Merge patch series "fs: harden anon inodes"

Merge patch series "fs: harden anon inodes"

Christian Brauner <brauner@kernel.org> says:

* Anonymous inodes currently don't come with a proper mode causing
  issues in the kernel when we want to add useful VFS debug assert. Fix
  that by giving them a proper mode and masking it off when we report it
  to userspace which relies on them not having any mode.

* Anonymous inodes currently allow to change inode attributes because
  the VFS falls back to simple_setattr() if i_op->setattr isn't
  implemented. This means the ownership and mode for every single user
  of anon_inode_inode can be changed. Block that as it's either useless
  or actively harmful. If specific ownership is needed the respective
  subsystem should allocate anonymous inodes from their own private
  superblock.

* Port pidfs to the new anon_inode_{g,s}etattr() helpers.

* Add proper tests for anonymous inode behavior.

The anonymous inode specific fixes should ideally be backported to all
LTS kernels.

* patches from https://lore.kernel.org/20250407-work-anon_inode-v1-0-53a44c20d44e@kernel.org:
  selftests/filesystems: add fourth test for anonymous inodes
  selftests/filesystems: add third test for anonymous inodes
  selftests/filesystems: add second test for anonymous inodes
  selftests/filesystems: add first test for anonymous inodes
  anon_inode: raise SB_I_NODEV and SB_I_NOEXEC
  pidfs: use anon_inode_setattr()
  anon_inode: explicitly block ->setattr()
  pidfs: use anon_inode_getattr()
  anon_inode: use a proper mode internally

Link: https://lore.kernel.org/20250407-work-anon_inode-v1-0-53a44c20d44e@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>