The most frustrating thing to ever happen to the concepts in plan9 is the funhouse mirror distortions of them that have landed in the linux world. They look just enough like plan9 that people think they represent those ideas, and so they have no idea how powerful those ideas can actually be and don't even know that they don't know it.
We'll never have any of the things it really promised until we give up on POSIX, tbh.
Plan 9 fell to second system syndrome. This will hurt Plan 9 enthusiasts and probably get me flamed but it's my opinion.
Not necessarily bloated or inelegant, but over engineered and over confident and missing reasons the first system was successful.
Purifying and perfecting some of the concepts from unix is nice, but someone running a file server or CAD program or editing source code or compiling code or running shell scripts and piping a lot of commands together to do cool things with data just does not see a whole lot of incremental benefit beyond what unix gave them.
Unix was successful because it was there and accessible and pretty easy and pretty good and evolved quickly (if not always elegantly) to new meet new requirements.
For example: purists talk about sockets as some kind of catastrophe. But in all honesty they're not that bad and once you have a few networking tools you can use in shell pipelines you really don't have to have absolute everything as a file. Simple standard composable tools is more important for practical use than everything is a file.
Oh I don't disagree with this at all. The main reason I wouldn't ever actually use plan9 day to day on a desktop, which is to some extent the primary use it was designed for (well, thin clients workstations really), is that the whole windowing system is just too alien.
I think most of the missteps in plan9 are above the "everything is a file" layer though. And while sockets aren't that bad, I do think I would still rather interact with them in the plan9 way than the unix way if I could.
I would call it maybe vageuly similar, but it is by no means a subset. It has some features in common and many that are completely nonsense in a posix context.
The most pressing problem for implementing plan9-like semantics in a POSIX system is the permission system. In particular, setuid as a mechanism for privilege escalation. This is a big part of why users can't make their own namespaces on linux without help/intervention from root-owned processes (like dockerd or systemd).
Think about it: if you can make the file namespace any shape you want, and then run `sudo`, which is a setuid process that looks at /etc/sudo.conf to decide whether your escalation is allowed, how do you secure it?
How do you even begin to do distributed permissions if everything's looking at /etc/passwd and /etc/group in the current process' namespace to decide who you are?
POSIX is very much built on the idea of a canonical view of the filesystem, and plan9 is built on a vfs that may as well be sand.
They're a good step but they're really a step in a different direction, even though capabilities are at the heart of how plan9 does permissions as well. Plan9 capabilities are more like kerberos tokens, so you get them from privileged services and then can use them to perform privileged actions.
Linux capabilities don't really change any of the issues around namespace security because they don't inherently provide a way to elevate privileges without setuid.
In modern Linux this is no problem. You can now give a process its own UID namespace. In the calling namespace its UID is non-zero, but in its own namespace it's root.
I dunno about the sibling comment's "backwards" comment but the thing here is that the goal isn't to prevent a user process from obtaining privileges, which is what uid namespaces are for.
This is the kind of thing I mean about "not knowing what you don't know," because you're looking at namespaces through the lens linux does, which is that they exist to limit capabilities.
Plan 9 uses namespaces to allow users to control their own environment. It's not a special operation, it's just a thing you do all the time.
For a small practical example: there's no PATH environment variable in plan9. You just union mount things into /bin, and /bin is where your shell looks for things to run. It's that much of an every day operation.
If you put a user under a uid namespace in linux, and then give them the right to create their own filesystem namespaces then sure, you've enabled them to potentially do things like this. But you've also blocked them from escalating their privileges, because now they can't use setuid binaries to obtain "real root" or whatever.
So you're left with one or the other: either you can manage your own namespace, but you have to be protected from potentially breaching root security through a setuid or cap flag on a binary; or you have to be prevented from managing your namespace outright in order to avoid lying to sudo about who can do what.
The thing about `sudo` is confusing because you don't even need to modify the namespace to overwrite `/etc/sudoers` if you have root. You just need to write to the file.
What it sounds like plan9 is doing is giving a local view of the root that local processes see. Which Linux can do too. Not with the same use-cases in mind as Plan9 though, as such capabilities were added for sandboxing/containerization. But the mechanisms are probably(?) general enough to do Plan9 in Linux.
