I understand that a file system can be visualized as a "tree" of files and directories. I also understand that "mounting" a file system means to placing or rooting that tree within an existing directory. https://askubuntu.com/questions/20680/what-does-it-mean-to-mount-something
With that said, I have mounted an implementation of python fuse that is similar to this one. When I run my implementation, it runs to the end of the init method and then just shows a blinking cursor. So fuse is getting mounted. I know that fuse is mounted because of what happens when I run $mount
$mount
...
FuseHandler on /home/memsql/mount
So now that I've mounted fuse, how do I access the files.
The linked tutorial says
You will see all files in /your/dir under /mnt/point and be able to
manipulate them exactly as if they were in the original filesystem.
How exactly do you do this? Can somebody show me, syntactically, how to instantiate and query Fuse? How do you perform a read or write? What does code that performs those operations look like?
As l4mpi notes, you’ve now mounted a file system, so it will respond to the standard Unix file system API, and FUSE will handle the file system operations. You can cd to it, ls in it, read or creat files in it, etc.
Related
I am creating a fuse-based filesystem very similar to the example passthrough_fh. Where I log some statistics in my handlers before calling the underlying system call.
I use this with a debian Wheezy chroot image from debboostrap. The idea is to mirror wheezy/ into my mountpoint, then a process will chroot into the mountpoint and all activities will be recorded through my fuse fs.
The OS seems to handle path resolution with chroot nicely. That is, if the chrooted process does stat("/bin/ls"), from my fuse process I see stat("wheezy/bin/ls").
However I'm not sure how to handle symlinks. For example the file
wheezy/lib64/ld-linux-x86-64.so.2
points to
/lib/x86_64-linux-gnu/ld-2.13.so
So when I call stat("wheezy/lib64/ld-linux-x86-64.so.2") it won't just work, since the OS will try to dereference the symlink /lib/x86_64-linux-gnu/ld-2.13.so instead of the correct wheezy/lib/x86_64-linux-gnu/ld-2.13.so.
This is a simplified example, I can't just prepend wheezy/ to all paths, I want to also support applications which do not chroot, or chroot multiple times.
I can think of some less than ideals ways to do this, e.g. check /proc/pid/root/ to get the root of the process in case of chroot, but then I have to always check if a file is a symbolic link.
Is there a better way or general way fuse based file systems handle this problem?
After contacting the fuse-devel mailing list, I received the following response:
If you are performing this stat(2) for GETATTR or LOOKUP, you should
be using lstat(2) instead. This will tell the kernel that you found a
symlink and it should keep managing path resolution correctly for you.
That is, use lstat(2) when handling LOOKUP or GETATTR, use the results of lstat to fill the fuse struct. From there, the kernel will automatically handle the name resolution (even for symbolic links, and processes running inside a chroot).
What is the libmount equivalent function to mount a FUSE file-system. I understand that FUSE is not a real file system and my strace of mount.fuse shows opening a /dev/fuse file and doing some complicated manipulations.
I tried seeing how the mount.fuse works by reading it's source code but not only it is needlessly complicated by string manipulations in C, it is a GPL program.
My question is, am I missing the obvious API to mount fuse file systems?
The kernel interface for mounting a FUSE filesystem is described in "linux/Documentation/filesystems/fuse.txt" (for example, see here).
In a nutshell, you call mount(2) as you would to mount any filesystem. However, the key difference is that you must provide a mount option fd=n where n is a file descriptor you've obtained by opening /dev/fuse and which will be used by the userspace process implementing the filesystem to respond to kernel requests.
In particular, this means that the mount is actually performed by the user space program that implements the filesystem. Specifically, most FUSE filesystems use libfuse and call the function fuse_main or fuse_session_mount to perform the mount (which eventually call the internal function fuse_mount_sys in mount.c that contains the actual mount(2) system call).
So, if you want to mount a FUSE filesystem programmatically, the correct way to do this is to fork and exec the corresponding FUSE executable (e.g., sshfs) and have it handle the mount on your behalf.
