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Viewing / Editing File: fd_ops.py

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"""fd-based file operations for symlink-attack mitigation.

All helpers in this module use O_NOFOLLOW and dir_fd-relative syscalls
so that no path-based resolution can be redirected by a concurrent
symlink swap.

This module is intentionally kept separate from utils/__init__.py to
avoid loading these OS-specific helpers into every agent component.
"""

import errno
import logging
import os
import stat
from contextlib import contextmanager, suppress
from pathlib import Path

logger = logging.getLogger(__name__)


def rmtree_fd(dir_fd) -> None:
    """Remove all contents of a directory using fd-relative operations.

    Every entry is opened with ``O_NOFOLLOW`` so symlinks inside the tree
    are unlinked rather than followed.  The directory referenced by
    *dir_fd* itself is **not** removed — the caller should ``os.rmdir()``
    the parent entry after this call returns.

    Uses an iterative approach with an explicit stack to avoid hitting
    Python's recursion limit on adversarial deeply-nested trees.

    *dir_fd* must be an open ``O_RDONLY | O_DIRECTORY`` descriptor.
    """
    # Each stack frame is (fd, name_to_rmdir_after_close) where
    # name_to_rmdir_after_close is the entry name that should be
    # rmdir'd from the parent once this fd is fully processed.
    # The initial fd is managed by the caller, so its rmdir entry is None.
    stack = [(dir_fd, None)]
    try:
        while stack:
            current_fd, _ = stack[-1]
            pushed = False
            with os.scandir(current_fd) as entries:
                for entry in entries:
                    if entry.is_dir(follow_symlinks=False):
                        child_fd = os.open(
                            entry.name,
                            os.O_RDONLY | os.O_DIRECTORY | os.O_NOFOLLOW,
                            dir_fd=current_fd,
                        )
                        stack.append((child_fd, entry.name))
                        pushed = True
                        break  # restart scan from the new directory
                    else:
                        os.unlink(entry.name, dir_fd=current_fd)
            if not pushed:
                # All entries in current directory have been removed.
                fd, name = stack.pop()
                if name is not None:
                    # Close the child fd and rmdir it from the parent.
                    os.close(fd)
                    parent_fd, _ = stack[-1]
                    os.rmdir(name, dir_fd=parent_fd)
    except BaseException:
        # On error, close any fds we opened (but not the caller's dir_fd).
        for fd, name in stack:
            if name is not None:
                os.close(fd)
        raise


def open_dir_no_symlinks(path) -> int:
    """Open a directory, refusing symlinks at every path component.

    Walks the absolute *path* one component at a time, opening each with
    ``O_NOFOLLOW | O_DIRECTORY`` relative to the parent fd.  This guards
    against symlink attacks at *any* depth in the hierarchy, not just the
    leaf.

    Returns an ``O_RDONLY`` file descriptor for the final directory.
    The caller is responsible for closing it.
    """
    path = os.path.abspath(os.fspath(path))
    parts = Path(path).parts  # ('/', 'home', 'user', ...)
    fd = os.open(parts[0], os.O_RDONLY | os.O_DIRECTORY)
    try:
        for part in parts[1:]:
            new_fd = os.open(
                part,
                os.O_RDONLY | os.O_DIRECTORY | os.O_NOFOLLOW,
                dir_fd=fd,
            )
            os.close(fd)
            fd = new_fd
        return fd
    except BaseException:
        os.close(fd)
        raise


@contextmanager
def open_nofollow(path, flags=os.O_RDONLY, *, dir_fd=None):
    """Open a file with O_NOFOLLOW, closing the fd on exit.

    Yields the raw file descriptor.  Rejects symlinks at the leaf
    component (raises ELOOP).

    When *dir_fd* is provided, *path* is resolved relative to that
    directory descriptor.
    """
    kw = {"dir_fd": dir_fd} if dir_fd is not None else {}
    fd = os.open(str(path), flags | os.O_NOFOLLOW, **kw)
    try:
        yield fd
    finally:
        os.close(fd)


@contextmanager
def safe_dir(path):
    """Open a directory with symlink protection, closing the fd on exit.

    Walks every path component with O_NOFOLLOW via open_dir_no_symlinks
    and yields the resulting fd.
    """
    fd = open_dir_no_symlinks(path)
    try:
        yield fd
    finally:
        os.close(fd)


def atomic_rewrite_fd(
    filename,
    data: bytes,
    *,
    uid,
    gid,
    allow_empty_content,
    permissions,
    dir_fd: int,
) -> bool:
    """dir_fd-relative implementation of atomic_rewrite.

    The caller opens the directory with O_NOFOLLOW before any file I/O
    begins.  All file operations use dir_fd so that a concurrent rename
    of the directory to a symlink cannot redirect writes to a privileged
    path.
    """
    _, basename = os.path.split(filename)

    # Read current content without following symlinks.
    try:
        content_fd = os.open(
            basename, os.O_RDONLY | os.O_NOFOLLOW, dir_fd=dir_fd
        )
        with os.fdopen(content_fd, "rb") as f:
            old_content = f.read(len(data) + 1)
        if old_content == data:
            return False
    except FileNotFoundError:
        pass  # file does not exist yet; will be created
    except OSError as exc:
        if exc.errno == errno.ELOOP:
            pass  # existing entry is a symlink; overwrite it
        else:
            raise

    if not allow_empty_content and not data:
        logger.error("empty content: %r for file: %s", data, filename)
        return False

    if permissions is None:
        try:
            st = os.stat(basename, dir_fd=dir_fd, follow_symlinks=False)
            if stat.S_ISLNK(st.st_mode):
                raise OSError(errno.ELOOP, os.strerror(errno.ELOOP), basename)
            permissions = stat.S_IMODE(st.st_mode)
        except FileNotFoundError:
            current_umask = os.umask(0)
            os.umask(current_umask)
            permissions = 0o666 & ~current_umask

    # Create temp file atomically inside the directory referenced by dir_fd.
    # O_NOFOLLOW + O_EXCL ensures the name cannot be a pre-existing symlink.
    tmp_basename = None
    tmp_fd = -1
    for _ in range(100):
        tmp_basename = f"{basename}_{os.urandom(4).hex()}.i360edit"
        try:
            tmp_fd = os.open(
                tmp_basename,
                os.O_WRONLY | os.O_CREAT | os.O_EXCL | os.O_NOFOLLOW,
                0o600,
                dir_fd=dir_fd,
            )
            break
        except FileExistsError:
            continue
    else:
        raise FileExistsError("Could not create temporary file (100 attempts)")

    try:
        view = memoryview(data)
        written = 0
        while written < len(data):
            written += os.write(tmp_fd, view[written:])
        if uid is not None and gid is not None:
            os.chown(tmp_fd, uid, gid)
        os.chmod(tmp_fd, permissions)
        os.fsync(tmp_fd)
        os.close(tmp_fd)
        tmp_fd = -1
        # Atomic rename entirely within the directory we hold open.
        os.rename(tmp_basename, basename, src_dir_fd=dir_fd, dst_dir_fd=dir_fd)
        tmp_basename = None  # rename succeeded; no cleanup needed
    finally:
        if tmp_fd >= 0:
            os.close(tmp_fd)
        if tmp_basename is not None:
            with suppress(FileNotFoundError):
                os.unlink(tmp_basename, dir_fd=dir_fd)

    return True
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Viewing / Editing File: fd_ops.py

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