Please explain this statement:
directory_path=dentry_path_raw(lower_dentry,buf,buflen);
also give an example of how does it work
What fields does struct dentry contain?
What does the s_root field point to?
What does the field s_root contain?
You can check the fields of dentry here. dentry_path_raw puts the full path of the directory entry from the root of the file system into the buffer. s_root is a member of super block defined in linux/fs.h and is the root of the file system, which is a dentry.
char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen)
The above function returns full pathname from root of the file system into the buffer.Each field is explained as below.
#dentry-dentry of a file to lookup
#buf-buffer to store path to
#buflen-length of #buf
For more detail check fs/dcache.c file.
Related
I have a vnode and I want to get the protection bits (sticky bit, write bit, etc.) of its associated file. I know I can get the vnode's vattr struct which contains a field called u_short va_mode; /* files access mode and type */.
However, I was wondering if this is the same thing as the vnode's associated file's mode_t st_mode; /* inode protection mode */. Does anybody know if these would be the same for a given vnode and its associated file?
My final goal is to see if a sticky bit is set for a vnode's assocated file.
The st_mode in struct stat is set from the vnode's va_mode, with the addition of a flag based on vnode type (eg VREG or VDIR). Take a look at http://fxr.watson.org/fxr/source/kern/vfs_vnops.c#L1428; that's the piece of code that fills in the struct stat.
I am pulling data from a server and one of the folder name is longer than 256 bytes, so my CentOS is throwing an error that the name is too long. I have searched all over the internet but couldn't find any way to create a folder/file name with size of over 256 bytes under ext2/ext3/ext4 file systems.
However, One solution had suggested to create reiserfs file system alongside ext4 to handle files\folder with longer names. This solution might work, but I just read in one of the book that it is possible to extend the limit of filename size from 255 characters to 1012 characters if needed.
The maximal file name size is 255 characters. This limit could be extended to `1012` if needed.
Source: Google
But I couldn't find any website that explains how the filesystem could be modified to extend the size to 1012?
Can someone please help me with that?
Don't know where 1012 was found (mentioned in http://e2fsprogs.sourceforge.net/ext2intro.html - Design and Implementation of the Second Extended Filesystem, ISBN 90-367-0385-9. 1995), but in modern Linux kernel file name is fixed in struct ext2_dir_entry_2 with maximum of 255 chars (bytes):
https://elixir.bootlin.com/linux/v4.10/source/fs/ext2/ext2.h#L600
/*
* The new version of the directory entry. Since EXT2 structures are
* stored in intel byte order, and the name_len field could never be
* bigger than 255 chars, it's safe to reclaim the extra byte for the
* file_type field.
*/
struct ext2_dir_entry_2 {
__le32 inode; /* Inode number */
__le16 rec_len; /* Directory entry length */
__u8 name_len; /* Name length */
__u8 file_type;
char name[]; /* File name, up to EXT2_NAME_LEN */
};
There was struct ext2_dir_entry with longer file name length, but extra byte of name_len was redefined as file_type.
__le16 name_len; /* Name length */
So, current maximum file name length for ext2 is 255
https://elixir.bootlin.com/linux/v4.10/source/include/linux/ext2_fs.h#L22
#define EXT2_NAME_LEN 255
https://elixir.bootlin.com/linux/v4.10/source/fs/ext2/namei.c#L62
if (dentry->d_name.len > EXT2_NAME_LEN)
return ERR_PTR(-ENAMETOOLONG);
Same for ext3/ext4:
https://elixir.bootlin.com/linux/v4.10/source/fs/ext4/ext4.h#L1892
/*
* Structure of a directory entry
*/
#define EXT4_NAME_LEN 255
https://elixir.bootlin.com/linux/v4.10/source/fs/ext4/namei.c
* `len <= EXT4_NAME_LEN' is guaranteed by caller.
if (namelen > EXT4_NAME_LEN)
return NULL;
if (dentry->d_name.len > EXT4_NAME_LEN)
return ERR_PTR(-ENAMETOOLONG);
Ondisk format is described with 8 bit file_name too (file_type uses only 3 bits in older docs - EXT2_FT_MAX, but modern driver will not handle 255+ file names. ext4 has extra FT of 0xde):
http://www.nongnu.org/ext2-doc/ext2.html#IFDIR-NAME-LEN "4.1.3. name_len - 8bit unsigned value indicating how many bytes of character data are contained in the name."
http://cs.smith.edu/~nhowe/262/oldlabs/ext2.html#direntry "The file_type field indicates what kind of file the entry is referring to... The maximum length of a file name is EXT2_NAME_LEN, which is usually 255."
https://oss.oracle.com/projects/ocfs2/dist/documentation/disklayout.pdf#page=16 "__u8 name_len"
See https://en.wikipedia.org/wiki/Comparison_of_file_systems#Limits, not a lot of filesystems handle filenames of more than 255 bytes.
While it may not be a direct reply to your question, I do not think you should try to go this route (changing the maximum length).
From what server do you retrieve your files? Why not changing their name when doing the retrieval?
Are you sure that the whole path name is relevant, if you extract the first or last 250 characters isn't it enough to reference each file without ambiguity?
