[PATCH v3] proc,fcntl: introduce F_SET_DESCRIPTION

[Pascal Bouchareine <kalou@tfz.net>]

This command attaches a description to a file descriptor for
troubleshooting purposes. The free string is displayed in the
process fdinfo file for that fd /proc/pid/fdinfo/fd.

One intended usage is to allow processes to self-document sockets
for netstat and friends to report

Signed-off-by: Pascal Bouchareine <kalou@tfz.net>
---
 Documentation/filesystems/proc.rst |  3 +++
 fs/fcntl.c                         | 19 +++++++++++++++++++
 fs/file_table.c                    |  2 ++
 fs/proc/fd.c                       |  5 +++++
 include/linux/fs.h                 |  3 +++
 include/uapi/linux/fcntl.h         |  5 +++++
 6 files changed, 37 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 996f3cfe7030..ae8045650836 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -1918,6 +1918,9 @@ A typical output is::
 	flags:	0100002
 	mnt_id:	19
 
+An optional 'desc' is set if the process documented its usage of
+the file via the fcntl command F_SET_DESCRIPTION.
+
 All locks associated with a file descriptor are shown in its fdinfo too::
 
     lock:       1: FLOCK  ADVISORY  WRITE 359 00:13:11691 0 EOF
diff --git a/fs/fcntl.c b/fs/fcntl.c
index 2e4c0fa2074b..c1ef724a906e 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -319,6 +319,22 @@ static long fcntl_rw_hint(struct file *file, unsigned int cmd,
 	}
 }
 
+static long fcntl_set_description(struct file *file, char __user *desc)
+{
+	char *d;
+
+	d = strndup_user(desc, MAX_FILE_DESC_SIZE);
+	if (IS_ERR(d))
+		return PTR_ERR(d);
+
+	spin_lock(&file->f_lock);
+	kfree(file->f_description);
+	file->f_description = d;
+	spin_unlock(&file->f_lock);
+
+	return 0;
+}
+
 static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 		struct file *filp)
 {
@@ -426,6 +442,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	case F_SET_FILE_RW_HINT:
 		err = fcntl_rw_hint(filp, cmd, arg);
 		break;
+	case F_SET_DESCRIPTION:
+		err = fcntl_set_description(filp, argp);
+		break;
 	default:
 		break;
 	}
diff --git a/fs/file_table.c b/fs/file_table.c
index 656647f9575a..6673a48d2ea1 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -272,6 +272,8 @@ static void __fput(struct file *file)
 	eventpoll_release(file);
 	locks_remove_file(file);
 
+	kfree(file->f_description);
+
 	ima_file_free(file);
 	if (unlikely(file->f_flags & FASYNC)) {
 		if (file->f_op->fasync)
diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 81882a13212d..60b3ff971b2b 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -57,6 +57,11 @@ static int seq_show(struct seq_file *m, void *v)
 		   (long long)file->f_pos, f_flags,
 		   real_mount(file->f_path.mnt)->mnt_id);
 
+	spin_lock(&file->f_lock);
+	if (file->f_description)
+		seq_printf(m, "desc:\t%s\n", file->f_description);
+	spin_unlock(&file->f_lock);
+
 	show_fd_locks(m, file, files);
 	if (seq_has_overflowed(m))
 		goto out;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index f5abba86107d..09717bfa4e3b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -980,6 +980,9 @@ struct file {
 	struct address_space	*f_mapping;
 	errseq_t		f_wb_err;
 	errseq_t		f_sb_err; /* for syncfs */
+
+#define MAX_FILE_DESC_SIZE 256
+	char                    *f_description;
 } __randomize_layout
   __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */
 
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 2f86b2ad6d7e..f86ff6dc45c7 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -55,6 +55,11 @@
 #define F_GET_FILE_RW_HINT	(F_LINUX_SPECIFIC_BASE + 13)
 #define F_SET_FILE_RW_HINT	(F_LINUX_SPECIFIC_BASE + 14)
 
+/*
+ * Set file description
+ */
+#define F_SET_DESCRIPTION	(F_LINUX_SPECIFIC_BASE + 15)
+
 /*
  * Valid hint values for F_{GET,SET}_RW_HINT. 0 is "not set", or can be
  * used to clear any hints previously set.
-- 
2.25.1