[PATCH 0/4] pidfs: implement file handle support

[Erin Shepherd]

Since the introduction of pidfs, we have had 64-bit process identifiers 
that will not be reused for the entire uptime of the system. This greatly 
facilitates process tracking in userspace.

There are two limitations at present:

 * These identifiers are currently only exposed to processes on 64-bit 
   systems. On 32-bit systems, inode space is also limited to 32 bits and 
   therefore is subject to the same reuse issues.
 * There is no way to go from one of these unique identifiers to a pid or 
   pidfd.

Patch 1 & 2 in this stack implements fh_export for pidfs. This means 
userspace  can retrieve a unique process identifier even on 32-bit systems 
via name_to_handle_at.

Patch 3 & 4 in this stack implement fh_to_dentry for pidfs. This means 
userspace can convert back from a file handle to the corresponding pidfd. 
To support us going from a file handle to a pidfd, we have to store a pid 
inside the file handle. To ensure file handles are invariant and can move 
between pid namespaces, we stash a pid from the initial namespace inside 
the file handle.

I'm not quite sure if stashing an initial-namespace pid inside the file 
handle is the right approach here; if not, I think that patch 1 & 2 are 
useful on their own.

Erin Shepherd (4):
  pseudofs: add support for export_ops
  pidfs: implement file handle export support
  pid: introduce find_get_pid_ns
  pidfs: implement fh_to_dentry

 fs/libfs.c                |  1 +
 fs/pidfs.c                | 57 +++++++++++++++++++++++++++++++++++++++
 include/linux/pid.h       |  1 +
 include/linux/pseudo_fs.h |  1 +
 kernel/pid.c              | 10 +++++--
 5 files changed, 68 insertions(+), 2 deletions(-)

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2.46.1