Re: [PATCH] selftests/bpf: fix compile errors with older glibc

[Yonghong Song <yhs@fb.com>]

On 4/18/19 11:00 AM, Wang YanQing wrote:
> On Thu, Apr 18, 2019 at 05:09:53AM +0000, Yonghong Song wrote:
>>
>>
>> On 4/17/19 10:48 AM, Wang YanQing wrote:
>>> The older glibc (for example, 2.23) doesn't handle __UAPI_DEF_*
>>> in libc-compat.h properly, and it bring below compile errors:
>>
>> I have an even old glibc 2.17 and it still works. Not sure
>> why it failed here. Could you explain more?
> 
> We will meet these errors with the combination of some versions of kernel headers and
> some versions of glibc headers.
> 
> After some research on the git history of glibc and kernel, I find the reason behind
> the scene is a little complex:
> There are some same definitions between glibc's netinet/in.h and kernel's linux/in6.h,
> IPPROTO_HOPOPTS, etc.
> These same definitions willn't bring trouble when we include both of them if kernel and
> glibc coordinates with each other well, but the reality is the coordination is poor and
> unsynchronous in history.
> 
> Kernel and glibc uses guard macros to detect whether need to export their definitions,
> linux/in6.h includes libc-compat.h which will check the guard macro, _NETINET_IN_H, and
> netinet/in.h includes bits/in.h which will check the guard macro, _UAPI_LINUX_IN6_H
> (glibc-2.19~821 6c82a2f8d7c8e21e39237225c819f182ae438db3 "Coordinate IPv6 definitions for Linux and glibc"),
> 
> the problem is in the installation process of kernel headers, the "_UAPI" in _UAPI_LINUX_IN6_H
> in linux/in6.h will be stripped due to commit 56c176c9cac9
> ("UAPI: strip the _UAPI prefix from header guards during header installation").
> 
> The good news is the glibc fix this the trouble by checking the guard macro, _LINUX_IN6_H, too.
> (glibc c9bd40daaee18cf1d9824e4a7ebaebe321e0a5a8 "Bug 20214: Fix linux/in6.h and netinet/in.h sync.").

Thanks for explanation, my system contains this glibc fix (_LINUX_IN6_H 
is checked) so that is why I did not have the issue.

> 
> My environment still have this trouble:
> lsb_release -a:
> No LSB modules are available.
> Distributor ID:	Ubuntu
> Description:	Ubuntu 16.04.6 LTS
> Release:	16.04
> Codename:	xenial
> 
> dpkg -l | grep libc-dev:
> ii  libc-dev-bin              2.23-0ubuntu11           amd64        GNU C Library: Development binaries
> ii  linux-libc-dev:amd64      4.4.0-145.171            amd64        Linux Kernel Headers for development.
> 
> 
> I will send out the v2 which I will change some words in changelog.
> 
> Thanks for review.
>