[issue13734] Add a generic directory walker method to avoid symlink attacks

[Antoine Pitrou]

Antoine Pitrou <pitrou at free.fr> added the comment:

> Since walkdir is currently entirely based on returning filesystem
> paths as strings (just like os.walk()) and hence shares the pervasive
> symlink attack vulnerability, I'm particularly interested in the
> question of whether or not the various *at APIs can be used to avoid
> symlink attacks if we just have a os.walkfd() API that emits a (dirfd,
> subdirs, files) triple instead of the os.walk style (dirpath, subdirs,
> files).

Be aware that you have to manage dirfd's lifetime, which can make things
interesting.
Also be aware that symlinks mean sometimes you won't have a dirfd: if
you have a symlink that points to another directory, you can't open that
directory using openat from the symlink's directory. So if you follow
symlinks (or have an option to do so) you must also take that case into
account.

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<http://bugs.python.org/issue13734>
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