Struct file linux kernel

Реализация struct file в ядре

Всем снова привет.

В ядре, в заголовочном файле fs.h есть struct file, которая ответственна собственно за сам файл.
Если тут есть специалисты по системному программированию, прошу подсказать, каким образом система понимает что файл является директорией?
Какой флаг, структура или что-то ещё ответственны за это?

Я понимаю что есть системный вызов readdir, но не очень понимаю где эта информация хранится в ядре.
Подскажите пожалуйста, куда копать, какие структуры изучать?

Реализация семафора в ядре Linux
SUSE Linux 11.1 ядро 2.6.27.7-9 * интересует где находится реализация семафора и как.

Struct sockaddr vs. struct sockaddr_in
Вопрос,связанный с переносимостью кода на другие платформы. Читаю эту книжку.

typedef struct Foo или struct Foo
В чём разница между: typedef struct < int a; >Foo; и struct Foo

Сплайсинг в ядре
Добрый день. Интересует реализация сплайсинга в драйвере ядра, скажем, для функции sys_mkdir. Я.

Сама структура inode и что она из себя представляет, а то grep очень много результатов даёт.
Например, я хочу внести изменения и добавить новый элемент в само понятие файла. Это возможно?
Добавить какой-нибудь параметр и т.д.
Я ищу в файлах ядра, но там ничего не вижу.

Похожее нашел в /usr/include/linux/fs.h

Добавлено через 2 минуты
Здесь пример использования поля (проверка на директория, или нет), для выставления нужных обработчиков для inod’ы:
http://lxr.free-electrons.com/. =3.6#L3025
Кури.

Добавлено через 2 минуты

Если интересно, можете глянуть на моем гитхабе, как я написал простой драйвер в ядре линукс для символьного девайса. В коде учавствует и struct file и inode. Могут при необходимости дать подробный комментарий по коду.

А здесь я сделал простое чтение файла (девайса /dev/urandom)
https://github.com/thatskriptk. ernel_IO.c

Хорошо, всё понял.
Есть какая-нибудь удобночитаемая книга по структуре ядра?
У меня после работы не очень много времени, чтобы читать очередного Страуструпа.

Драйвер заценил, смотрю.

Решение

1) К превиликому сожалению, самая лучшая книга по ядру — это исходники. Лучший сайт по исходникам ядра, я считаю

Можете искать по идентификатору, например вводите struct file и вперед, смотреть как и что.

2) Начнем с русского языка. Очень хорошо описал написание дров, модулей и проччих вещей в ядре — Олег Цилюрик. Очень редко можно встретить более менее актуальный материал по ядру на русском! Очень рекомендую.
http://rus-linux.net/MyLDP/BOO. index.html

3) На opennet.ru тоже есть материал на русском, но там немного посложнее, средний уровень подготовки я считаю, но мне как новичку помогло.

4) Теперь перейдем к топовым книгам. Коротенькая, информативная, стандарт для разработчиков ядра, это конечно же Linux Device Driver, 3 издание и Adison.Wesley.Linux.Kernel.Development.3rd.Edition . По первой (LDD) я делал мноиге примеры, очень рекомендую ее. Вторая как раз по структуре ядра, идеальная прям, открываете оглавление и читаете то что интересно.

Первая книга есть в октрытом доступе, вот оффициальная ссылка.
http://lwn.net/Kernel/LDD3/

5) По написанию дров под линь, очень мало современных материалов и книг, очень прям мало. Потому что ядро развивается очень быстро, каждый день сотни коммитов, поэтому уследить трудно. Но есть еще одна хорошенькая книга. Называется Essential Linux Device Drivers.

6) Фундаментальный труд по ядру, это конечно же книга — Undestanding the linux kernel,3rd edition (почти 1000 страниц)

Все же рекомендую бегло пройтись по Adison.Wesley.Linux.Kernel.Development, ну и по ЦИрюльнику. Вторую я приложил в виде pdf (добрые люди все в один файл собрали)

Если необходимо, могу подкинуть еще ссылок.

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In Linux, how can I get the filename from the «struct file» structure, while stepping thru the kernel with kgdb?

I’m trying to view the filename via kgdb, so I cannot call functions and macros to get it programatically. I need to find it by manually inspecting data structures.

