git branch in cmd.exe prompt

04 Jul 2013

I really wanted the current git branch name in the prompt on Windows. It turns out that getting this was … somewhat non-trivial.

I thought at first that some wrappers around git commands might be sufficient, but in practice that doesn’t work because there’s many ways to change the current branch, and they’re not all going to go through the wrappers. And so it doesn’t work consistently, which makes it almost worse than not having it at all (because you can’t trust it).

Poking around in cmd.exe’s prompt /?, I found the variable $M. This “displays the remote name associated with the current drive letter”. Windbg’ing cmd.exe showed that functionality is implemented by calling WNetGetConnection. This returned string gets embedded into the prompt which is similar to what I want, and since I didn’t even know about $M, I probably wouldn’t miss the original functionality

Helpfully, there’s only one call to WNetGetConnectionW too, so the original functionality doesn’t need to be conditionally maintained. So, I tried IAT patching WNetGetConnectionW to replace it with a hardcoded string as a test. Unfortunately the call to that function is guarded by a call to GetDriveType (to check that the drive is actually a remote drive). GetDriveType is a bit trickier; it’s used for a variety of different reasons in many places, so it can’t be unconditionally replaced without breaking other functionality.

The final solution looks like this (eliding a few complexities for differences between the Windows 7 and Windows 8 binaries):

  1. Loader exe uses CreateRemoteThread() with a ThreadProc of LoadLibrary to inject a DLL into the running cmd.exe.
  2. (Now inside the injected DLL during attach) Push a PROMPT variable into the environment that contains the $M variable.
  3. Overwrite the first instruction of kernel32!GetDriveType with a HLT, and install a vectored exception handler to trap that invalid instruction when it executes.
  4. Almost right away we’ll try to render that prompt which will call GetDriveType in the $M handling code, which will fault. In the fault handler, walk up the stack, and if the code at the callsite matches the disassembly that we’re expecting (a comparison with DRIVE_REMOTE, a particular form of jmp), then patch the comparison to compare to DRIVE_FIXED instead. This allows us to get to the WNetGetConnectionW call.
  5. Restore the original operation of GetDriveType, and remove the vectored exception handler.
  6. Patch the import address table for WNetGetConnectionW and have it jump to our “get git branch information” function instead.
  7. Implement the actual git-related functionality via libgit2.

Because we don’t have control over the working directory and don’t want to affect the normal cmd.exe operation too much, I did a little extra packaging of the various binaries and dlls into one big exe that extracts to TEMP and runs from there, and uses various path and DLL functions to avoid DLL loading problems.

Also, I use Console2 to wrap cmd.exe. It’s x86-only and so the cmd.exe that I end up running is x86. Because of that and step #4 above, the patching code currently only works on C:\Windows\SysWOW64\cmd.exe (that is, the x86 version). It should be easy to extend to x64 though. And of course, step #4 is generally just crazy-hacky, so it almost surely doesn’t work on versions of Windows that I haven’t tried it on (fully patched Windows 7 and Windows 8).

Anyway, after all that morally questionable mucking around, the end result makes me happy. From an existing cmd.exe:

[master] d:\src\cmdEx>

Or, a bit more fancy during a conflicted rebase:

[refs/heads/wps_impl 1/3|REBASE] d:\src\cr3\src>

meaning rebasing in-progress on step 1 of 3 on the named branch. And there’s no janky delay in the prompt like there is if you do this with bash on Windows.

Code is at github.