A `sudo` that is seeing a local view of the root is going to have privileged access to that local root, not to the global system root. And that is correct. That is what sudo does. It gives root access to the same root that contains /etc, not to any "outer" or "more global" root.
It doesn't mean you can't have any access to the global root from the local root though. There are many ways to arrange such privilege escalation. (They do have to be arranged, of course, by someone writing the userspace code -- like sudo had to be written.)
>If you put a user under a uid namespace in linux, and then give them the right to create their own filesystem namespaces then sure, you've enabled them to potentially do things like this. But you've also blocked them from escalating their privileges, because now they can't use setuid binaries to obtain "real root" or whatever.
Privileged processes can have a global view of the namespace while the user does not. An ordinary setuid binary on a filesystem the user controls can't get a global view, only because the user does not (should not) have authority to do that. A process with the global view and root can grant the authority though.
The important thing, it seems to me, is that the global outer namespace can grant to the process local namespace any capabilities available through the outer namespace. I'm not sure if this is 100% completed but the ongoing containerization efforts do involve reaching toward that 100% mark.
> The thing about `sudo` is confusing because you don't even need to modify the namespace to overwrite `/etc/sudoers` if you have root. You just need to write to the file.
The point is that you DONT have root, and you DONT have access to write to the file. But you're free to rearrange your namespace WITHOUT having root, and you want to arrange for SOME users to escalate privileges and, say, debug the kernel. Or do something else dangerous that requires elevated privileges in the global context.
> A `sudo` that is seeing a local view of the root is going to have privileged access to that local root, not to the global system root. And that is correct. That is what sudo does. It gives root access to the same root that contains /etc, not to any "outer" or "more global" root.
Yes, and that's a concise description of the flaw: you can't use suid+file based privilege escalation to modify system-wide configuration, without restricting the ability to manage freely your namespace.
This is what unix does by design, and why the authentication design from unix isn't going to work for systems used in the style of plan 9.
You can use suid programs to do what they do. You can use other mechanisms to do other things.
Suid programs on Plan9 could not possibly behave any differently. If the user rebinds `/bin` and then runs a suid program that calls other programs, that suid program cannot use the rebound `/bin`. That kind of binding simply can't be allowed to cross security contexts.
I don't know what you're trying to say here. You seem to be saying that "namespaces wouldn't work in plan9 if plan9 had setuid" which.. yes? That's exactly right. But plan9 does have namespaces, and does not have setuid, so it does indeed work there.
I mean it's supposed to be a problem for Linux if a setuid binary just has a different namespace view than the caller. But in Plan9 there's no setuid binary at all yet it's not a problem?
In neither Plan9, nor Linux, nor any other potential system, could you have the caller of a privileged program control the namespaces accessed under the privilege of that program.
Like the whole thing about rebinding /bin instead of $PATH... well neither rebinding, nor editing $PATH, nor any other such thing, $LD_PRELOAD, anything, that would affect how the process found files to execute, could be secure if allowed to affect a privileged process. It only means you disable environment sharing, and you disable namespace sharing, etc., any other kind of sharing (no matter how implemented... Linux way, Plan9 way, anything). Plan9 has no better way than that, no way to make /bin rebinding work in a way that makes sense for privilege escalation.
> But in Plan9 there's no setuid binary at all yet it's not a problem?
I think you mean, 'and therefore, it's not a problem.'
In plan 9, the program that allows you to switch users needs almost no privileges.
> Plan9 has no better way than that, no way to make /bin rebinding work in a way that makes sense for privilege escalation.
Processes aren't allowed to become privileged without obtaining a cryptographic capability token via negotiation with the authentication agent. The auth agent which was started at boot will write the secret to the kernel, and give you the hash of it so you can prove you are the rightful recipient of that uid switch request. If you don't have the right secrets, you don't get a token.
You can't swap the devcap in the authenticators namespace, and negotiating with a rebound devcap is simply not going to work, because your authentication token wasn't written into it. You don't have the ability to change what the program doing authentication sees.
In summary: There's no information attached to the binary, and no capability grants in a namespace you can rebind.
If you want to namespace this sort of authentication agent, it's also possible -- you can authenticate, escalate permissions, and start your own agent talking to devcap -- but the capability to start a functional authentication agent is guarded by capability tokens. You need to be authenticated to allow authentication.
