[PATCH 0/2] procfs: Add file path and size to /proc/<pid>/fdinfo

[Kalesh Singh]

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

-----

There was some concern about exposing the file path in the RFC[2], to that
effect the change was split into separte patches. Also retrieving the file
path from fdinfo is guarded by the same capability (PTRACE_MODE_READ) as
/proc/<pid>/maps which also exposes file path, so this may not be an issue.

[1] https://lore.kernel.org/r/20210308170651.919148-1-kaleshsingh@google.com/
[2] https://lore.kernel.org/r/20220519214021.3572840-1-kaleshsingh@google.com/


Kalesh Singh (2):
  procfs: Add 'size' to /proc/<pid>/fdinfo/
  procfs: Add 'path' to /proc/<pid>/fdinfo/

 Documentation/filesystems/proc.rst | 22 ++++++++++++++++++++--
 drivers/dma-buf/dma-buf.c          |  1 -
 fs/proc/fd.c                       | 13 +++++++++----
 3 files changed, 29 insertions(+), 7 deletions(-)


base-commit: 8ab2afa23bd197df47819a87f0265c0ac95c5b6a
-- 
2.36.1.255.ge46751e96f-goog