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

[Wang YanQing <udknight@gmail.com>]

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.").

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.