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

Rob Jones
2014-08-06T15:57:03Z

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@codethink.co.uk>
> Signed-off-by: Rob Jones <rob.jones@codethink.co.uk>
> ---
> 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@codethink.co.uk
tel:+44 161 236 5575