Like if I had a breakpoint here in gdb, how could I look around with gdb and find the filename?

I’ve tried looking around in filp.f_path , filp.f_inode , etc. I cannot see the filename anywhere.

4 Answers 4

You can get the filename from struct file *filp with filp->f_path.dentry->d_iname .

To get the full path call dentry_path_raw(filp->f_path.dentry,buf,buflen) .

In the Linux kernel, the file structure is essentially how the kernel «sees» the file. The kernel is not interested in the file name, just the inode of the open file. This means that all of the other information which is important to the user is lost.

EDIT: This answer is wrong. You can get the dentry using filp->f_path.dentry . From there you can get the name of the dentry or the full path using the relevant FS flags.

The path is stored in the file->f_path structure as it’s name implies. Just not in a plain-text form, but parsed into objects that are more useful for kernel operation, namely a chain of dentry structures, and the vfsmount structure pointing to the root of the current subtree.

You can use the d_path function to regenerate a human-readable path name for a struct path like file->f_path. Note that however this is not a cheap operation and it may slow down your workload significantly.

The above mentioned issues about open but unlinked files, multiple hardlinks and similar are valid for mapping from and inode to a pathname, and open file always has a path associated with it. If the file has been unlinked d_path will prepend a » (deleted)» to the name, and if the filename it has been opened with has been changed to something else using rename since it was opened d_path will not print the original name, but the current name of the entry that was used for opening it.

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struct file in linux driver

I am currently learning how to write Linux device drivers and I have trouble understanding «struct file«. I am using the book Linux Device Drivers 3rd edition to help me out.

This is what I understood.

a. struct file represents an open file thus, when open is called in the device driver module, the kernel will create a struct file that includes everything related to the device driver.

b. If you want to pass around this instance of the device driver then one has to pass a pointer to the particular struct file that was created by the kernel after open()

c. file->private_data will always return a pointer to the device.

Another question related to this is the field «f_pos«. The book says that the driver can read this value if it wants to know the current position in the file. This is what I understand from it.

d. If struct foo_dev and if the total amount of memory used by this driver to store data is X then f_pos points to the current position in that block of memory reserved by the driver.

How much of what I understood is right and please correct me where I am wrong.

2 Answers 2

The struct file is created by the kernel and represents the kernels view of your device it allows the kernel to map from a file handle to the device.

The struct file only contains the data the kernels upper layers needs, this is unlikely to be everything you need for your driver, if you need extra storage to track your devices status (and generally you will) you need to allocate the memory for your structure yourself either in the open function or more normally when you detect your hardware.

If you do allocate storage then you can use the file->private_data to allow you to get from the struct file thats passed to your driver by read / write / etc to your structure.

How the file->private_data is used is up to the driver, the kernel doesn’t touch it. Its just there for the drivers use.

The f_pos field is a legacy from the kernel using the same struct file for devices and files. It is an index into a file were the next operation will happen, it depends on your device if this makes sense, if your device supports some form of random access (say a ram device) then using f_pos and implementing lseek might make sense, if you hardware is sequential then f_pos is normally irrelevant.

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The seq_file Interface¶

This file is originally from the LWN.net Driver Porting series at https://lwn.net/Articles/driver-porting/

There are numerous ways for a device driver (or other kernel component) to provide information to the user or system administrator. One useful technique is the creation of virtual files, in debugfs, /proc or elsewhere. Virtual files can provide human-readable output that is easy to get at without any special utility programs; they can also make life easier for script writers. It is not surprising that the use of virtual files has grown over the years.

Creating those files correctly has always been a bit of a challenge, however. It is not that hard to make a virtual file which returns a string. But life gets trickier if the output is long — anything greater than an application is likely to read in a single operation. Handling multiple reads (and seeks) requires careful attention to the reader’s position within the virtual file — that position is, likely as not, in the middle of a line of output. The kernel has traditionally had a number of implementations that got this wrong.

The 2.6 kernel contains a set of functions (implemented by Alexander Viro) which are designed to make it easy for virtual file creators to get it right.

The seq_file interface is available via
. There are three aspects to seq_file:

An iterator interface which lets a virtual file implementation step through the objects it is presenting.

