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Date:   Sat, 27 Oct 2018 12:11:52 +0000
From:   Ed Maste <emaste@freebsd.org>
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     David Drysdale <drysdale@google.com>, linux-kernel@vger.kernel.org
Subject: : [PATCH v2 1/3] namei: implement O_BENEATH-style AT_* flags
Message-ID: <20181027121152.GA70269@freebsd.org>
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> What is the proposed semantic of O_BENEATH with absolute paths -- I
> believe you don't have an openat(2) on FreeBSD (but please feel free to
> correct me)?

openat(2) is necessary for capability mode (since open(2) is not permitted),
but it turns out it was actually added to FreeBSD earlier than that - the
work was done by a student in the 2007 Google Summer of Code.

From the proposed man page change:
| If the specified path is absolute, O_BENEATH
| allows arbitrary prefix that ends up in the starting directory,
| after which all further resolved components must be under it.

For open, and openat(AT_FDCWD), cwd is used as the top / starting directory.

Once the absolute path traverses the "top" directory in namei all
components must remain within.  The proposed patch allows paths that perhaps
enter, exit, and re-enter the hierarchy -- for example, if /a/b is the top
then /c/d/../../a/b/file would be allowed.

(If you're interested in taking a look the proposed change is available at
https://reviews.freebsd.org/D17714 )

In capability mode absolute paths will always be disallowed.