WSL2-Linux-Kernel/Documentation/filesystems/seq_file.rst

.. SPDX-License-Identifier: GPL-2.0

======================
The seq_file Interface
======================

	Copyright 2003 Jonathan Corbet <corbet@lwn.net>

	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 <linux/seq_file.h>. 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::

    dd if=/proc/sequence of=out1 count=1
    dd if=/proc/sequence skip=1 of=out2 count=1

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::

    -	entry = create_proc_entry("sequence", 0, NULL);
    -	if (entry)
    -		entry->proc_fops = &ct_file_ops;
    +	entry = proc_create("sequence", 0, NULL, &ct_file_ops);

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::

	static void *ct_seq_start(struct seq_file *s, loff_t *pos)
	{
	        loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
	        if (! spos)
	                return NULL;
	        *spos = *pos;
	        return spos;
	}

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::

	static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
	{
	        loff_t *spos = v;
	        *pos = ++*spos;
	        return spos;
	}

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::

	static void ct_seq_stop(struct seq_file *s, void *v)
	{
	        kfree(v);
	}

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

	static int ct_seq_show(struct seq_file *s, void *v)
	{
	        loff_t *spos = v;
	        seq_printf(s, "%lld\n", (long long)*spos);
	        return 0;
	}

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::

	static const struct seq_operations ct_seq_ops = {
	        .start = ct_seq_start,
	        .next  = ct_seq_next,
	        .stop  = ct_seq_stop,
	        .show  = ct_seq_show
	};

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::

	seq_putc(struct seq_file *m, char c);
	seq_puts(struct seq_file *m, const char *s);
	seq_escape(struct seq_file *m, const char *s, const char *esc);

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::

	int seq_path(struct seq_file *m, const struct path *path,
		     const char *esc);
	int seq_path_root(struct seq_file *m, const struct path *path,
			  const struct path *root, const char *esc)

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::

	bool seq_has_overflowed(struct seq_file *m);

and avoid further seq_<output> 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::

	static int ct_open(struct inode *inode, struct file *file)
	{
		return seq_open(file, &ct_seq_ops);
	}

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.::

	static int ct_open(struct inode *inode, struct file *file)
	{
		return seq_open_private(file, &ct_seq_ops,
					sizeof(struct mystruct));
	}

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.::

	static int ct_open(struct inode *inode, struct file *file)
	{
		struct mystruct *p =
			__seq_open_private(file, &ct_seq_ops, sizeof(*p));

		if (!p)
			return -ENOMEM;

		p->foo = bar; /* initialize my stuff */
			...
		p->baz = true;

		return 0;
	}

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::

	static const struct file_operations ct_file_ops = {
	        .owner   = THIS_MODULE,
	        .open    = ct_open,
	        .read    = seq_read,
	        .llseek  = seq_lseek,
	        .release = seq_release
	};

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::

	static int ct_init(void)
	{
	        struct proc_dir_entry *entry;

	        proc_create("sequence", 0, NULL, &ct_file_ops);
	        return 0;
	}

	module_init(ct_init);

And that is pretty much it.


seq_list
========

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

	struct list_head *seq_list_start(struct list_head *head,
	       		 		 loff_t pos);
	struct list_head *seq_list_start_head(struct list_head *head,
			 		      loff_t pos);
	struct list_head *seq_list_next(void *v, struct list_head *head,
					loff_t *ppos);

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::

	int single_open(struct file *file,
	                int (*show)(struct seq_file *m, void *p),
	                void *data);

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.