[RFC 1/5] seq_file: introduce seq_open_data helper

There are quite a few callers of seq_open that could be simplified by
setting the ->private member via the seq_open call instead of fetching
file->private_data afterwards.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
I've just included a few examples of possible users of this helper,
there are many more similar cases. As a bonus, the first two fix
potential NULL derefs (if one believes that seq_open can actually
fail).

seq_open_private would have been a better name, but that one is
already taken...

Documentation/filesystems/seq_file.txt | 9 +++++----
fs/seq_file.c                          | 9 ++++++++-
include/linux/seq_file.h               | 1 +
3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/seq_file.txt b/Documentation/filesystems/seq_file.txt
index 9de4303201e1..68571b8275d8 100644
--- a/Documentation/filesystems/seq_file.txt
+++ b/Documentation/filesystems/seq_file.txt

@@ -234,10 +234,11 @@ 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.
+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. Using the wrapper seq_open_data()
+allows you to set the initial value for that field.

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

diff --git a/fs/seq_file.c b/fs/seq_file.c
index eea09f6d8830..f2145cb6e23d 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c

@@ -45,7 +45,7 @@ static void *seq_buf_alloc(unsigned long size)
 *	Note: seq_open() will allocate a struct seq_file and store its
 *	pointer in @file->private_data. This pointer should not be modified.
 */
-int seq_open(struct file *file, const struct seq_operations *op)
+int seq_open_data(struct file *file, const struct seq_operations *op, void *data)

{
	struct seq_file *p;

@@ -59,6 +59,7 @@ int seq_open(struct file *file, const struct seq_operations *op)

	mutex_init(&p->lock);
	p->op = op;
+	p->private = data;

	// No refcounting: the lifetime of 'p' is constrained
	// to the lifetime of the file.

@@ -85,6 +86,12 @@ int seq_open(struct file *file, const struct seq_operations *op)

}
EXPORT_SYMBOL(seq_open);

+int seq_open(struct file *file, const struct seq_operations *op)
+{
+	return seq_open_data(file, op, NULL);
+}
+EXPORT_SYMBOL(seq_open_data);

+
static int traverse(struct seq_file *m, loff_t offset)
{
	loff_t pos = 0, index;

diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index ab437dd2e3b9..f5ff376fa62b 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h

@@ -107,6 +107,7 @@ void seq_pad(struct seq_file *m, char c);

char *mangle_path(char *s, const char *p, const char *esc);
int seq_open(struct file *, const struct seq_operations *);
+int seq_open_data(struct file *, const struct seq_operations *, void *);
ssize_t seq_read(struct file *, char __user *, size_t, loff_t *);
loff_t seq_lseek(struct file *, loff_t, int);
int seq_release(struct inode *, struct file *);
--
2.15.1