19

My opinion is yes, it does, because all useful exposure to the outside world (non-privileged processor mode) would first require a process running in the outside world. That would require a file system, even a temporary, in-RAM, file system.

Another engineer disagrees with me, but I can't seem to prove this beyond all (unknown to me) cases.

Does the answer to this question depend on the definition of 'running'?

26

That's rather an odd question because you don't run the kernel like you run a program. The kernel is a platform to run programs on. Of course there is setup and shutdown code but it's not possible to run the kernel on its own. There must always be a main "init" process. And the kernel will panic if it's not there. If init tries to exit the kernel will also panic.

These days the init process is something like systemd. If not otherwise specified the kernel will try to run a program from a list of locations starting with /sbin/init. See the init Param here http://man7.org/linux/man-pages/man7/bootparam.7.html in an emergency you can boot Linux with init=/bin/bash . But notice how you always specify a file on the file system to run.

So the kernel will panic if it starts up an has no file system because without one there is no way to load init.

Some confusion may arise because of a chicken and egg situation where the kernel must load drivers to access it's file system. To get round this an initial ramdisk is loaded from an image on disk containing vital drivers and setup scripts. These are executed before the file system is loaded. But make no mistake the initial ramdisk is itself a file system. With an initial ramdisk /init is called (which is stored on the initial ramdisk). In many distributions it is ultimately this which calls /sbin/init. Again without a file system, this is impossible.

  • Isn't there a condition where the kernel gives up trying to initialize hardware and load a known file system (not initrd passed into the kernel via init params), then drops into a very limited shell (without init=/bin/bash)? Also, since you bring up /bin/bash, would the kernel always have that minimal file system available, even if it was built with other .config options that could potentially eliminate this? – Peter L. Mar 22 at 0:40
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    @PeterL. that limit shell is some shell from the initrd/initramfs/whatever that kernel booted with, IIRC. – muru Mar 22 at 2:04
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    Note that you can build the initramfs (a CPIO archive that is extracted into a ramfs or tmpfs filesystem) into the kernel. Whether or not that counts as the kernel "needing a filesystem" is up to you, since it means you can boot the kernel and nothing but the kernel and have a functional (if a bit limited) system. Also note that, even if you patch the kernel to no longer require an init, it will still create internal virtual filesystems that are never exposed. – forest Mar 22 at 5:37
  • @forest The system doesn't have to be "limited" -- you can pack systemd and gnome into your initrd (together with stuff which is actually useful ;-)). One limitation of initramfs was (still is?) that it wasn't supporting extended attributes -- I did work around it on android by including an ext4 image in the initrd cpio archive which was then mounted as a loop device from the init.$DEV.rc script. – Uncle Billy Mar 22 at 8:48
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    @IsmaelMiguel, nope, an initramfs as such is a cpio archive. Squashfs is a good choice for embedded filesystems, and one could make an initrd (vs an initramfs) that uses it (the terms are often used interchangably but they're not quite the same thing), but it's not the format Linux unpacks into its initramfs. (Indeed, a squashfs image doesn't need to be unpacked before it can be used at all; it's properly indexed). – Charles Duffy Mar 22 at 17:23
16

The answer is going to depend on whether you literally mean without a file system or if the question is intended to be interpreted a little different from how it is actually stated. The answers for slight variations in how the question is interpreted are:

  • Running Linux without any block devices is entirely feasible and useful for some specialized use cases.
  • Running Linux without any file system is going to require rewriting some parts of the kernel code and it's unlikely to be a useful effort.
  • Running Linux without using any file descriptors is going to require a lot of effort. I'm pretty sure that's not going to be worth the effort.

The reasons you'd have to rewrite parts of the kernel code to make a working system without a file system are:

  • Every thread has a root directory and a current working directory which must point to some file system.
  • Programs are started by the execve system call which needs an executable from a file system.
  • The kernel creates a memory based file system during the boot process.

After a program has been started using execve it is possible for it to unmap the executable from which it was started, though in order to do so it without immediately crashing it first have to create an executable memory mapping which isn't backed by a file, and it has to initialize that with some useful code before jumping to it and unmapping the executable.

