Re: [regresssion 4.15] Userspace compilation broken by uapi/linux/if_ether.h update

On Fri, Jan 26, 2018 at 11:51:38AM +0100, Guillaume Nault wrote:

On Thu, Jan 25, 2018 at 11:21:34PM +0100, Hauke Mehrtens wrote:

quoted

On 01/25/2018 03:58 PM, Guillaume Nault wrote:

quoted

Now that linux/libc-compat.h is included in linux/if_ether.h, it is
processed before netinet/in.h. Therefore, it sets the relevant
__UAPI_DEF_* guards to 1 (as _NETINET_IN_H isn't yet defined).
Then netinet/in.h is included, followed by linux/in.h. The later
doesn't realise that what it defines has already been included by
netinet/in.h because the __UAPI_DEF_* guards were set too early.

This is about this commit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6926e041a8920c8ec27e4e155efa760aa01551fd

On option would be to move this into include/uapi/linux/if_ether.h and
remove the include for libc-compat.h:
#ifndef __UAPI_DEF_ETHHDR
#define __UAPI_DEF_ETHHDR		1
#endif

This will only work if netinet/if_ether.h is included before
linux/if_ether.h, but I think this is very likely.

I don't see what makes its likely. That's not directly related to your
point, but for example, glibc guarantees the opposite as it includes
linux/if_ether.h at the beginning of netinet/if_ether.h.

quoted

I think we can do this because we do not need some special libc handling
like it is done for other symbols as __UAPI_DEF_ETHHDR is currently only
needed by musl and not by glibc.

That's ok for me as long as existing projects keep compiling. But all
__UAPI_DEF_* are currently centralised in libc-compat.h. Adding
__UAPI_DEF_ETHHDR in if_ether.h looks like defeating the purpose of
libc-compat.h and I wonder if that'd be accepted. Maybe with a
different name.

In any case, we're really late in the release cycle. If more discussion
is needed, it's probably better to revert and take time to work on a
solution for the next release.

Hi David,

I just realise you've sent your last pull request for this release. I
was waiting for feedbacks in order to avoid a revert. Should I send a
revert now or do you prefer to sort this out later and backport a fix
in 4.15.1?