Re: [PATCH] seq_file: Allow private data to be supplied on seq_open

From: Rob Jones
Date: Wed Aug 06 2014 - 11:57:07 EST

Does anyone have any feedback on this?

I would have thought it was quite uncontroversial.

On 29/07/14 18:39, Rob Jones wrote:
Create a function seq_open_priv() that is identical to seq_open() except
that it accepts a void * parameter that it stores in the private field
of the struct seq_file.

Document seq_open_priv().

Some consumers of the seq_file interface need to pass data to their
iterators that is best obtained at the time the seq_file is opened.

At the moment these consumers have to obtain the struct seq_file pointer
(stored by seq_open() in file->private_data) and then store a pointer to
their own data in the private field of the struct seq_file so that it
can be accessed by the iterator functions.

Although this is not a long piece of code it is unneccessary boilerplate.

seq_open() remains in place and its behaviour remains unchanged so no
existing code should be broken by this patch.

Signed-off-by: Ian Molton <ian.molton@xxxxxxxxxxxxxxx>
Signed-off-by: Rob Jones <rob.jones@xxxxxxxxxxxxxxx>
---
Documentation/filesystems/seq_file.txt | 9 +++++++++
fs/seq_file.c | 30 ++++++++++++++++++++++++++++--
include/linux/seq_file.h | 1 +
3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/seq_file.txt b/Documentation/filesystems/seq_file.txt
index a1e2e0d..128ffee 100644
--- a/Documentation/filesystems/seq_file.txt
+++ b/Documentation/filesystems/seq_file.txt
@@ -226,6 +226,15 @@ 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 function seq_open_priv() which behaves identically to
+seq_open() except that it takes an additional void * parameter that it
+stores in the private field of the seq_file structure, thereby making it
+available to the start function and thus all subsequent iterator functions
+Note that a corresponding wrapper function for seq_release() may need to
+be created to free any resources allocated by an open function that uses
+this capability (although, for simple cases, seq_release_private() may
+suffice).
+
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:
diff --git a/fs/seq_file.c b/fs/seq_file.c
index 1d641bb..9a0db94 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -31,9 +31,10 @@ static void seq_set_overflow(struct seq_file *m)
}

/**
- * seq_open - initialize sequential file
+ * seq_open_priv - initialize sequential file with private data
* @file: file we initialize
* @op: method table describing the sequence
+ * @d: private data to be made available to the iterator functions
*
* seq_open() sets @file, associating it with a sequence described
* by @op. @op->start() sets the iterator up and returns the first
@@ -43,8 +44,12 @@ static void seq_set_overflow(struct seq_file *m)
* ERR_PTR(error). In the end of sequence they return %NULL. ->show()
* returns 0 in case of success and negative number in case of error.
* Returning SEQ_SKIP means "discard this element and move on".
+ *
+ * Supplying @d allows data that is only available at the time the file
+ * is opened to be supplied to @op->start() (and thereby to @op->next()
+ * as well).
*/
-int seq_open(struct file *file, const struct seq_operations *op)
+int seq_open_priv(struct file *file, const struct seq_operations *op, void *d)
{
struct seq_file *p = file->private_data;

@@ -57,6 +62,7 @@ int seq_open(struct file *file, const struct seq_operations *op)
memset(p, 0, sizeof(*p));
mutex_init(&p->lock);
p->op = op;
+ p->private = d;
#ifdef CONFIG_USER_NS
p->user_ns = file->f_cred->user_ns;
#endif
@@ -80,6 +86,26 @@ int seq_open(struct file *file, const struct seq_operations *op)
file->f_mode &= ~FMODE_PWRITE;
return 0;
}
+EXPORT_SYMBOL(seq_open_priv);
+
+/**
+ * seq_open - initialize sequential file
+ * @file: file we initialize
+ * @op: method table describing the sequence
+ *
+ * seq_open() sets @file, associating it with a sequence described
+ * by @op. @op->start() sets the iterator up and returns the first
+ * element of sequence. @op->stop() shuts it down. @op->next()
+ * returns the next element of sequence. @op->show() prints element
+ * into the buffer. In case of error ->start() and ->next() return
+ * ERR_PTR(error). In the end of sequence they return %NULL. ->show()
+ * returns 0 in case of success and negative number in case of error.
+ * Returning SEQ_SKIP means "discard this element and move on".
+ */
+int seq_open(struct file *file, const struct seq_operations *op)
+{
+ return seq_open_priv(file, op, NULL);
+}
EXPORT_SYMBOL(seq_open);

static int traverse(struct seq_file *m, loff_t offset)
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index 52e0097..fce87af 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -95,6 +95,7 @@ static inline void seq_setwidth(struct seq_file *m, size_t size)
void seq_pad(struct seq_file *m, char c);

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


--
Rob Jones
Codethink Ltd
mailto:rob.jones@xxxxxxxxxxxxxxx
tel:+44 161 236 5575
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