Removing suid binaries with config files as a method for privilege escalation is the right path. They don't play well with namespaces.
Cryptographic capability tokens that you can delegate to other programs do.
Linux lets you create a namespace with no capability grants in it. That's what I said in my top post here. Linux lets you do it, sudo will even work it (and grant no real global capabilities) yet somehow it's a problem FOR LINUX that Linux has a userland process called sudo that couldn't ever work on Plan9.
> You don't have the ability to change what the program doing authentication sees.
Obviously not. It has to behave just like sudo in this regard! It's the only option!
Since you can't do that, your rebindings are just as localized as what Linux permits.
> Processes aren't allowed to become privileged without obtaining a cryptographic capability token via negotiation with the authentication agent. The auth agent which was started at boot will write the secret to the kernel, and give you the hash of it so you can prove you are the rightful recipient of that uid switch request. If you don't have the right secrets, you don't get a token.
This is pretty cool, and also something that could be put into a Linux userspace daemon to authenticate privileged operations. I mean this is all userspace details from a Linux perspective. Systemd could do this or some PAM module.
> Removing suid binaries with config files as a method for privilege escalation is the right path. They don't play well with namespaces.
Suid binaries aren't used that commonly for privilege escalation anyway. Much more ordinary is to use privilege inherited from init.
> Linux lets you create a namespace with no capability grants in it. That's what I said in my top post here. Linux lets you do it, sudo will even work it (and grant no real global capabilities) yet somehow it's a problem FOR LINUX that Linux has a userland process called sudo that couldn't ever work on Plan9.
Again, going back to the example I was using: How does that help with securely allowing `$get_permissions debug-my-kernel`?
Sudo is a HOLE IN THE SIDE OF A BOAT. It is not a problem for Plan 9, because plan 9 does not have a hole, and is therefore not doing contortions to avoid filling with water. Designing a boat without a hole in its side is generally considered a good idea. Designing a security model without a suid in its side is a similarly good idea.
If you want `auth/as`, it's there. But it does not use suid, and therefore does not have the problems created by suid.
The problem is that the setuid program does have the same view of the namespace as its caller. When you run a setuid binary it inherits all aspects of the process that exec()'d it, including its namespaces.
Ok, so let's make this more concrete. The scenario we're discussing is a user who wishes to (illegitimately) elevate their privileges, and has the ability to mount, and in particular bind/union mount:
> mkdir ~/my-etc
> echo 'badperson ALL=NOPASSWD:(ALL:ALL) ALL' > ~/my-etc/sudoers
> mount --bind ~/my-etc /etc # note: you could also insert a command to bump us into a new filesystem namespace before this if you want, or assume that say logind did it for us, it wouldn't change anything about what happens next
> sudo -s
# rm -rf /*
This happens because sudo is a setuid program, and it inherits the filesystem namespace of the shell that ran it. In that namespace, the real sudoers file has been masked with a fake one that says badperson can sudo to any user they want, and so it lets them.
That this is the mechanism of privilege escalation that linux uses is fundamental to why mounting (and bind mounting) is a privileged operation. Plan9 does not have either root, nor setuid programs, nor filesystem-stored capabilities, and so does not suffer from this and you can manipulate your namespace to your heart's content.
> Plan9 has no better way than that, no way to make /bin rebinding work in a way that makes sense for privilege escalation.
> But the mechanisms are probably(?) general enough to do Plan9 in Linux.
They are not. The whole point of this subthread is that the ability to create namespaces as an unprivileged user[1] would be key to actually 'doing plan9 in linux'. You can not believe that if you want, but I'd suggest you read up a bit more on plan9 if so, because it becomes obvious pretty quick that it's the case.
[1] And here by 'unprivileged user' I mean someone who is still a user of the machine, and not a user who has been containered away into a separate user namespace, let alone into a whole docker-style container.
I feel like plan9 nerds (of which I’m one) are missing the point. People are asking “what’s so special about plan9?” and the best response is some esoteric point about a thing people empirically don’t want to do? Who cares?