Some utility functions for formatting objects for output without needing to worry about things like output buffers.

A set of canned file_operations which implement most operations on the virtual file.

We’ll look at the seq_file interface via an extremely simple example: a loadable module which creates a file called /proc/sequence. The file, when read, simply produces a set of increasing integer values, one per line. The sequence will continue until the user loses patience and finds something better to do. The file is seekable, in that one can do something like the following:

Then concatenate the output files out1 and out2 and get the right result. Yes, it is a thoroughly useless module, but the point is to show how the mechanism works without getting lost in other details. (Those wanting to see the full source for this module can find it at https://lwn.net/Articles/22359/).

Deprecated create_proc_entry¶

Note that the above article uses create_proc_entry which was removed in kernel 3.10. Current versions require the following update:

The iterator interface¶

Modules implementing a virtual file with seq_file must implement an iterator object that allows stepping through the data of interest during a “session” (roughly one read() system call). If the iterator is able to move to a specific position — like the file they implement, though with freedom to map the position number to a sequence location in whatever way is convenient — the iterator need only exist transiently during a session. If the iterator cannot easily find a numerical position but works well with a first/next interface, the iterator can be stored in the private data area and continue from one session to the next.

A seq_file implementation that is formatting firewall rules from a table, for example, could provide a simple iterator that interprets position N as the Nth rule in the chain. A seq_file implementation that presents the content of a, potentially volatile, linked list might record a pointer into that list, providing that can be done without risk of the current location being removed.

Positioning can thus be done in whatever way makes the most sense for the generator of the data, which need not be aware of how a position translates to an offset in the virtual file. The one obvious exception is that a position of zero should indicate the beginning of the file.

The /proc/sequence iterator just uses the count of the next number it will output as its position.

Four functions must be implemented to make the iterator work. The first, called start() , starts a session and takes a position as an argument, returning an iterator which will start reading at that position. The pos passed to start() will always be either zero, or the most recent pos used in the previous session.

For our simple sequence example, the start() function looks like:

The entire data structure for this iterator is a single loff_t value holding the current position. There is no upper bound for the sequence iterator, but that will not be the case for most other seq_file implementations; in most cases the start() function should check for a “past end of file” condition and return NULL if need be.

For more complicated applications, the private field of the seq_file structure can be used to hold state from session to session. There is also a special value which can be returned by the start() function called SEQ_START_TOKEN; it can be used if you wish to instruct your show() function (described below) to print a header at the top of the output. SEQ_START_TOKEN should only be used if the offset is zero, however. SEQ_START_TOKEN has no special meaning to the core seq_file code. It is provided as a convenience for a start() funciton to communicate with the next() and show() functions.

The next function to implement is called, amazingly, next(); its job is to move the iterator forward to the next position in the sequence. The example module can simply increment the position by one; more useful modules will do what is needed to step through some data structure. The next() function returns a new iterator, or NULL if the sequence is complete. Here’s the example version:

The next() function should set *pos to a value that start() can use to find the new location in the sequence. When the iterator is being stored in the private data area, rather than being reinitialized on each start() , it might seem sufficient to simply set *pos to any non-zero value (zero always tells start() to restart the sequence). This is not sufficient due to historical problems.

Historically, many next() functions have not updated *pos at end-of-file. If the value is then used by start() to initialise the iterator, this can result in corner cases where the last entry in the sequence is reported twice in the file. In order to discourage this bug from being resurrected, the core seq_file code now produces a warning if a next() function does not change the value of *pos . Consequently a next() function must change the value of *pos , and of course must set it to a non-zero value.

The stop() function closes a session; its job, of course, is to clean up. If dynamic memory is allocated for the iterator, stop() is the place to free it; if a lock was taken by start() , stop() must release that lock. The value that *pos was set to by the last next() call before stop() is remembered, and used for the first start() call of the next session unless lseek() has been called on the file; in that case next start() will be asked to start at position zero:

Finally, the show() function should format the object currently pointed to by the iterator for output. The example module’s show() function is:

If all is well, the show() function should return zero. A negative error code in the usual manner indicates that something went wrong; it will be passed back to user space. This function can also return SEQ_SKIP, which causes the current item to be skipped; if the show() function has already generated output before returning SEQ_SKIP, that output will be dropped.

