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

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Am 01.06.22 um 00:48 schrieb Stephen Brennan:
Kalesh Singh <kaleshsingh@xxxxxxxxxx> writes:
On Tue, May 31, 2022 at 3:07 PM Stephen Brennan
<stephen.s.brennan@xxxxxxxxxx> 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.
Interesting, thanks for explaining. It seems weird to have a duplicate
interface for the same information but such is life.

Yeah, the size change is really straight forward but for this one I'm not 100% sure either.

Probably best to ping some core fs developer before going further with it.

BTW: Any preferred branch to push this upstream? If not I can take it through drm-misc-next.

Regards,
Christian.


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")),
I really should have read through that function before commenting,
thanks for teaching me something new :)

Stephen

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@xxxxxxxxxx>
---

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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