linux-kernel - Re: [GIT PULL] remove ksys_mount() and ksys

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Date:   Tue, 17 Dec 2019 22:57:43 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jesse Barnes <jsbarnes@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] remove ksys_mount() and ksys_dup()

On Tue, Dec 17, 2019 at 02:21:03PM -0800, Jesse Barnes wrote:
> > and yes,that particular problem only triggers when you have some odd
> > root filesystem without a /dev/console. Or a kernel config that
> > doesn't have those devices enabled at all.
> >
> > I delayed pulling it for a couple of days, but the branch was not in
> > linux-next, so my delay didn't make any difference, and all these
> > things only became obvious after I pulled. And while it was all
> > horribly buggy, it was only buggy for the "these cases don't happen in
> > a normal distro" case, so the regular use didn't show them.
> >
> > My bad. I shouldn't have pulled this, but it all looked very obvious
> > and trivial.
> 
> Oh I should have caught that too, I was looking right at it...
> 
> But anyway it looks like a nice cleanup with a few more fixes.
> Hopefully we can get there soon...

FWIW, this is precisely what I'd been talking about[*] - instead of
a plain "we are reusing the damn syscall, with fixed interface and
debugged by userland all the time" we'd got an open-coded analogue
that will be a headache (and a source of bitrot) for years.

It's not a normal part of the kernel, and I bloody well remember
what kind of headache it had been before it got massaged to use
of plain syscalls.  Constant need to remember that a change in
VFS guts might break something in the code that is hell to
debug - getting test coverage for it is not fun at all.  As we
are seeing right now...

Seriously, these parts of init/* ought to be treated as userland code
that runs in kernel mode mostly because it's too much PITA to arrange
building a static ELF binary and linking it into the image.

[*] "IMO it's not a good idea.  Exposing the guts of fs/namespace.c to
what's essentially a userland code that happens to run in kernel thread
is asking for trouble - we'd been there and it had been hell to untangle."

My fault, I guess - should've been more specific than that ;-/