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feat(fs): support O_CLOEXEC, fd table limit, and fix pipe2/dup3/dup

StellaOS requested to merge feature/fs_syscall into main

  • O_CLOEXEC (close-on-exec):

    • Add FdEntry wrapper around Arc with per-fd cloexec field, keeping FD_CLOEXEC independent from the shared open file description
    • FdTable::close_on_exec() closes all cloexec-marked fds during exec()
    • sys_openat/sys_pipe2: parse O_CLOEXEC from user flags, set on new fd; strip O_CLOEXEC before passing to VfsFile.flags
    • sys_dup3: set/clear cloexec based on flags argument
    • sys_dup: always clear cloexec on new fd (POSIX)

  • O_NONBLOCK for pipe2:

    • make_pipe() now accepts OpenFlags, passes O_NONBLOCK to VfsFile via new_dev_with_flags(); read end uses O_RDONLY, write end O_WRONLY
    • Existing WouldBlock -> EAGAIN logic in sys_read/sys_write applies

  • FdTable limit (RLIMIT_NOFILE):

    • Add limit field (default 256), try_alloc_fd/try_insert return Option
    • All syscall-layer callers return EMFILE when fd table exhausted
    • sys_dup3 checks newfd < limit, returns EBADF on violation
    • Manual Default impl ensures limit is set correctly

  • Rename SysErrNo -> SyscallErrNo for clarity

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