linux-kernel - Re: Official Linux system wrapper library?

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Message-Id: <1C1B38FC-1266-4A91-B8F6-20A0C49ED1E2@amacapital.net>
Date:   Wed, 14 Nov 2018 06:46:47 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Szabolcs Nagy <Szabolcs.Nagy@....com>
Cc:     Dave P Martin <Dave.Martin@....com>,
        Daniel Colascione <dancol@...gle.com>, nd <nd@....com>,
        Florian Weimer <fweimer@...hat.com>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        Willy Tarreau <w@....eu>, Vlastimil Babka <vbabka@...e.cz>,
        Carlos O'Donell <carlos@...hat.com>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>
Subject: Re: Official Linux system wrapper library?




> On Nov 14, 2018, at 3:58 AM, Szabolcs Nagy <Szabolcs.Nagy@....com> wrote:
> 
>> On 13/11/18 19:39, Dave Martin wrote:
>>> On Mon, Nov 12, 2018 at 05:19:14AM -0800, Daniel Colascione wrote:
>>> We should adopt a similar approach. Shipping a lower-level
>>> "liblinux.so" tightly bound to the kernel would not only let the
>>> kernel bypass glibc's "editorial discretion" in exposing new
>>> facilities to userspace, but would also allow for tighter user-kernel
>>> integration that one can achieve with a simplistic syscall(2)-style
>>> escape hatch. (For example, for a long time now, I've wanted to go
>>> beyond POSIX and improve the system's signal handling API, and this
>>> improvement requires userspace cooperation.) The vdso is probably too
>>> small and simplistic to serve in this role; I'd want a real library.
>> 
>> Can you expand on your reasoning here?
> 
> such lib creates a useless abi+api layer that
> somebody has to maintain and document (with or
> without vdso).

I’m not so sure it’s useless. Historically, POSIX systems have, in practice and almost by definition, been very C focused, but the world is changing. A less crufty library could be useful for newer languages:

> 
> it obviously cannot work together with a posix
> conform libc implementation for which it would
> require knowledge about
> 
> thread cancellation internals,

Thread cancellation is a big mess, and we only really need to support it because on legacy code. The whole mechanism should IMO be considered extremely deprecated.

> potentially TLS
> for errno,

errno is IMO a libc thing, full stop. A lower level library should *not* support errno.

> know libc types even ones that are
> based on compile time feature macros (and expose
> them in headers in a way that does not collide
> with libc headers),

This one is tricky. I wonder if we could instead get a C compiler extension to set libc declare that a given struct is a layout-compatible variant of another.

> abi variants the libc supports
> (e.g. softfp, security hardened abi),

Hmm.

> libc
> internal signals (for anything that's changing
> signal masks),

This is nasty, but see my cancellation comment above.

> thread internals for syscalls that
> require coordination between all user created
> threads (setxid),

We should just deal with this in the kernel. The current state of affairs is nuts.

> libc internal state for syscalls
> that create/destroy threads.

I disagree. If you make or destroy threads behind libc’s back, I think you get to keep both pieces.