There has to be a better answer than “Wow, I can make my computer that can’t run anything people want to run secure in a hypothetical hierarchical organization structure of permissions that can each have their own subtree sudo. I even call it treedo, ha-ha!” .. it just doesn’t resonate.
I dunno I think the positives are all actually pretty practical. Probably even more so today where heterogeneous computing is so much more common than it was in the 90s.
I would frequently love to have the ability to just mount a bunch of cpus off a beefier machine onto my laptop and take advantage of that to speed up my builds. I can use DISTCC but holy hell is it a lot more complicated to set up.
Or like, mounting a zip file as a directory, without needing a whole enormous systemd or gnome hairball along with fuse or gfs to make it happen as a regular user. Or hell, mount a usb stick even!
These are literally things I wish were easier every day as a software developer. The vaguely plan9-shaped bits that have been added to linux over the years have brought me no closer to them.
Fuse is how it works on Linux. Fuse is the mechanism the kernel provides to do this. So the proviso "without needing ... fuse ... to make it happen as a regular user" is terminal.
You can't mount even a fuse filesystem without root privileges (or an equivalent narrow capability). Mount is a privileged operation in linux, and it has to be for the reasons I've outlined.
In plan9 mount is not a privileged operation. Anyone can do it at any time for any reason. It does not impact or interact with the security of the system (except that you can implicitly remove access to things by unmounting them).
I mean, yes? People are 'trying', and they're building an ever higher house of cards on which to rest this functionality. Obviously on a turing complete machine anything is possible given enough LoC, but as we've seen recently with a CVE for privilege escalation that takes advantage of one of these new 'unprivileged user can get privileges in a namespace' tricks, there's a lot of complexity and likely a lot of security issues hiding behind it.
Plan9's design makes all of this very simple, and a big part of why is the specific choice to eschew standard UNIX semantics and use a different kind of mechanism for privilege management that allowed for flexible namespaces managed outside the kernel. It also has the advantage of moving all filesystem operations out of the kernel.
So this really backs up my point, rather than contradicts it: It's only through mitigating and otherwise contradicting traditional POSIX semantics that linux is able to approach this kind of thing.
The problem here is boring and practical: how do you make `bind` (or, in Linux land) `mount --bind` secure and still allow you to authenticate as a separate user, elevate permissions to change kernel config, and so on?
The authentication mechanisms on most Linuxes are based around suid binaries that read configuration files in order to decide on what to do, so if you can bind in a namespace, you can fool the authentication mechanisms.
In plan 9, this is solved with the kernel capability device. It's not particularly exciting, it's just one of the things that need to happen when you remove the concept of a global 'root user' from the system.
I think you're right that I don't understand the problem.
If you want to give actual real global root, I think you can do it by having a gifting process put the real global root process into the same process namespace as the giftee process.
POSIX standarized and attempted to unify a dozen different incompatible systems that developed independently on top of the original unix from bell labs. Those systems were developed by building new functionality on top of what unix provided. In order to keep at least some sort of compatibility the old and at times obsolete functionality was kept in the system.
Plan 9 on the other hand intentionally broke compatibility with its predecessors and had those same features that were glued recklessly on top of each other in various unices thoughtfully redesigned from scratch, often omitting stuff that didn't seem relevant enough to its authors.
For example: consider how you'd write some generic code to forward all ioctls transparently across the network. Keep in mind that the data attached to the ioctls is machine dependent, driver dependent, and has no information about how it's formatted. Every ioctl for every driver is its own special case.
Meanwhile, faithfully forwarding all devices in a plan 9 system is trivial. Control messages aren't strictly formatted -- but they're done via reads and writes on file descriptors, so sending them to the devices that understand them, and relaying back the result, is trivial. It's just 9p: https://man.9front.org/5/0intro
Doing this fully, for all devices (except /srv, which is a bit magical) is implemented here, in a short shell script. This is the remote login program used by 9front, which gives you something resembling ssh or vnc, but with full access to the data and devices on your local machine, graphics, audio, mouse, keyboard, USB, network, and anything else, even if it hasn't been implemented yet. It does both client and server side:
The client side sets itself as a file server, using exportfs. It exports everything in its namespace, including /dev, over to the server.
