linux-kernel - Re: new ...at() flag: AT_NO

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Message-ID: <CALCETrXhOhG0tRDDOROwT9ghvQvKziM2PBN=CX5Soa2m7=0cFw@mail.gmail.com>
Date:   Sat, 29 Apr 2017 16:17:18 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: new ...at() flag: AT_NO_JUMPS

On Sat, Apr 29, 2017 at 3:04 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> New AT_... flag - AT_NO_JUMPS
>
> Semantics: pathname resolution must not involve
>         * traversals of absolute symlinks
>         * traversals of procfs-style symlinks
>         * traversals of mountpoints (including bindings, referrals, etc.)
>         * traversal of .. in the starting point of pathname resolution.

Can you clarify this last one?  I assume that ".." will be rejected,
but what about "a/../.."?  How about "b" if b is a symlink to ".."?
How about "a/b" if a is a directory and b is a symlink to "../.."?

> Right now I have it hooked only for fstatat() and friends; it could be
> easily extended to any ...at() syscalls.  Objections?

I like it, assuming the answers to all the questions above are that
they will be rejected.