[PATCH] proc: extend /proc/<pid>/fdinfo/<fd>

[Eugene Teo <eugeneteo@kernel.sg>]

This patch extends /proc/<pid>/fdinfo/<fd> to report information about open
files, and pathname. This information can be useful to know when debugging an
application.

For each file descriptor, the information is provided in the following format:

kerndev: ~/code/kernel# cat /proc/`pgrep pickup`/fdinfo/6
mode:   0622
dev:    253,0
ino:    21463057
uid:    89
gid:    89
rdev:   0,0
pos:    0
flags:  04002 FD_CLOEXEC # if close-on-exec flag is set
path:   /var/spool/postfix/public/pickup

You can also use this to find out more information about locked open files:

kerndev: /proc# cat /proc/locks
1: POSIX  ADVISORY  WRITE 2663 fd:00:21398205 0 EOF
                          ^^^^       ^^^^^^^^
2: FLOCK  ADVISORY  WRITE 2654 fd:00:21398203 0 EOF
3: FLOCK  ADVISORY  WRITE 2564 fd:00:21463056 0 EOF
4: FLOCK  ADVISORY  WRITE 2266 fd:00:21398151 0 EOF
kerndev: /proc# cd 2663/fdinfo/
kerndev: /proc/2663/fdinfo# grep 21398205 * -B2 -A6
               ^^^^
3-mode:	0644
3-dev:	253,0
3:ino:	21398205
        ^^^^^^^^
3-uid:	0
3-gid:	0
3-rdev:	0,0
3-pos:	5
3-flags:	02 FD_CLOEXEC
3-path:	/var/run/atd.pid

Signed-off-by: Eugene Teo <eugeneteo@kernel.sg>
---
 fs/proc/base.c |   47 +++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index c59852b..dde617f 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1402,7 +1402,7 @@ out:
 	return ~0U;
 }
 
-#define PROC_FDINFO_MAX 64
+#define PROC_FDINFO_MAX 256
 
 static int proc_fd_info(struct inode *inode, struct dentry **dentry,
 			struct vfsmount **mnt, char *info)
@@ -1411,6 +1411,7 @@ static int proc_fd_info(struct inode *inode, struct dentry **dentry,
 	struct files_struct *files = NULL;
 	struct file *file;
 	int fd = proc_fd(inode);
+	int ret = -ENOENT;
 
 	if (task) {
 		files = get_files_struct(task);
@@ -1424,24 +1425,58 @@ static int proc_fd_info(struct inode *inode, struct dentry **dentry,
 		spin_lock(&files->file_lock);
 		file = fcheck_files(files, fd);
 		if (file) {
+			ret = 0;
+
 			if (mnt)
 				*mnt = mntget(file->f_path.mnt);
 			if (dentry)
 				*dentry = dget(file->f_path.dentry);
-			if (info)
+			/* called by proc_fdinfo_read() */
+			if (info) {
+				struct fdtable *fdt = files_fdtable(files);
+				struct dentry *d = dget(file->f_path.dentry);
+				struct vfsmount *v = mntget(file->f_path.mnt);
+				char *page = (char *)__get_free_page(GFP_TEMPORARY);
+				int gcoe = FD_ISSET(fd, fdt->close_on_exec);
+				int dev_nr = d->d_inode->i_sb->s_dev;
+				int rdev_nr = d->d_inode->i_rdev;
+
+				if (!page) {
+					ret = -ENOMEM;
+					goto out;
+				}
+
 				snprintf(info, PROC_FDINFO_MAX,
+					 "mode:\t0%o\n"
+					 "dev:\t%d,%d\n"
+					 "ino:\t%ld\n"
+					 "uid:\t%d\n"
+					 "gid:\t%d\n"
+					 "rdev:\t%d,%d\n"
 					 "pos:\t%lli\n"
-					 "flags:\t0%o\n",
+					 "flags:\t0%o %s\n"
+					 "path:\t%s\n",
+					 d->d_inode->i_mode & 0777,
+					 MAJOR(dev_nr), MINOR(dev_nr),
+					 d->d_inode->i_ino,
+					 file->f_uid, file->f_gid,
+					 MAJOR(rdev_nr), MINOR(rdev_nr),
 					 (long long) file->f_pos,
-					 file->f_flags);
+					 file->f_flags, gcoe ? "FD_CLOEXEC" : "",
+					 d_path(d, v, page, PAGE_SIZE));
+				free_page((unsigned long)page);
+out:
+				mntput(v);
+				dput(d);
+			}
 			spin_unlock(&files->file_lock);
 			put_files_struct(files);
-			return 0;
+			return ret;
 		}
 		spin_unlock(&files->file_lock);
 		put_files_struct(files);
 	}
-	return -ENOENT;
+	return ret;
 }
 
 static int proc_fd_link(struct inode *inode, struct dentry **dentry,