The server takes the client's namespace, and mounts it over /mnt/term. Then, it takes /mnt/term/dev/cons and binds that over /dev/cons and starts a shell. That means that every time a program is run, it opens /dev/cons to interact with the user, using the client's mouse, keyboard, and so on, forwarding all the operations transparently over the network.
The idea can go further; Instead of using network translation layers, for example, a plan 9 machine would import a different machine's network stack and mount it over itself:
# whats my ip?
% cat /net/ipselftab
192.168.1.11 01 4u
% hget https://api.ipify.org
74.{home.address}
# ok, let's import another machine's network stack and use it.
% rimport orib.dev /net/ /net
# what's my ip now? look ma, I'm proxying!
% cat /net/ipselftab
144.202.1.203 01 4u
% hget https://api.ipify.org
144.202.1.203
There are no special hooks in the network stack for this. It doesn't know. This happens for free because the network stack being accessible through the filesystem API.
This kind of thing happens everywhere, because everything goes through 9p, and everything can be namespaced. There isn't any other special case to consider: If you forward 9p, you forward all operations you can do with a device. Or any other file server.
If everything is in a namespace, you don't pull the devices other programs are using out from under them, so you can put one login in one sandbox with a remote mouse and keyboard, and a different one in a different sandbox with a different network stack.
This falls apart when you have the 53,719 special cases bundled with posix. If you need a special case for each operation you through the network, or interpose in userspace, you're in for a rough time.
Plan 9 works because it's relatively simple and uniform.
Hard drives are cheap, so space is not an argument anymore, for reasonable uses of disk space. And most uses are reasonable!
Ah, but you might say, if a shared library is compromised, it's easy to push a fix! But how to you think it got so widely compromised in the first place? Perhaps because it was a widely shared library? Sharing is a double-edged sword.
The impetus behind the virtual environments for scripting languages, like Python's venv and Ruby's RVM, is isolation from the base system. Untold developer hours have been lost in attempts to run software with different dependencies than the base system. It's a total mess.
We shouldn't expect an operating system to be a monolith that dictates the dependency versions for all the code that runs on it. Code should be deployed in sandboxes and it should be independent of the base system. When the code is removed, it should be like it was never there.
Hint: static linking predates Plan 9. Static linking even predates shared libraries. Back in the '80s there were impassioned quarrels over whether anything should be shared. Laura Creighton was static linking's most impassioned voice. The fact that a shared library may be mapped once and used by all the programs that call it carried the day, back when main memory was measured in megabytes, and GUI libraries were huge and getting bigger.
If there is anything we should thank Plan 9 for, it has to be UTF-8.
> Ah, but you might say, if a shared library is compromised, it's easy to push a fix! But how to you think it got so widely compromised in the first place? Perhaps because it was a widely shared library? Sharing is a double-edged sword.
Not sure I understand. I imagine without dynamic linking/shared libraries, most widely used shared libraries would be widely used statically linked libraries, and so a vulnerability in them would indeed be harder to fix, as you'd need to relink all the binaries using them instead of just the dynamically linked library?
(Also, memory usage seems more concerning to me than disk space. Shared libraries are called "shared" because all their non-mutable pages in memory are shared across processes. To even approximate the same with static libraries, you'd pretty much have to have deduplication of pages in memory. Link-time optimization might then spoil even that plan entirely. Of course on the other hand, dynamic linking precludes LTO for that.)
You don't re-link binaries. The vendors ship you a new build. Your OS should not have any libraries other than the ones it needs to function by itself.
Well, that's the point. The vendor needs to relink, and now you need to wait for an update from the vendor of each individual client of a library. With shared libraries, replacing the shared library is enough.
What I did not understand is how static linking would have precluded the problem in the first place. I don't think that would have made those libraries less widely used.
> With shared libraries, replacing the shared library is enough.
Yes, but if a shared library update includes a bug, it affects all your programs! That's why it is a double-edged sword: The same mechanism that solves the bugs can also deliver bugs.
This comes up constantly and is pretty easily refuted by opening pretty much any app on an Android or iOS phone and observing just how many pages of the system shared libraries are touched by even the most trivial apps. You can do this by e.g. purging the FS cache, disabling readahead, launching an app, and then observing how many code pages are resident.
All of that code would have to be brought into app binary itself, making mobile apps even larger in code size than they already are. Dead code elimination could eliminate some of that bloat, but I doubt it'd eliminate much of it, given that we're only measuring resident pages to begin with.