We will look at seq_printf() in a moment. But first, the definition of the seq_file iterator is finished by creating a seq_operations structure with the four functions we have just defined:

This structure will be needed to tie our iterator to the /proc file in a little bit.

It’s worth noting that the iterator value returned by start() and manipulated by the other functions is considered to be completely opaque by the seq_file code. It can thus be anything that is useful in stepping through the data to be output. Counters can be useful, but it could also be a direct pointer into an array or linked list. Anything goes, as long as the programmer is aware that things can happen between calls to the iterator function. However, the seq_file code (by design) will not sleep between the calls to start() and stop() , so holding a lock during that time is a reasonable thing to do. The seq_file code will also avoid taking any other locks while the iterator is active.

The iterater value returned by start() or next() is guaranteed to be passed to a subsequent next() or stop() call. This allows resources such as locks that were taken to be reliably released. There is no guarantee that the iterator will be passed to show(), though in practice it often will be.

Formatted output¶

The seq_file code manages positioning within the output created by the iterator and getting it into the user’s buffer. But, for that to work, that output must be passed to the seq_file code. Some utility functions have been defined which make this task easy.

Most code will simply use seq_printf(), which works pretty much like printk() , but which requires the seq_file pointer as an argument.

For straight character output, the following functions may be used:

The first two output a single character and a string, just like one would expect. seq_escape() is like seq_puts(), except that any character in s which is in the string esc will be represented in octal form in the output.

There are also a pair of functions for printing filenames:

Here, path indicates the file of interest, and esc is a set of characters which should be escaped in the output. A call to seq_path() will output the path relative to the current process’s filesystem root. If a different root is desired, it can be used with seq_path_root(). If it turns out that path cannot be reached from root, seq_path_root() returns SEQ_SKIP.

A function producing complicated output may want to check:

and avoid further seq_ calls if true is returned.

A true return from seq_has_overflowed means that the seq_file buffer will be discarded and the seq_show function will attempt to allocate a larger buffer and retry printing.

Making it all work¶

So far, we have a nice set of functions which can produce output within the seq_file system, but we have not yet turned them into a file that a user can see. Creating a file within the kernel requires, of course, the creation of a set of file_operations which implement the operations on that file. The seq_file interface provides a set of canned operations which do most of the work. The virtual file author still must implement the open() method, however, to hook everything up. The open function is often a single line, as in the example module:

Here, the call to seq_open() takes the seq_operations structure we created before, and gets set up to iterate through the virtual file.

On a successful open, seq_open() stores the struct seq_file pointer in file->private_data. If you have an application where the same iterator can be used for more than one file, you can store an arbitrary pointer in the private field of the seq_file structure; that value can then be retrieved by the iterator functions.

There is also a wrapper function to seq_open() called seq_open_private(). It kmallocs a zero filled block of memory and stores a pointer to it in the private field of the seq_file structure, returning 0 on success. The block size is specified in a third parameter to the function, e.g.:

There is also a variant function, __seq_open_private(), which is functionally identical except that, if successful, it returns the pointer to the allocated memory block, allowing further initialisation e.g.:

A corresponding close function, seq_release_private() is available which frees the memory allocated in the corresponding open.

The other operations of interest — read(), llseek(), and release() — are all implemented by the seq_file code itself. So a virtual file’s file_operations structure will look like:

There is also a seq_release_private() which passes the contents of the seq_file private field to kfree() before releasing the structure.

The final step is the creation of the /proc file itself. In the example code, that is done in the initialization code in the usual way:

And that is pretty much it.

seq_list¶

If your file will be iterating through a linked list, you may find these routines useful:

These helpers will interpret pos as a position within the list and iterate accordingly. Your start() and next() functions need only invoke the seq_list_* helpers with a pointer to the appropriate list_head structure.

The extra-simple version¶

For extremely simple virtual files, there is an even easier interface. A module can define only the show() function, which should create all the output that the virtual file will contain. The file’s open() method then calls:

When output time comes, the show() function will be called once. The data value given to single_open() can be found in the private field of the seq_file structure. When using single_open(), the programmer should use single_release() instead of seq_release() in the file_operations structure to avoid a memory leak.

© Copyright The kernel development community.

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