chroot: mediate path-based xattr syscalls under fs_mount

[congwang-mk]

Fixes #84.

Problem

Under chroot/fs_mount, the supervisor rewrites paths for openat/statx/newfstatat/readlinkat/etc., but not the xattr family. Path-based getxattr/lgetxattr/listxattr on a path under an fs_mount target therefore resolved against the real (empty) mount point and returned ENOENT, even though statx on the same path succeeded. This surfaced as ls -l printing a spurious No such file or directory (it probes xattrs for the ACL + / SELinux . indicator) and exiting non-zero.

Root cause

chroot_path_syscalls() enumerated every path-bearing syscall except the xattr family, so those syscalls never raised SECCOMP_RET_USER_NOTIF and fell straight through to the kernel with an un-rewritten path.

Fix

Three production changes, mirroring the existing statx/readlink handlers:

1. context.rs — add the 8 path-based xattr syscalls ({,l}{get,set,list,remove}xattr) to the chroot notify set. The fd-based f*xattr variants are intentionally left unmediated (their fd already points at the resolved file).
2. chroot/dispatch.rs — handle_chroot_xattr: a single unified handler that classifies the syscall into one of four ops, resolves the path through the chroot/fs_mount/COW layers, gates on can_read/can_write, then re-issues the real syscall on the rewritten host path, marshalling the value/name/list buffers across the child boundary.
3. seccomp/dispatch.rs — register the handler for all 8 syscalls.

Edge cases handled: size == 0 probe semantics preserved (passes NULL, returns the kernel's length); output buffer sized from the child's size arg and clamped to XATTR_MAX (64 KiB), which bounds allocation and can never cause a spurious ERANGE; failures on the resolve path return a proper errno rather than falling through to the kernel (which would re-expose the bug).

Tests

TDD: added getxattr/listxattr/setxattr applets to the test helper and three regression tests in test_fs_mount.py (read, list, and write-through), each watched failing (ENOENT, exit 1) before implementing. They skip cleanly on filesystems without user-xattr support.

Verified: exact issue repro now exits 0 with empty stderr; full suites green (315 core lib + 221 chroot integration + 328 Python).