Only allowing static linking might make sense on servers (it certainly makes deployment a lot easier). But it wouldn't work on mobile devices without significantly changing the way mobile apps and OSes are architected.
Including XPC, D-BUS and COM into the picture, which allow for dynamic linking, or if one wants static linking like experience with out-of-process servers, and still there are some issues with it.
I don't much care for static linking. I do care about it being too difficult to have multiple versions of the same shared library, and no easy distribution mechanism when I just want to shoot a binary over to some system and run it.
In an ideal world, archives would be well structured executables and my system would automatically deduplicate libraries out of them when they landed on the local filesystem. The linker would automatically pull a whitelist of "known bad" and do overrides for me from my package manager/security source automatically.
> Ah, but you might say, if a shared library is compromised, it's easy to push a fix! But how to you think it got so widely compromised in the first place? Perhaps because it was a widely shared library? Sharing is a double-edged sword.
By this logic, every game developer should be writing their own versions of Vulkan for every GPU they want to target. They would have to ship their own set of GPU drivers with every game.
Also, every app that wants to make a TLS connection would have to develop its own cryptography primitives. Hope you enjoy writing ASN.1 parsers (in C, because that's the Plan 9 way).
Sorry, but this "no dependencies" utopia is completely impractical.
Unix with its "worse is better" simplicity steamrollered vastly more complex operating systems (Multics above all).
It even steamrollered its own successor. Plan 9 is brilliant, but Unix already served most people's needs so why change.
Mental game: If they had managed to quickly push the whole thing out as what is now called open source, while Unix was still proprietary, how would the world look now?
> Mental game: If they had managed to quickly push the whole thing out as what is now called open source, while Unix was still proprietary, how would the world look now?
An AT&T that was willing to do that would have been willing to let BSDI slide. Linux would have died in the crib and we'd all be using BSD right now.
Computer system security would be vastly superior, I imagine. Webpages would be mounted file systems with restricted permissions systems, and browser apps would be command line utilities. Both would have benefited from the same security that UNIX systems use these days.
We've had sandboxing for a decade on mainstream operating systems with mainstream browsers. I doubt that just adopting Plan 9 would offer any meaningful security improvements over the status quo. It would just be switching a big pile of non-memory-safe C++ for a big pile of non-memory-safe C.
Given that webpages aren't static today, it seems just as likely that in that universe sites would expect you to execute them natively, mounted over 9P. This actually could still work out since Plan 9 did could easily sandbox things, though the security angle would still make me nervous.
Plan 9 would have to contend with a huge install base issue. Remember academics had been using Unix freely since the 1960's and had a massive pile of code plus every machine worth mentioning. Plan 9 would be trying to catch up from a very delayed position.
Woah there, UNIX only reached academic world after UNIX V6, released in 1975 and its first Assembly version was born in 1969, only being released to the world in 1973.
Fewer bugs I imagine. I was reading up on symlinks (trying to find a good reference for how much of a bad idea they are) and found this: https://9p.io/sys/doc/lexnames.html
Kind of sad that Plan 9 got rid of them literally decades ago and we're still stuck dealing with their mess.
Read the page, but essentially `/foo/bar/..` is not the same as `/foo`. It makes loads of things way more complicated than you'd expect, e.g. normalising paths now requires filesystem access, and normalising paths that only partially exist is really complicated.
Another annoying issue is symlink loops, but I'm not sure if Plan 9's solution solves that.
I have another mental game, if UNIX had been sold at the same price as Multics and VMS, how would the world look now?
UNIX only steamrollered other OSes, becasue it was already "open source" during its early decade until AT&T got back the rights to sell, and the BSD lawsuit came to be alongside the prohibition of UNIX V6 annotated source book.
This. So much this. I've used Plan9 on a Raspberry Pi with a mouse, but have never been really able to use it on any kind of modern hardware simply because of the mouse chording anachronisms (which the community is steadfast in considering "perfectly normal")
For example there is this mandatory covid testing. So each department is handed an excel file and they log the tests. And there is another excel which summarizes those dept’s ones.
We'll never have any of the things it really promised until we give up on POSIX, tbh.