Unpacking a snap with mode-555 top-level squashfs-root in a subdirectory of a subdirectory of /tmp fails

[Daniel Manrique]

Given the attached evil snap (whose only evilness is that its top-level squashfs-root directory is mode 555), this works:

PYTHONPATH=$PYTHONPATH:~/click-reviewers-tools/ ~/click-reviewers-tools/bin/unpack-package ./kooo.snap /tmp/top-level-1

Successfully unpacked to '/tmp/top-level-1'

But this fails (notice the extra directory level):

mkdir /tmp/top-level-2
PYTHONPATH=$PYTHONPATH:~/click-reviewers-tools/ ~/click-reviewers-tools/bin/unpack-package ./kooo.snap /tmp/top-level-2/sub-level-1

Traceback (most recent call last):
  File "/usr/lib/python3.6/shutil.py", line 544, in move
    os.rename(src, real_dst)
PermissionError: [Errno 13] Permission denied: '/tmp/review-tools-zj9goluu' -> '/tmp/top-level-2/sub-level-1'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/ubuntu/click-reviewers-tools/bin/unpack-package", line 15, in <module>
    common.unpack_pkg(pkg, dir)
  File "/home/ubuntu/click-reviewers-tools/clickreviews/common.py", line 666, in unpack_pkg
    return _unpack_snap_squashfs(fn, dest, item)
  File "/home/ubuntu/click-reviewers-tools/clickreviews/common.py", line 635, in _unpack_snap_squashfs
    return _unpack_cmd(cmd, d, dest)
  File "/home/ubuntu/click-reviewers-tools/clickreviews/common.py", line 591, in _unpack_cmd
    shutil.move(d, dest)
  File "/usr/lib/python3.6/shutil.py", line 556, in move
    rmtree(src)
  File "/usr/lib/python3.6/shutil.py", line 480, in rmtree
    _rmtree_safe_fd(fd, path, onerror)
  File "/usr/lib/python3.6/shutil.py", line 438, in _rmtree_safe_fd
    onerror(os.unlink, fullname, sys.exc_info())
  File "/usr/lib/python3.6/shutil.py", line 436, in _rmtree_safe_fd
    os.unlink(name, dir_fd=topfd)
PermissionError: [Errno 13] Permission denied: 'command-hello.wrapper'

This is a minimal snap that reproduces the issue but we've managed to repro this with any snap whose squashfs-root is not writable by the owner (mode 555 is the reason we've encountered). This is rare but has happened with manually-created snaps that result when squashfs'ing a whole filesystem whose root (/) is mode 555. Ubuntu doesn't have this, but some other distros do.

[Jamie Strandboge (jdstrand)]

This is fix in r1152.