linux-kernel - Re: execve(NULL, argv, envp) for nommu?

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Message-ID: <CAMuHMdUPUaLfbbFF1kZoEUy7or-9sVOt=ykAHT+S6NBvFy5V=g@mail.gmail.com>
Date:   Tue, 5 Sep 2017 11:00:55 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Rob Landley <rob@...dley.net>
Cc:     Linux Embedded <linux-embedded@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: execve(NULL, argv, envp) for nommu?

CC Oleg, lkml

On Tue, Sep 5, 2017 at 9:34 AM, Rob Landley <rob@...dley.net> wrote:
> For years I've wanted an execve() system call modification that let me
> pass a NULL as the first argument to say "re-exec this program please".
> Because on nommu you've got to exec something to unblock vfork(), and
> daemons (or things like busybox and toybox) want to re-exec themselves.
> I just hit this again trying to implement a nommu-friendly strace(): the
> one on github doesn't SIGSTOP the child before the execve() of the
> process to trace because vfork(), and just races and misses the first
> few system calls on nommu instead...)
>
> The problem with exec /proc/self/exe is A) I haven't necessarily got
> /proc mounted, B) in a chroot the original binary might not be in scope
> anymore. But I'm already _running_ this program. If I could fork() I
> could already get a second copy of the sucker and call main() again
> myself if necessary, but I can't, so...
>
> I'm aware there's a possible "but what if it was suid and it's already
> dropped privileges" argument, and I'm fine with execve(NULL) not
> honoring the suid bit if people feel that way. I just wanna unblock
> vfork() while still running this code. (A way to detect I did this would
> be great too, but the normal tweaking of argv[] or envp[] to let main
> know we're a child still works.)
>
> Is there a _reason_ the kernel doesn't do this, or has nobody bothered
> to code it up yet?
>
> Rob