I am modifying ext4 filesystem to add a simple encryption to files contents.
I started by changing read and write system calls to change the contents right before write and after read system calls.
now copying files in terminal is working just as I expected but when I try to copy a file using a GUI based file manager (pcmanfm in this case) it is corrupting the contents.
my question is: what system calls are used for reading/writing files besides normal .read and .write?
mmap, sendfile, etc
If you want crypto in ext4, you should probably look at the google's recent patch to Linux just for that,
http://www.phoronix.com/scan.php?page=news_item&px=EXT4-Encryption-Support
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Today I just realized in my Ubuntu Linux, I can mount and store files on my newly purchased hard drive as a raw device without a file system. (as long as I partitioned the disk correctly)
So, I am not sure if my below statement is correct, looking for expert to answer:
Looks like it's not required to create a file system on a disk in order to use it in Linux? Is it correct?
I have some very basic understanding of how a file system works. In Linux, is the concept of "inode" a file system feature or a Linux feature?
I understand that the "inode" file system works unlike NTFS or FAT32 that it tries to spread out the data across the disk so that Linux/Unix doesn't need as Windows like "defgramentation" program to keep data in consecutive chunks. My question is, if I am storing my data on a raw device without a file system, and if "inode" is a file system feature not a Linux feature, what will the actual data layout look like on the raw device then?
Thanks in advance
What are the system calls that can be used to delete a file on Linux? I am not referring to just the system calls used by the libc-wrapper(which in-turn are used by command line tools).
Other than unlink and unlinkat what are the system calls that could be used to delete files on a Linux machine?
rename() and renameat() can be used to delete a file by renaming another file over it.
If you consider making a file empty to be a form of deletion, a variety of system calls, including truncate() and open() with O_TRUNC, can do that.
I want to create a file from kernel and this file must be accessed from user space. Other ways of communication (for example ioctl) is not suitable, because the user space application needs only files, and I don't have the source code of it.
I need to do this on MAC. If I were using Linux, I would use sysfs for it, but on MacOs they dont have sysfs, so I decided to end up with devfs
I created the sample soultion and everything works great, but the problem is that the device file (devfs file) does not have size. The user-space code checks for file size and skips this file. I know how big the size will be, but I dont know how to set it to devfs file.
I dont want to create the file in real filesystem, because it can be quite big. All I want is to redirect reads and writes to my internal functions.
FUSE (http://en.wikipedia.org/wiki/Filesystem_in_Userspace) would be ideal for be, but this involves user-space daemon.
Any suggestions?
Say I want to synchronize data buffers of a file system to disk (in my case the one of an USB stick partition) on a linux box.
While searching for a function to do that I found the following
DESCRIPTION
sync() causes all buffered modifications to file metadata and
data to be written to the underlying file sys‐
tems.
syncfs(int fd) is like sync(), but synchronizes just the file system
containing file referred to by the open file
descriptor fd.
But what if the file system has no file on it that I can open and pass to syncfs? Can I "abuse" the dot file? Does it appear on all file systems?
Is there another function that does what I want? Perhaps by providing a device file with major / minor numbers or some such?
Yes I think you can do that. The root directory of your file system will have at least one inode for your root directory. You can use the .-file to do that. Play also around with ls -i to see the inode numbers.
Is there a possibility to avoid your problem by mounting your file system with sync? Does performance issues hamper? Did you have a look at remounting? This can sync your file system as well in particular cases.
I do not know what your application is, but I suffered problems with synchronization of files to a USB stick with the FAT32-file system. It resulted in weird read and write errors. I can not imagine any other valid reason why you should sync an empty file system.
From man 8 sync description:
"sync writes any data buffered in memory out to disk. This can include (but is not
limited to) modified superblocks, modified inodes, and delayed reads and writes. This
must be implemented by the kernel; The sync program does nothing but exercise the sync(2)
system call."
So, note that it's all about modification (modified inode, superblocks etc). If you don't have any modification, it don't have anything to sync up.
In Linux file system, there are files such as /dev/zero and /dev/random which are not real files on hard disk.
Is there any way that we can create a similar file and tell it to get ouput from executing a program?
For example, can I create file, say /tmp/tarfile, such that any program reading it actually gets the output from the execution of a different program (/usr/bin/tar ...)?
It is possible to create such a file/program, but it would require creation of a special filesystem in order to insert hooks into the VFS so that accesses can be detected and handled properly.