bug: fails to fully extract a readonly directory

[Alexander Monakov]

Hello,

When extracting a directory, GNU tar tries to postpone chmod'ing a freshly
created directory until after extracting all files and subdirectories under
it (otherwise extracting files under a readonly directory would fail).

Unfortunately, it chmods as soon as it encounters an item in the archive
that would fall outside the directory. It usually works (as long as GNU
tar uses a DFS-like walk to create recursive archives), but if the archive
was appended to, or created by another producer, it may cause a failure.

In practice, this causes errors when extracting Archlinux rootfs archives
from a non-root user. The "bsdtar" tool from libarchive handles them without
issue (by postponing chmod operations until end of extraction).

The following commands perform a minimal demonstration:

# Create the directory/file structure to be archived
$ mkdir -p dir/subdir
$ touch dir/subdir/file1 dir/file2
$ chmod a-w dir/subdir

# Using '--no-recursion' for the sake of minimal example, in practice
# a problematic archive may be created in other ways too
$ tar --no-recursion -cvf t.tar dir/{subdir,file2,subdir/file1}
dir/subdir/
dir/file2
dir/subdir/file1

# Move the original out of the way
$ mv dir dir~

# Attempt to extract the archive
$ tar xvf t.tar
dir/subdir/
dir/file2
dir/subdir/file1
tar: dir/subdir/file1: Cannot open: Permission denied
tar: Exiting with failure status due to previous errors


Please keep me Cc'ed, I am not subscribed.

Regards,
Alexander