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

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Date:   Mon, 12 Nov 2018 12:45:26 +0000
From:   Szabolcs Nagy <Szabolcs.Nagy@....com>
To:     Daniel Colascione <dancol@...gle.com>,
        Florian Weimer <fweimer@...hat.com>
CC:     nd <nd@....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 11/11/18 14:22, Daniel Colascione wrote:
> On Sun, Nov 11, 2018 at 3:09 AM, Florian Weimer <fweimer@...hat.com> wrote:
>> We had a patch for the membarrier system call, but the kernel developers
>> could not tell us what the system call does in therms of the C/C++
>> memory model
> [snip]
>> A lot of the new system calls lack clear specifications or are just
>> somewhat misdesigned.  For example, pkey_alloc
> [snip]
>> getrandom still causes boot delays
> [snip]
>> For copy_file_range, we still have debates whether the system call (and
>> the glibc emulation) should preserve holes or not,
> [snip]
> 
> These objections illustrate my point. glibc development is not the
> proper forum for raising post-hoc objections to system call design.
> Withholding wrappers will not un-ship these system calls. Applications
> are already using them, via syscall(2). Developers and users would be
> better served by providing access to the system as it is, with
> appropriate documentation caveats, than by holding out for some
> alternate and more ideal set of system calls that may or may not
> appear in the future. This resistance to exposing the capabilities of
> the system as they are, even in flawed and warty form, is what I meant
> by "misplaced idealism" in my previous message. If the kernel provides
> a system call, libc should provide a C wrapper for it, even if in the
> opinion of the libc maintainers, that system call is flawed.

flaws can be worked around.

it's just more work to do that, hence wrappers are delayed.

(while new flawed syscalls get added, there are missing
syscalls for implementing posix semantics or for better libc
quality, so are the priorities of linux right?)

> I agree with the proposals mentioned above to split system interface
> responsibility, having glibc handle higher-level concerns like stdio
> while punting system call wrappers and other low-level facilities to a
> kernel-provided userspace library that can move faster and more
> explicitly conform to the Linux kernel's userspace ABI.

consuming linux uapi headers is a huge problem (not just for
glibc): the libc has to repeat uapi definitions under appropriate
feature macros using proper libc types etc, this usually creates
conflict between linux and libc headers and a lot of duplicated
work at every linux release. the situation would be worse if all
new types were exposed for new syscalls when they appeared.

the proposal mentioned above does not solve this in any way.