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Message-ID: <20150118164010.GC4574@brightrain.aerifal.cx>
Date: Sun, 18 Jan 2015 11:40:10 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: musl and android
On Sun, Jan 18, 2015 at 03:01:19PM +0700, Рысь wrote:
> В Sun, 18 Jan 2015 01:44:40 -0500
> Rich Felker <dalias@...c.org> пишет:
>
> > On Sun, Jan 18, 2015 at 01:32:37PM +0700, Рысь wrote:
> > > В Thu, 15 Jan 2015 16:13:22 +0700
> > > Рысь <lynx@...server.ru> пишет:
> > >
> > > > Hello all!
> > > >
> > > > Are there any efforts or even a project which aims to port musl to
> > > > android platform?
> > > >
> > > > For a year I slowly had built and running a couple of C tty only
> > > > programs such as iptables, tcpdump and of course busybox and many
> > > > others including my own with musl libc linked statically. They
> > > > work perfectly (no much differences between an ARM phone and, for
> > > > example, raspberry pi SoC) but a few items I missed:
> > > >
> > > > + Proper DNS resolving is not available
> > > > + Translation of android special user names is not done
> > > >
> > > > As an advanced Linux user I know that android is not friendly
> > > > enough to plain C stuff and it's libc is even not finished now so
> > > > I aimed to port at least listed things to musl.
> > > >
> > > > Because I am not going beyond listed items, a patch can be
> > > > developed just to support these inside musl-linked binaries.
> > > >
> > > > I am first here or there is already someone who done this before?
> > >
> > > I made a patch to implement all three things for musl on android.
> > >
> > > I do not post it here as attachment to this mail because it normally
> > > should not be integrated with musl. The patch itself is here:
> > > http://lynxlynx.tk/prg/patches/musl-1.1.4_android.patch, notes about
> > > it: http://lynxlynx.tk/eng/musl_android/.
> > >
> > > Patch is invasive enough I think and not optimized and I agree with
> > > Rich that it must go as a special treat. I tried however move away
> > > all translating code to src/android and headers to include/android.
> > >
> > > Patch is still in it's alpha stage, and probably will be changed
> > > often.
> > >
> > > Modifications made for musl 1.1.4.
> > >
> > > If anyone interested to audit it - welcome!
> >
> > Thanks. If nothing else, this is very informative as to how Android
> > does things. I noticed a couple things that could be a lot less
> > invasive, I think, particularly the builtin users/groups. Couldn't you
> > just fmemopen() a constant string inside libc in place of /etc/passwd
> > or /etc/group and use the existing code to parse it?
>
> This sounds more interesting idea, I forgot that get*ent() open fd and
> there is fmemopen() function. But app_/ux_ax names still need a
> dedicated translation function, probably the same code which will form
> a passwd-like string and feed it into get*ent().
The intent is that get{gr,pw}{nam,uid} will eventually support
alternate backends via sending a request and receiving a response in
passwd/group line format. On normal systems this would take place via
a unix socket in some reasonable standard directory, but for Android
you could in principle build a reply mechanism into the binary in
place of it. Note that this would probably not support get{pw,gr}ent
enumeration, but I don't see that as a problem. Traditionally
enumeration does not work with backends like nis or ldap, as far as I
know, and it would be an unreasonable/abusive idiom on such large user
databases.
> > Another general question I have is why can't the /system issue be
> > fixed? Is there any good reason Android doesn't have symlinks like
> > /bin->/system/bin, /etc->/system/etc, /lib->/system/lib,
> > /tmp->/data/tmp, etc.? The gratuitous incompatibility is really
> > disgusting.
>
> First, android has /etc -> /system/etc in place, but, unfortunately, no
> more :(
>
> I made /etc -> /system/etc change just because I distrust symlinks when
> doing security things. It is usually stable across devices, but it's
> just symlink.
>
> It even does not have usual Unix /tmp directory or it's surrogate
> somewhere in filesystem. /data/tmp is my adoption, though it's
> non-standard on android.
>
> Second, android has a very different bootup process, and all those
> links in / are created during bootup (on a tmpfs?). Sadly only /etc is
> defined by android developers to be symlink to /system/etc.
>
> Third, there are probably many differences between many android device
> vendors, android OS types (different android versions including ancient
> ones, different android forks that exist today, vendor customisations
> of fs layout and in future many others). In fact, android is _very_
> fragmented OS.
>
> All this is based on my experience with some android devices I working
> (and worked) with.
If / is a tmpfs, I don't see why proper symlinks can't be created, or
why /tmp can't just be created as a directory on / or as another tmpfs
mount on /tmp. Obviously you can't do these kinds of things as a
normal application installing on Android, but if you're doing your own
system programming on Android or if you have root, this seems like the
obvious correct fix...
I'd really like to hear from the Android side what their motive is for
breaking this by not providing the proper symlinks.
Rich
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