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20370a5f5d
Add a file descriptor based counterpart to chroot(2). This has been overdue for a long time. It is the natural companion to fchdir() and avoids re-resolving a path that the caller already holds a file descriptor to. No TOCTOU between resolving the target and changing the root. It composes with modern fd-based APIs meaning it works with O_PATH file descriptors and file descriptors to detached mount trees created via open_tree(OPEN_TREE_CLONE). The permission model is identical to chroot(2). The caller must have CAP_SYS_CHROOT in its user namespace, must pass MAY_EXEC | MAY_CHDIR permission checks on the target directory, and LSMs are consulted via the same security_path_chroot() hook. The system call takes a flags argument for future extensibility which must currently be zero. Link: https://patch.msgid.link/20260724-work-failfs-v2-3-485dabbae185@kernel.org Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>