linux-kernel - Re: [PATCH v2 7/8] exec: Generic execfd support

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Date:   Thu, 21 May 2020 22:28:39 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Rob Landley <rob@...dley.net>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        linux-fsdevel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        linux-security-module@...r.kernel.org,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v2 7/8] exec: Generic execfd support


Rob Landley <rob@...dley.net> writes:

> On 5/20/20 11:05 AM, Eric W. Biederman wrote:

> Toybox would _like_ proc mounted, but can't assume it. I'm writing a new
> bash-compatible shell with nommu support, which means in order to do subshell
> and background tasks if (!CONFIG_FORK) I need to create a pipe pair, vfork(),
> have the child exec itself to unblock the parent, and then read the context data
> that just got discarded through the pipe from the parent. ("Wheee." And you can
> quote me on that.)

Do you have clone(CLONE_VM) ?  If my quick skim of the kernel sources is
correct that should be the same as vfork except without causing the
parent to wait for you.  Which I think would remove the need to reexec
yourself.

>> The file descriptor is stored in mm->exe_file.
>> Probably the most straight forward implementation is to allow
>> execveat(AT_EXE_FILE, ...).
>
> Cool, that works.
>
>> You can look at binfmt_misc for how to reopen an open file descriptor.
>
> Added to the todo heap.

Yes I don't think it would be a lot of code.

I think you might be better served with clone(CLONE_VM) as it doesn't
block so you don't need to feed yourself your context over a pipe.

Eric