Honor O_PATH semantics across fd consumers

[jserv]

Linux O_PATH descriptors carry only path reference: read, write, lseek, ftruncate, fsync/fdatasync, flock, ioctl, fchmod, fchown, getdents64, fsetxattr, and fremovexattr must all return EBADF. Our translate_open_flags maps O_PATH to a plain O_RDONLY host fd, so without explicit gating the host call would silently succeed and diverge from Linux semantics on every one of those paths.

Add host_fd_ref_open_io in syscall/internal.h that snapshots the fd entry and rejects FD_PATH with -EBADF. Wire it into sys_lseek and sys_ftruncate (fs.c), sc_fsync_common and sc_flock (syscall.c), and sys_fsetxattr / sys_fremovexattr (fs-xattr.c). sys_getdents64 gains an explicit FD_PATH gate ahead of the dir==NULL check so an O_PATH directory reports EBADF rather than ENOTDIR. io.c's host_fd_ref_open_regular_io now delegates to the shared helper. fchdir, fstat, fstatfs, close, dup, fcntl(CLOEXEC/DUPFD/GETFL), and *at() dirfd usage stay allowed, matching Linux.

Audit also surfaced an aarch64 syscall-number bug. SYS_fgetxattr was 16 (real value 10) and SYS_fremovexattr was 18 (real value 16) per include/uapi/asm-generic/unistd.h. Guest fremovexattr was being dispatched to the fgetxattr handler, which then ran with garbage size/value pointers from the unused argument registers; the missing FD_PATH check kept this hidden until tests/test-opath.c exercised it. Numbers in src/syscall/abi.h now match upstream.

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Summary by cubic

Enforce Linux O_PATH semantics so ops that need a real file now return EBADF on O_PATH fds, and fix aarch64 xattr syscall numbers. This aligns behavior with Linux and prevents silent success.

• Bug Fixes
  • Added host_fd_ref_open_io to reject FD_PATH; used by lseek, ftruncate, fsync/fdatasync, flock, fsetxattr, fremovexattr; getdents64 now returns EBADF for O_PATH dirs; io.c delegates to the helper.
  • Kept allowed O_PATH ops: fstat, fstatfs, close, dup, fcntl(CLOEXEC/DUPFD/GETFL), fchdir, and use as a *at() dirfd.
  • Corrected aarch64 xattr syscall numbers to match asm-generic (fgetxattr=10, fremovexattr=16) to fix mis-dispatch.
  • Added test-opath and updated test manifest.

^{Written for commit b23f003. Summary will update on new commits.}

[cubic-dev-ai]

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