procfs: add 'path' to /proc/<pid>/fdinfo/

procfs: add 'path' to /proc/<pid>/fdinfo/

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...".

To be cautious, only expose the paths for anonymous inodes, and this also
avoids printing path names with strange characters.

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.

Link: https://lkml.kernel.org/r/20220623220613.3014268-3-kaleshsingh@google.com
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian König <christian.koenig@amd.com>
Cc: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Colin Cross <ccross@google.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Ioannis Ilkos <ilkos@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Paul Gortmaker<paul.gortmaker@windriver.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Brennan <stephen.s.brennan@oracle.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>