
Sat Mar 7 12:44:24 UTC 2015
Here's the promised suggestion regarding how we handle versioning
problems in reactos. It has some relationship to the tree restructure.
Since some time we now run into issues with our targeted Windows
version. This is both wine dlls, as well as applications that refuse to
run due to reactos being limited to Windows server 2003 SP2.
I think many of us, me included, see more in ReactOS than an academic
research project, or a nice way for 3rd party companies to cheaply get
insight into how the Windows kernel works. So we are interested in
making it an actually useful operating system. To achieve this goal, it
is obviously important to make it run modern Windows applications.
The current approach of pure Windows 2003 Server SP2 compatibility on
user mode side is a dead end. Our target OS version is starting to
become a fossil. With time more and more applications will simply refuse
to work on it. Even wine DLLs start to require Vista APIs and their
number will most likely increase.
So what can we do? It is obvious, that we cannot instantaniously switch
all user mode to Windows 7/8/10 compatibility, due to the amount of
required work, especially regarding missing kernel features.
The wine approach is just adding whatever is needed, creating a Windows
version chimera. It has already been discussed here and shown to be a
problem, since it can easily fool applications into believing they run
on Vista or Windows 7, making them demand all the modern features, which
we cannot provide, thus failing to run, while they would run flawlessly,
when being provided a pure Windows 2003 environment, restricting itself
to this functionality. So this is also not a very good way, either.
So the conclusion is, that we need a mechanism, that allows us to
control this, providing individual applications with what they require,
while leaving others in a more restricted environment. And at the same
time allowing our internal/wine DLLs to make use of higher version
1. We need a method to specify which application should be run in which
environment. We should probably use the same mechanism that is used on
Windows. Compatibility information is stored in a registry key
HKCU\Software\Microsodt\Windows NT\CurrentVersion\AppCompatFlags\... The
trick is to make this easy / transparent for the user. A right-click ->
properties -> compatibility approach should for now probably be the
easiest thing, even if it requires the user to actively make this
setting. A larger app compatibility database would be nice, but it would
be difficult to figure out what application is being run. And it's also
a problem to maintain such a list. Potential solutions: detect failures
to load due to missing imports and app crashes and invoke a
"compatibility assistent" in that case. Detect first-run of a new
application and try to identify it, either based on a hash or based on
PE version information.
2. We need a way to provide the application transparently with the
environment we want to give it. In terms of DLL exports this could be
done on the loader side, making it chose the right DLL, potentially
adding a suffix to the DLL name or selecting a different folder other
than system32. While this will most likely work good in the majority of
cases, it is not 100% transparent. Therefore a mechanism in the kernel,
using file system redirection, like it already exists on 64 bit Windows
for WoW64, seems to be a more promising approach. The file system
redirection would redirect system32 into merged folders, containing the
version specific DLLs, while everything that is not existing in this
folder will be taken from the original system32. Potential naming
scheme: system32.601 system32.602, etc.
3. We need a method to create and maintain the required DLLs for
different OS versions. Preferably avoiding bloat by sharing common code
in common "parent" DLLs. But also allowing to still plug the DLLs into
the related Windows version for testing. This can be tricky. I suggest a
DLL import forwarding scheme. This is both to avoid bloat, i.e. avoid to
compile and deploy all full blown DLLs for all OS versions, as well as
creating a better organized system. So each DLL, lets say ADVAPI32, as
located in different version specific system32 folders, would
mainly/only consist of forwarders to a "parent" DLL. On Windows we can
see this being developed similarly, using "api sets" and redirections
made by the loader. Cloning this mechanism 1:1 might not be the right
thing though, since it does not address all our requirements. So instead
I suggest proving our own custom "parent DLLs". While these could be
organized the same way as on Windows 2003, this is probably not optimal.
Instead I suggest merging stuff together into 1 or few DLLs (similar to
how stuff was combined in kernelbase.dll)
This might looks like this (note, that the names are just quickly made
up names, I don't claim that they are good)
- user32/gdi32 -> ros-win32-core.dll
- kernel32/advapi32 -> ros-kernel-base.dll
- msvcr* -> ros-crt.dll
- ntdll -> ros-ntdll (the kernel would need to load this one)
This also allows our DLLs to make use of higher version APIs, by linking
them to the parent DLL.
Now this obviously introduces a problem when trying to run individual
DLLs within a Windows system. To still be able to do this for testing
purposes, we need to make sure that they still run there.
First, if they import from a custom ros-* DLL, it won't run on Windows
without that DLL. So we need the possibility to either statically link
these functions, or compile a "helper DLL", that contains these
functions, so our DLLs can be run on Windows.
If we used a common parent DLL, this also creates the problem of DLL
initialization. e.g. the DLL parent for kernel32 and advapi32 would do
the initialization for both of these, so this would not be suitable to
use when replacing only one of the DLLs. Instead DLL initialization
could be done by calling a specific initialization function in the
parent DLL from the child DLL. So kernel32!DllMain would call
ros-kernel-base!DllMain_kernel32, passing the original parameters as
well as a version number, that the parent DLL can use to do version
specific initialization. This way The parent DLL would only do the
kernel32 initialization, when the related kernel32 child DLL was used,
the advapi32 initialization would not be done.
There is still a problem: relocation. So we would need to make sure we
chose base addresses that still allow us to plug the stuff into Windows
without causing everything to relocate, which often simply doesn't work.
As an alternative, we should provide a compile time switch to compile
specific DLLs in a self-contained way.
In terms of structure we could use the MS api-sets as a base for static
libraries. Then we can link these either into the parent DLLs like
ros-kernel-base.dll or - when a compile switch is given - to link
together fully self contained DLLs.
I am really not interested in answers like "This is not what WIndows
does!", "THIS CANNOT WORK!!!", "You are a ***** even suggesting this",
"What if application x parses the import table and disassembles the DLL
to hook into internal functions, ...":
I am only interested in *constructive* comments.
Everything else: -> /dev/null
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