[patch 3/9] procfs: Add ability to plug in auxiliary fdinfo providers

This patch brings ability to print out auxiliary data associated
with file in procfs interface /proc/pid/fdinfo/fd.

In particular further patches make eventfd, evenpoll, signalfd
and fsnotify to print additional information complete enough
to restore these objects after checkpoint.

To simplify the code we add show_fdinfo callback inside
struct file_operations (as Al and Pavel are proposing).

Signed-off-by: Cyrill Gorcunov <redacted>
CC: Pavel Emelyanov <redacted>
CC: Al Viro <viro@ZenIV.linux.org.uk>
CC: Alexey Dobriyan <redacted>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: James Bottomley <redacted>
CC: "Aneesh Kumar K.V" <redacted>
CC: Alexey Dobriyan <redacted>
CC: Matthew Helsley <redacted>
CC: "J. Bruce Fields" <redacted>
CC: "Aneesh Kumar K.V" <redacted>
---
 fs/proc/fd.c       |    5 +++++
 include/linux/fs.h |    3 +++
 2 files changed, 8 insertions(+)

Index: linux-2.6.git/fs/proc/fd.c
===================================================================

--- linux-2.6.git.orig/fs/proc/fd.c
+++ linux-2.6.git/fs/proc/fd.c

@@ -23,8 +23,13 @@ struct proc_fdinfo {
 static int seq_show(struct seq_file *m, void *v)
 {
 	struct proc_fdinfo *fdinfo = container_of(m, struct proc_fdinfo, m);
+
 	seq_printf(m, "pos:\t%lli\nflags:\t0%o\n",
 		   (long long)fdinfo->fd_file->f_pos, fdinfo->f_flags);
+
+	if (fdinfo->fd_file->f_op->show_fdinfo)
+		return fdinfo->fd_file->f_op->show_fdinfo(m, fdinfo->fd_file);
+
 	return 0;
 }
 

Index: linux-2.6.git/include/linux/fs.h
===================================================================

--- linux-2.6.git.orig/include/linux/fs.h
+++ linux-2.6.git/include/linux/fs.h

@@ -1775,6 +1775,8 @@ struct block_device_operations;
 #define HAVE_COMPAT_IOCTL 1
 #define HAVE_UNLOCKED_IOCTL 1
 
+struct seq_file;
+
 struct file_operations {
 	struct module *owner;
 	loff_t (*llseek) (struct file *, loff_t, int);

@@ -1803,6 +1805,7 @@ struct file_operations {
 	int (*setlease)(struct file *, long, struct file_lock **);
 	long (*fallocate)(struct file *file, int mode, loff_t offset,
 			  loff_t len);
+	int (*show_fdinfo)(struct seq_file *m, struct file *f);
 };
 
 struct inode_operations {