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126 lines
4.6 KiB
Plaintext
126 lines
4.6 KiB
Plaintext
December 6, 2014.
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The idea is to make a proper ssh implementation for Android. Important
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features:
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* it should run happily without root (on a non-root port)
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* it should be a regular android app requiring no special permissions,
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and not requiring any 'magic' executable files
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* should not rely on busybox
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* preferably support sftp
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* open source
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The existing apps are either expensive, don't work, need root, or too
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complicated, or a mix of all of the above. And none of them are open
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source.
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I figure I'll start with dropbear, which I will run through JNI instead
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of putting it in its own binary (because making such a binary executable
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is a bit of a hack).
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So that's the plan........
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December 14, 2014.
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I got dropbear to compile under the Android NDK, so now it's time to work
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on the Android side of it.
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I need:
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* a Service that can be started, stopped, and queried for whether it's
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running or not
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* a Thread to implement the Service's work (by calling into dropbear's
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main()), which can also be stopped.
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* a config UI with at least these choices:
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- bool: start on boot (def: false)
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- number: port number (def: 2222)
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- string: path to authorized_keys file (def: /sdcard/ssh)
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- string: name of default shell (def: /system/bin/sh -l)
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- string: default path for HOME (def: /sdcard/ssh)
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- button: start or (if it's running) stop
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December 15, 2014.
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Getting to the fun part. Process management...
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To start sshd, it seems like I can startService(). Then in the Service's
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onStartCommand(), call startForeground() so it won't be killed (return
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START_STICKY too?).
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The question is if dropbear's main() should run under a separate Thread,
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or a separate Process. The trouble with a Thread is that it might be
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hard to kill. The trouble with a process is that there is no way to
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report back status (such as a failure to start sshd).
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Connectbot starts a new process for its shell -- it really doesn't have a
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choice because the shell binary isn't linked with Connectbot, and exec()
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in a thread stinks. To stop it, it just closes stdin/stdout!!! So
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zombies can (and do) linger.
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I suppose dropbear could be in its own process if it had something like
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stdin/stdout to communicate failure? Or it could just write error
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messages to (i.e.) /sdcard/ssh/log. To stop the service, it would just
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use kill().
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I am curious how the main waiting-for-connections loop looks, but even if
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it uses select(), I'm not sure how I would honor Thread.interrupt() or
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whatever. It's not guaranteed to interrupt select(), and I'm not keen on
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adding an arbitrary timeout/polling feature to it.
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December 20, 2014.
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So, I added a builtin scp endpoint. It was pretty straight forward,
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except dropbear defaults to vfork(), which blocks the parent until the
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child runs execve()!!
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Anyways, I noticed that scp doesn't quote its arguments to the remote
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scp. That means you can't conveniently copy a remote file with a space in
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its name (it becomes two files). But the upside is that this is where
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wildcards are handled -- by the shell!
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So I need to either run it as a separate executable launched through the
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shell, or make my own implementation of wildcards.
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It is easy, using a $(BUILD_EXECUTABLE) script, to get ndk to build an
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executable. But it is only packaged up if it is named "gdbserver" (and
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debug apk), or "libfoo.so". The good news is that libfoo.so can be
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executed in /data/data/org.galexander.sshd/lib/libfoo.so, so that is a
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viable option.
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Doing the expansion myself is not necessarily hard either, though. I
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need a library function called glob(), which is apparently not part of
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bionic. But I have the idea some cut and paste would resolve that with
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very little extra work on my part.
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December 21, 2014.
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Well, bionic libc *does* provide fnmatch(), and even scandir() (a
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shortcut for readdir). In the best case, though, that still leaves me
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with a bit of a path parsing conundrum (I have to tell scandir which
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directory to operate on). And also a bit of an escape character
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conundrum -- \* and "*" should not act like wildcards.
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Those are not insurmountable but I think I've talked myself out of it.
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So then the question is, do I figure out how to ship an executable, or
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do I do some hack like open a pipe to "/system/bin/sh echo filespec" and
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use the shell solely for expansion?
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I'm developing the idea that it's actually pretty easy to ship an
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executable, I just need to find some -pre-package step where I can do
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"mv scp libscp.so" and then it will ship. ndk-build will not let me make
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a target with a "." in it directly.
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XXX - sftp (?)
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XXX - rsync
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XXX - document (ga.org/software/simplesshd/)
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XXX - upload
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