Thus a running user mode program can exist in a state where it has no memory mappings backed by files and it can close all file descriptors backed by files. It cannot stop having a root directory and current working directory, but it can refrain from those.

So though in this state you could implement kernel code to rip the file system away from under the program and have it keep running, it doesn't sound like it's useful. And getting into that final state without going through an intermediate state of using a file system is going to be even more work for no useful benefit.

A useful setup for some specialized use cases

Avoiding the use of block devices can be useful. During boot the kernel creates a memory file system, and it can also populate that file system with contents from a cpio archive before executing init. That way you can run a system entirely from a memory based file system without any block device to back it.

This can be useful for systems where you don't want to preserve any state and like the system to start from a clean slate upon rebooting.

Of course the kernel and cpio archive have to get somehow exist in memory before the kernel is given control. How they got there is a job for the boot loader. The boot loader could have loaded those from a block device even though the final running system doesn't use block devices. But it's also possible for the boot loader to acquire the kernel and cpio archive without using a block device for example by booting over the network.

  • 1
    The question is if a Linux kernel in any built configuration (without re-writing anything) can 'run' without any file system. It does not have to do anything useful or preserve a state. From all of the answers, I'm understanding that some sort of file system is provided and assumed within the kernel itself, at least until shutdown. Even '/' is a file system. So, I think to simplify the answer it is 'yes'. – Peter L. Mar 22 at 14:58
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    @PeterL. Yes, if you don't rewrite anything Linux will require a file system. When people talk about practical uses for Linux without a file system they would usually be referring to those backed by a block device and you can run Linux without a file system backed by a block device. You'd still have some sort of file system. – kasperd Mar 22 at 22:11
3

In Linux, nearly every device is a file, so you have to have a filesystem to run it.

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    But of course the device drivers exist inside the kernel irrespective of whether or not a device file points to them. – Philip Couling Mar 22 at 0:09
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    Not every device is a file. Network interfaces (eth0, wlan0 etc.) aren't, for example. – Ruslan Mar 22 at 4:35
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    This is a common misconception. While in theory, everything is a file in UNIX and UNIX-like systems, it is only completely true for highly specialized systems like Plan 9 (though it is far more true than for Windows). For Linux, quite a few things are not files. This is getting more and more true as many drivers have begun to use netlink rather than ioctls on character devices (which are files). – forest Mar 22 at 5:34
  • @forest Plan 9 is not a "highly specialized" system -- it was supposed to be a general purpose system like Unix or windows (why it failed at replacing Unix and remained a research system is a completely different story). Anyways, just as Linux, plan9 is exposing virtualized interfaces to its hardware (and doesn't have any ioctls -- I don't see how using netlink vs ioctls factor in all this), even if it strives to be more consistent (eg. the network interfaces are accessible via the filesystem). With the introduction of namespaces, Linux is already more like plan9 than classic unix. – Uncle Billy Mar 22 at 9:11
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    Very nice argument: either there's devfs, which is a filesystem per definition, or there's no devfs, in which case you need a filesystem to host device nodes ... – pmf Mar 22 at 14:47
-1

A kernel IS a program, just like any other. By default the Linux kernel attempts to access file system, however this behavior can be trivially eliminated by kernel modification (actually just an addition of an "arch_call_rest_init()" function). In order to perform "useful work" then we expect that the developer might include kernel threads (kthreads), perhapos in a custom driver, to perform some desired initialization and application type workload. The Linux kernel contains many kthreads already ,but primarily to perform work ancillary to the kernel or drivers. The APIs available within the kernel context are quite different from those available in Linux user-space. A large fraction of system call functionality would become useless in a no-filesystem scenario.

Yes, Linux expects access to file systems by default. No, a modified kernel could certainly be made to perform useful work w/o any file system. The practical use of Linux w/o filesystem is IMO quite limited, but not nil. FWIW, in the past many real-time kernels were built into the same name-space & binary as the RT applications.

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