linux-kernel - Re: [musl] Re: [PATCH resent] uapi libc compat: allow non-glibc to opt out of uapi definitions

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Date:   Tue, 25 Apr 2017 08:45:43 +0200
From:   Hauke Mehrtens <hauke@...ke-m.de>
To:     musl@...ts.openwall.com, David Woodhouse <dwmw2@...radead.org>,
        Felix Janda <felix.janda@...teo.de>,
        linux-kernel@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>, linux-api@...r.kernel.org
Subject: Re: [musl] Re: [PATCH resent] uapi libc compat: allow non-glibc to
 opt out of uapi definitions

On 03/08/2017 05:39 PM, Carlos O'Donell wrote:
> Any header needing compat with a libc includes libc-compat.h (per the 
> documented way the model works). With this patch any included linux kernel
> header that also includes libc-compat.h would immediately define all 
> the __UAPI_DEF_* constants to 1 as-if it had defined those structures, 
> but it has not.
> 
> For example, with these two patches applied, the inclusion of linux/if.h
> would define __UAPI_DEF_XATTR to 1, but linux/if.h has not defined
> XATTR_CREATE or other constants, so a subsequent inclusion sys/xattrs.h
> from userspace would _not_ define XATTR_CREATE because __UAPI_DEF_XATTR set
> to 1 indicates the kernel has.
> 
> I don't want to read into the model you are proposing and would rather you
> document the semantics clearly so we can all see what you mean.

What about moving the code from libc-comapt.h into the specific header
files? This way including linux/if.h would not have an impact on
__UAPI_DEF_XATTR, because this is defined in linux/xattr.h. We would
still have a problem when the specific Linux header file gets included
before the libc header file, but at least musl does not support this anyway.

Hauke