You have many options, depending on your constraints:
either generate "random" names (or sequential ones), and just store in a text file the mapping between old true name and the new fake name
or split the initial name in 250 characters path elements (or something like that) and create intermediate directories with them
You can find similar discussion at https://serverfault.com/questions/264339/how-to-increase-filename-size-limit-on-ext3-on-ubuntu
I'm reading the Understand the Linux Kernel book, and it says that the list of file_lock is stored in the file's inode (of field i_flock).
But in the sys_flock() of Linux 2.6.11.12, which will eventually call flock_lock_file(). It uses filp->f_dentry->d_inode->i_flock to get the list of file_lock and filp->f_dentry is an dentry of the directory which "contains" the file.
int flock_lock_file(struct file *filp, struct file_lock *new_fl) {
// ...
struct inode * inode = filp->f_dentry->d_inode;
// ...
}
Suppose that the file_lock list are linked with filp->f_dentry->d_inode->i_flock, What will happen when a hard link exists:
/some_path/foo/file.txt
/another_path/bar/file_link
and file_link is a hard link to file.txt
When we use this two path to open the same file, sys_open() will set filp->f_dentry to foo and bar separately, isn't it? If my guess is right, how file_lock can work?
The file_lock is indeed stored in the inode of the corresponding file.
An inode can be referred by several directory entries, which linked in inode's i_dentry field. Even for a unique file might have different filp->f_dentry, filp->f_dentry->d_inode is all refer to the same inode.
I am implementing a system call for a guest operating system(Instructional OS) which could be used with below format in a user program:
syscall( syscall number , "string to be printed on console");
this system call should print the second parameter that is a string on console.
Since message can be char[ ] or char * it is treated as address in user space. and to access some address from user space in kernel space, I need to use user space memory access functions like- access_ok() and copy_from_user(). These functions are defined in uaccess.h header file of /usr/src/linux 3.2.0-65/arch/x86/include/asm/ folder.
i include below line in my program to access functions defined in uaccess.h :
#include<linux/uaccess.h>
But its not working.
On MAKE, its showing an error like
syscall.c:39:27: error: linux/uaccess.h: No such file or directory
Can anyone help me ...?
It is said that fork system call creates a clone of the calling process, and then (usually) the child process issues execve system call to change its image and running a new process. Why this two-step?
BTW, what does execve stand for?
The reason for the two-step is flexibility. Between the two steps you can modify the context of the child process that the newly exec'ed program will inherit.
Some things you may want to change are:
File descriptors
User/group ID
Process group and session IDs
Current directory
Resource limits
Scheduling priority and affinity
File creation mask (umask)
If you did not split up fork and exec and instead had a single spawn-like system call, it would need to take arguments for each of these process attributes if you wanted them set differently in a child process. For example, see the argument list to CreateProcess in the Windows API.
With fork/exec, you change whatever inheritable process attributes you want to in the child before you exec the new program.
Setting up file descriptors is one of the more common things to change in a child's process context. If you want to capture the output of a program, you will typically create a pipe in the parent with the pipe(2) system call, and after fork(2)ing, you will close the write end in the parent process and close the read end in the child process before calling execve(2). (You'll also use dup(2) to set the child end of the pipe to be file descriptor 1 (stdout)). This would either be impossible or restrictive in a single system call.
exec: Execute new process
v : use array of arguments
e: Specify as well the environment
Other variations of exec abound:
int execl(const char *path, const char *arg, ...);
int execlp(const char *file, const char *arg, ...);
int execle(const char *path, const char *arg,..., char * const envp[]);
int execv(const char *path, char *const argv[]);
int execvp(const char *file, char *const argv[]);
l: list arg on function
p: use $PATH to locate executable file
Each step is relatively simple.
In Unix, your process has two parts -- a read-only memory area with the application code ("text") and the read-write memory area ("data").
A fork clones the read-write area, leaving the text page alone. You now have two processes running the same code. They differ by a register value -- the return value from fork -- which separates parent from child.
An exec replaces the text page, leaving the data page alone. There are many forms of exec, depending on how much environment information you're passing to it. See http://linux.die.net/man/3/exec for an additional list of variants.
The "exec" family of functions replace the current process image(from where it is called) with a new process image, so the calling image is replaced by the new process image. For eg. if you were to run the 'ls' command from a shell(/bin/sh or /bin/csh) then the shell would fork to a new process which would then execute ls. Once the ls command exits it returns control to the parent process, which in this example is the shell.
If there were no fork functionality then the shell would be replaced by the 'ls' process which upon exit would leave you with an inaccessible terminal since the shell's image in memory was replaced upon the exec call to ls.
For variations in the 'exec' family look at 0x6adb015's answer.
What does execve stand for?
The 6 variations of the exec functions in C are are exec{l,v}{,e,p}. See function prototypes below for details.
Command-line arguments
v - Command-line arguments are passed to the function as an array (vector) of pointers.
l - Command-line arguments are passed individually (a list) to the function.
Environment variables (optional)
e - An array of pointers to environment variables is explicitly passed to the new process image
Locate the file to be executed (optional)
p - Uses the PATH environment variable to find the file named in the file argument to be executed
int execl (char const *path, char const *arg0, ...);
int execle(char const *path, char const *arg0, ..., char const *envp[]);
int execlp(char const *file, char const *arg0, ...);
int execv (char const *path, char const *argv[]);
int execve(char const *path, char const *argv[], char const *envp[]);
int execvp(char const *file, char const *argv[]);
Source