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Date: Tue, 13 Oct 2020 10:40:29 -0400
From: Rich Felker <dalias@...c.org>
To: libc-coord@...ts.openwall.com
Cc: Ariadne Conill <ariadne@...eferenced.org>
Subject: Re: [RFC] Add posix_spawnattr_chroot_np()
On Tue, Sep 29, 2020 at 04:01:09PM -0700, Robert Mustacchi wrote:
> Hi Ariadne,
>
> On 9/29/20 1:38 PM, Ariadne Conill wrote:
> > Hello,
> >
> > Package managers frequently have a feature where you can use the package
> > manager to build a new filesystem image in a sub-directory (usually
> > exposed as the --root option in the package manager). However, to
> > achieve this, we have to be able to chroot into the sub-directory to run
> > maintainer scripts.
> >
> > Right now, package managers usually do fork() + chroot() + execve() to
> > implement this. I find it desirable to migrate apk from this fork() +
> > chroot() + execve() pattern toward using posix_spawn() as that allows
> > for the underlying libc to take care of spawning the maintainer scripts
> > for us (and I assume posix_spawn() will always use the most efficient
> > method available for doing so).
> >
> > Accordingly, I propose adding the following interface:
> >
> > int posix_spawnattr_chroot_np(posix_spawnattr_t *attr, const char *chroot);
> >
> > This will, of course, set a chroot attribute in posix_spawnattr_t.
> >
> > Like the other interfaces, it will return 0 or an appropriate error number.
> >
> > Doing so would allow us to migrate maintainer script handling in package
> > managers to use posix_spawn, at least in apk this is the only missing
> > functionality we require.
>
> This seems like it could be a useful thing for folks and makes sense as
> something to add. Thanks for writing this up. A couple of questions
> about the proposal:
>
> * Semantically would one expect that the "path" argument (as POSIX calls
> it in
> https://pubs.opengroup.org/onlinepubs/007904975/functions/posix_spawn.html)
> to be located within the chroot? I'd imagine so?
>
> * When does the chroot occur with respect to the open actions taking
> place? Does one process all file actions before or after the chroot
> takes place or does this need to be interleaved in the file actions to
> give consumers more flexibility?
>
> * What are the expectations if multiple calls to
> posix_spawnattr_chroot_np() are made? Is it expected that they all must
> apply or that only the last one applies? Does passing NULL for a path
> effectively un-set a chroot request?
I don't like that this is storing unbounded-size state in the
attribute object, and the obvious solution to the first question (and
the second) is that chroot should not be an attribute but a file
action, the same way chdir (newly added) is.
Rich
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