[PATCH 2/2] procfs: Add 'path' to /proc/<pid>/fdinfo/

[Kalesh Singh <kaleshsingh@google.com>]

In order to identify the type of memory a process has pinned through
its open fds, add the file path to fdinfo output. This allows
identifying memory types based on common prefixes. e.g. "/memfd...",
"/dmabuf...", "/dev/ashmem...".

Access to /proc/<pid>/fdinfo is governed by PTRACE_MODE_READ_FSCREDS
the same as /proc/<pid>/maps which also exposes the file path of
mappings; so the security permissions for accessing path is consistent
with that of /proc/<pid>/maps.

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---

Changes from rfc:
  - Split adding 'size' and 'path' into a separate patches, per Christian
  - Fix indentation (use tabs) in documentaion, per Randy

 Documentation/filesystems/proc.rst | 14 ++++++++++++--
 fs/proc/fd.c                       |  4 ++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 779c05528e87..591f12d30d97 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -1886,14 +1886,16 @@ if precise results are needed.
 3.8	/proc/<pid>/fdinfo/<fd> - Information about opened file
 ---------------------------------------------------------------
 This file provides information associated with an opened file. The regular
-files have at least five fields -- 'pos', 'flags', 'mnt_id', 'ino', and 'size'.
+files have at least six fields -- 'pos', 'flags', 'mnt_id', 'ino', 'size',
+and 'path'.
 
 The 'pos' represents the current offset of the opened file in decimal
 form [see lseek(2) for details], 'flags' denotes the octal O_xxx mask the
 file has been created with [see open(2) for details] and 'mnt_id' represents
 mount ID of the file system containing the opened file [see 3.5
 /proc/<pid>/mountinfo for details]. 'ino' represents the inode number of
-the file, and 'size' represents the size of the file in bytes.
+the file, 'size' represents the size of the file in bytes, and 'path'
+represents the file path.
 
 A typical output is::
 
@@ -1902,6 +1904,7 @@ A typical output is::
 	mnt_id:	19
 	ino:	63107
 	size:	0
+	path:	/dev/null
 
 All locks associated with a file descriptor are shown in its fdinfo too::
 
@@ -1920,6 +1923,7 @@ Eventfd files
 	mnt_id:	9
 	ino:	63107
 	size:   0
+	path:	anon_inode:[eventfd]
 	eventfd-count:	5a
 
 where 'eventfd-count' is hex value of a counter.
@@ -1934,6 +1938,7 @@ Signalfd files
 	mnt_id:	9
 	ino:	63107
 	size:   0
+	path:	anon_inode:[signalfd]
 	sigmask:	0000000000000200
 
 where 'sigmask' is hex value of the signal mask associated
@@ -1949,6 +1954,7 @@ Epoll files
 	mnt_id:	9
 	ino:	63107
 	size:   0
+	path:	anon_inode:[eventpoll]
 	tfd:        5 events:       1d data: ffffffffffffffff pos:0 ino:61af sdev:7
 
 where 'tfd' is a target file descriptor number in decimal form,
@@ -1968,6 +1974,7 @@ For inotify files the format is the following::
 	mnt_id:	9
 	ino:	63107
 	size:   0
+	path:	anon_inode:inotify
 	inotify wd:3 ino:9e7e sdev:800013 mask:800afce ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:7e9e0000640d1b6d
 
 where 'wd' is a watch descriptor in decimal form, i.e. a target file
@@ -1992,6 +1999,7 @@ For fanotify files the format is::
 	mnt_id:	9
 	ino:	63107
 	size:   0
+	path:	anon_inode:[fanotify]
 	fanotify flags:10 event-flags:0
 	fanotify mnt_id:12 mflags:40 mask:38 ignored_mask:40000003
 	fanotify ino:4f969 sdev:800013 mflags:0 mask:3b ignored_mask:40000000 fhandle-bytes:8 fhandle-type:1 f_handle:69f90400c275b5b4
@@ -2018,6 +2026,7 @@ Timerfd files
 	mnt_id:	9
 	ino:	63107
 	size:   0
+	path:	anon_inode:[timerfd]
 	clockid: 0
 	ticks: 0
 	settime flags: 01
@@ -2042,6 +2051,7 @@ DMA Buffer files
 	mnt_id:	9
 	ino:	63107
 	size:   32768
+	path:	/dmabuf:
 	count:  2
 	exp_name:  system-heap
 
diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 464bc3f55759..8889a8ba09d4 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -60,6 +60,10 @@ static int seq_show(struct seq_file *m, void *v)
 	seq_printf(m, "ino:\t%lu\n", file_inode(file)->i_ino);
 	seq_printf(m, "size:\t%lli\n", (long long)file_inode(file)->i_size);
 
+	seq_puts(m, "path:\t");
+	seq_file_path(m, file, "\n");
+	seq_putc(m, '\n');
+
 	/* show_fd_locks() never deferences files so a stale value is safe */
 	show_fd_locks(m, file, files);
 	if (seq_has_overflowed(m))
-- 
2.36.1.255.ge46751e96f-goog