Note that /sbin/mount.fuse doesn't actually mount anything itself. It's just a wrapper to allow you to mount FUSE filesystems via entries in "/etc/fstab" via the mount command line utility or at boot time. That's why you can't find any mounting code there. It mounts FUSE filesystems the same way I described above, by running the FUSE executable for the filesystem in question to perform the actual mount.
Does Linux need a writeable file system to function correctly? I'm just running a very simple init programme. Presently I'm not mounting any partitions. The Kernel has mounted the root partition as read-only. Is Linux designed to be able run with just a read-only file system as long as I stick to mallocs, readlines and text to standard out (puts), or does Linux require a writeable file system in-order even to perform standard text input and output?
I ask because I seem to be getting kernel panics and complaints about the stack. I'm not trying to run a useful system at the moment. I already have a useful system on another partition. I'm trying to keep it as simple as possible so as I can fully understand things before adding in an extra layer of complexity.
I'm running a fairly standard x86-64 desktop.
No, writable file system is not required. It is theoretically possible to run GNU/Linux with the only read-only file system.
In practice you probably want to mount /proc, /sys, /dev, possibly /dev/pts to everything work properly. Note that even some bash commands requires writable /tmp. Some other programs - writable /var.
You always can mount /tmp and /var as ramdisk.
Yes and No. No it doesn't need to be writeable if it did almost nothing useful.
Yes, you're running a desktop so it's needed to be writeable.
Many processes actually need a writeable filesystem as many system calls can create files. e.g. Unix Domain Sockets can create files.
Also many applications write into /var, and /tmp
The way to get around this is to mount the filesystem read/only and use a filesystem overlay to overlay an in memory filesystem. That way, the path will be writable but they go to ram and any changes are thrown away on reboot.
See: overlayroot
No it's not required. For example as most distributions have a live version of Linux for booting up for a cd or usb disk with actually using and back end hdd.
Also on normal installations, the root partitions are changed to read-only when there are corruptions on the disk. This way the system still comes up as read-only partition.
You need to capture the vmcore and the stack trace of the panic form the dmesg output to analyse further.
I'm trying to build a new filesystem with deduplication using FUSE.
I tried running the fusexmp_fh.c provided in the example section of the FUSE. However after mounting the filesystem at a mount point, I can see all the existing directories inside the mount point. I dont need those directories. I want the mounted filesystem to be empty.
I tried searching through fusexmp_fh.c but could not find out where the existing directories get added.
Can someone explain to me how this works?
Also
can fusexmp_fh.c be taken as a base for building the filesystem?
Does it have all the basic functionalities?
Are you saying that you mounted the file system over a non-empty directory, and you can see the previous contents of the directory? (In which case, the answer is "don't mount to a non-empty mount point". Usually it throws an error to tell you not to do that.)
If what you're seeing is the directories and files in the directory that you are using as your base directory, that's the normal behavior for a loopback file system, which is what fusexmp_fh.c is. The example file system takes a mount point, and passes all commands on that mount point through to a backing directory. If you use a backing directory that has files in it, you will now see those files in two places, the original location and the mounted fuse directory.
If you want to understand how the directory filling works, start by taking a look at readdir, and see how the stat items that it returns are constructed. Each of those is a single directory entry.
Yes, you can use fusexmp_fh.c as the basis for a basic file system, it's got all the necessary pieces, although extended metadata isn't supported. (But adding it is fairly trivial for a loopback.)
As far as I know proc file system is a virtual file system. Is there any way to unmount the proc file system and even if I do that what will be the consequences after that.
You can check (as root) who is using a mounted filesystem like so:
fuser -m /proc
Typically, your box will not be very usable if you kill all the processes using /proc. Otherwise, there is no law saying it has to be mounted, beyond all and sundry developer assuming that it is.
umount will work like on any other file system (same conditions for a filesystem to be unmonted). You can expect a whole lot of this to stop working as soon as you do that though (including very simple utilities like ps).