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Subject[PATCH v2 0/2] procfs: Add file path and size to /proc/<pid>/fdinfo
Hi all,

This is v2 of the fdinfo patches. The main update is adding path
field only for files with anon inodes. Rebased on 5.19-rc3.

The previous cover letter is copied below for convenience.



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.

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/libfs.c | 9 +++++++++
fs/proc/fd.c | 18 ++++++++++++++----
include/linux/fs.h | 1 +
5 files changed, 44 insertions(+), 7 deletions(-)

base-commit: a111daf0c53ae91e71fd2bfe7497862d14132e3e

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