fs/seq_file.c: simplify seq_file iteration code and interface

The documentation for seq_file suggests that it is necessary to be able
to move the iterator to a given offset, however that is not the case.
If the iterator is stored in the private data and is stable from one
read() syscall to the next, it is only necessary to support first/next
interactions. Implementing this in a client is a little clumsy.

- if ->start() is given a pos of zero, it should go to start of
sequence.

- if ->start() is given the name pos that was given to the most recent
next() or start(), it should restore the iterator to state just
before that last call

- if ->start is given another number, it should set the iterator one
beyond the start just before the last ->start or ->next call.

Also, the documentation says that the implementation can interpret the
pos however it likes (other than zero meaning start), but seq_file
increments the pos sometimes which does impose on the implementation.

This patch simplifies the interface for first/next iteration and
simplifies the code, while maintaining complete backward compatability.
Now:

- if ->start() is given a pos of zero, it should return an iterator
placed at the start of the sequence

- if ->start() is given a non-zero pos, it should return the iterator
in the same state it was after the last ->start or ->next.

This is particularly useful for interators which walk the multiple
chains in a hash table, e.g. using rhashtable_walk*. See
fs/gfs2/glock.c and drivers/staging/lustre/lustre/llite/vvp_dev.c

A large part of achieving this is to *always* call ->next after ->show
has successfully stored all of an entry in the buffer. Never just
increment the index instead. Also:

- always pass &m->index to ->start() and ->next(), never a temp
variable

- don't clear ->from when ->count is zero, as ->from is dead when
->count is zero.

Some ->next functions do not increment *pos when they return NULL. To
maintain compatability with this, we still need to increment m->index in
one place, if ->next didn't increment it. Note that such ->next
functions are buggy and should be fixed. A simple demonstration is

dd if=/proc/swaps bs=1000 skip=1

Choose any block size larger than the size of /proc/swaps. This will
always show the whole last line of /proc/swaps.

This patch doesn't work around buggy next() functions for this case.

[[e-mail]: ensure ->from is valid]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: NeilBrown <[e-mail]>
Acked-by: Jonathan Corbet <[e-mail]> [docs]
Tested-by: Jann Horn <[e-mail]>
Cc: Alexander Viro <[e-mail]>
Cc: Kees Cook <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/filesystems/seq_file.txt

···
 
 The iterator interface
 
-Modules implementing a virtual file with seq_file must implement a simple
-iterator object that allows stepping through the data of interest.
-Iterators must be able to move to a specific position - like the file they
-implement - but the interpretation of that position is up to the iterator
-itself. A seq_file implementation that is formatting firewall rules, for
-example, could interpret position N as the Nth rule in the chain.
-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.
+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() takes a position as an argument and returns an iterator
-which will start reading at that position. For our simple sequence example,
+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)
···
 "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. 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.
+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.
 
 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
···
 	        return spos;
 	}
 
-The stop() function is called when iteration is complete; its job, of
-course, is to clean up. If dynamic memory is allocated for the iterator,
-stop() is the place to free it.
+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)
 	{

fs/seq_file.c

···
 
 static int traverse(struct seq_file *m, loff_t offset)
 {
-	loff_t pos = 0, index;
+	loff_t pos = 0;
 	int error = 0;
 	void *p;
 
 	m->version = 0;
-	index = 0;
+	m->index = 0;
 	m->count = m->from = 0;
-	if (!offset) {
-		m->index = index;
+	if (!offset)
 		return 0;
-	}
+
 	if (!m->buf) {
 		m->buf = seq_buf_alloc(m->size = PAGE_SIZE);
 		if (!m->buf)
 			return -ENOMEM;
 	}
-	p = m->op->start(m, &index);
+	p = m->op->start(m, &m->index);
 	while (p) {
 		error = PTR_ERR(p);
 		if (IS_ERR(p))
···
 		if (pos + m->count > offset) {
 			m->from = offset - pos;
 			m->count -= m->from;
-			m->index = index;
 			break;
 		}
 		pos += m->count;
 		m->count = 0;
-		if (pos == offset) {
-			index++;
-			m->index = index;
+		p = m->op->next(m, p, &m->index);
+		if (pos == offset)
 			break;
-		}
-		p = m->op->next(m, p, &index);
 	}
 	m->op->stop(m, p);
-	m->index = index;
 	return error;
 
 Eoverflow:
···
 {
 	struct seq_file *m = file->private_data;
 	size_t copied = 0;
-	loff_t pos;
 	size_t n;
 	void *p;
 	int err = 0;
···
 		size -= n;
 		buf += n;
 		copied += n;
-		if (!m->count) {
-			m->from = 0;
-			m->index++;
-		}
 		if (!size)
 			goto Done;
 	}
 	/* we need at least one record in buffer */
-	pos = m->index;
-	p = m->op->start(m, &pos);
+	m->from = 0;
+	p = m->op->start(m, &m->index);
 	while (1) {
 		err = PTR_ERR(p);
 		if (!p || IS_ERR(p))
···
 		if (unlikely(err))
 			m->count = 0;
 		if (unlikely(!m->count)) {
-			p = m->op->next(m, p, &pos);
-			m->index = pos;
+			p = m->op->next(m, p, &m->index);
 			continue;
 		}
 		if (m->count < m->size)
···
 		if (!m->buf)
 			goto Enomem;
 		m->version = 0;
-		pos = m->index;
-		p = m->op->start(m, &pos);
+		p = m->op->start(m, &m->index);
 	}
 	m->op->stop(m, p);
 	m->count = 0;
 	goto Done;
 Fill:
 	/* they want more? let's try to get some more */
-	while (m->count < size) {
+	while (1) {
 		size_t offs = m->count;
-		loff_t next = pos;
-		p = m->op->next(m, p, &next);
+		loff_t pos = m->index;
+
+		p = m->op->next(m, p, &m->index);
+		if (pos == m->index)
+			/* Buggy ->next function */
+			m->index++;
 		if (!p || IS_ERR(p)) {
 			err = PTR_ERR(p);
 			break;
 		}
+		if (m->count >= size)
+			break;
 		err = m->op->show(m, p);
 		if (seq_has_overflowed(m) || err) {
 			m->count = offs;
 			if (likely(err <= 0))
 				break;
 		}
-		pos = next;
 	}
 	m->op->stop(m, p);
 	n = min(m->count, size);
···
 		goto Efault;
 	copied += n;
 	m->count -= n;
-	if (m->count)
-		m->from = n;
-	else
-		pos++;
-	m->index = pos;
+	m->from = n;
 Done:
 	if (!copied)
 		copied = err;