[RFC PATCH] procfs: Add file path and size to /proc//fdinfo

Kalesh Singh
2022-05-19T21:40:38Z

Processes can pin shared memory by keeping a handle to it through a
file descriptor; for instance dmabufs, memfd, and ashsmem (in Android).

In the case of a memory leak, to identify the process pinning the
memory, userspace needs to:
- Iterate the /proc/<pid>/fd/* for each process
- Do a readlink on each entry to identify the type of memory from
the file path.
- stat() each entry to get the size of the memory.

The file permissions on /proc/<pid>/fd/* only allows for the owner
or root to perform the operations above; and so is not suitable for
capturing the system-wide state in a production environment.

This issue was addressed for dmabufs by making /proc/*/fdinfo/*
accessible to a process with PTRACE_MODE_READ_FSCREDS credentials[1]
To allow the same kind of tracking for other types of shared memory,
add the following fields to /proc/<pid>/fdinfo/<fd>:

path - This allows identifying the type of memory based on common
prefixes: e.g. "/memfd...", "/dmabuf...", "/dev/ashmem..."

This was not an issued when dmabuf tracking was introduced
because the exp_name field of dmabuf fdinfo could be used
to distinguish dmabuf fds from other types.

size - To track the amount of memory that is being pinned.

dmabufs expose size as an additional field in fdinfo. Remove
this and make it a common field for all fds.

Access to /proc/<pid>/fdinfo is governed by PTRACE_MODE_READ_FSCREDS
-- the same as for /proc/<pid>/maps which also exposes the path and
size for mapped memory regions.

This allows for a system process with PTRACE_MODE_READ_FSCREDS to
account the pinned per-process memory via fdinfo.

[1] https://lore.kernel.org/lkml/20210308170651.919148-1-kaleshsingh@google.com/

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
Documentation/filesystems/proc.rst | 22 ++++++++++++++++++++--
drivers/dma-buf/dma-buf.c | 1 -
fs/proc/fd.c | 9 +++++++--
3 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 061744c436d9..ad66d78aca51 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -1922,13 +1922,16 @@ 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 four fields -- 'pos', 'flags', 'mnt_id' and 'ino'.
+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.
+the file, 'size' represents the size of the file in bytes, and 'path'
+represents the file path.

A typical output is::

@@ -1936,6 +1939,8 @@ A typical output is::
flags: 0100002
mnt_id: 19
ino: 63107
+ size: 0
+ path: /dev/null

All locks associated with a file descriptor are shown in its fdinfo too::

@@ -1953,6 +1958,8 @@ Eventfd files
flags: 04002
mnt_id: 9
ino: 63107
+ size: 0
+ path: anon_inode:[eventfd]
eventfd-count: 5a

where 'eventfd-count' is hex value of a counter.
@@ -1966,6 +1973,8 @@ Signalfd files
flags: 04002
mnt_id: 9
ino: 63107
+ size: 0
+ path: anon_inode:[signalfd]
sigmask: 0000000000000200

where 'sigmask' is hex value of the signal mask associated
@@ -1980,6 +1989,8 @@ Epoll files
flags: 02
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,
@@ -1998,6 +2009,8 @@ For inotify files the format is the following::
flags: 02000000
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
@@ -2021,6 +2034,8 @@ For fanotify files the format is::
flags: 02
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
@@ -2046,6 +2061,8 @@ Timerfd files
flags: 02
mnt_id: 9
ino: 63107
+ size: 0
+ path: anon_inode:[timerfd]
clockid: 0
ticks: 0
settime flags: 01
@@ -2070,6 +2087,7 @@ DMA Buffer files
mnt_id: 9
ino: 63107
size: 32768
+ path: /dmabuf:
count: 2
exp_name: system-heap

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index b1e25ae98302..d61183ff3c30 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -377,7 +377,6 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
{
struct dma_buf *dmabuf = file->private_data;

- seq_printf(m, "size:\t%zu\n", dmabuf->size);
/* Don't count the temporary reference taken inside procfs seq_show */
seq_printf(m, "count:\t%ld\n", file_count(dmabuf->file) - 1);
seq_printf(m, "exp_name:\t%s\n", dmabuf->exp_name);
diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 913bef0d2a36..a8a968bc58f0 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -54,10 +54,15 @@ static int seq_show(struct seq_file *m, void *v)
if (ret)
return ret;

- seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\nino:\t%lu\n",
+ seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\nino:\t%lu\nsize:\t%zu\n",
(long long)file->f_pos, f_flags,
real_mount(file->f_path.mnt)->mnt_id,
- file_inode(file)->i_ino);
+ file_inode(file)->i_ino,
+ 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);

base-commit: b015dcd62b86d298829990f8261d5d154b8d7af5
--
2.36.1.124.g0e6072fb45-goog