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

On Tue, May 31, 2022 at 3:07 PM Stephen Brennan stephen.s.brennan@oracle.com wrote:

On 5/31/22 14:25, Kalesh Singh wrote:

...

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.

Hi Kalesh,

Hi Stephen,

Thanks for taking a look.

I think I see the value in the size field, but I'm curious about path, which is available via readlink /proc/<pid>/fd/<n>, since those are symlinks to the file themselves.

This could work if we are root, but the file permissions wouldn't allow us to do the readlink on other processes otherwise. We want to be able to capture the system state in production environments from some trusted process with ptrace read capability.

File paths can contain fun characters like newlines or colons, which could make parsing out filenames in this text file... fun. How would your userspace parsing logic handle "/home/stephen/filename\nsize:\t4096"? The readlink(2) API makes that easy already.

I think since we have escaped the "\n" (seq_file_path(m, file, "\n")), then user space might parse this line like:

if (strncmp(line, "path:\t", 6) == 0) char* path = line + 6;

Thanks, Kalesh

Is the goal avoiding races (e.g. file descriptor 3 is closed and reopened to a different path between reading fdinfo and stating the fd)?

Stephen

...

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))
  

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