netdev - Re: [regresssion 4.15] Userspace compilation broken by uapi/linux/if

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Date:   Thu, 25 Jan 2018 23:21:34 +0100
From:   Hauke Mehrtens <hauke@...ke-m.de>
To:     Guillaume Nault <g.nault@...halink.fr>, netdev@...r.kernel.org
Cc:     linux-api@...r.kernel.org
Subject: Re: [regresssion 4.15] Userspace compilation broken by
 uapi/linux/if_ether.h update

On 01/25/2018 03:58 PM, Guillaume Nault wrote:
> Hi,
> 
> Commit 6926e041a892 ("uapi/if_ether.h: prevent redefinition of struct ethhdr"),
> can break compilation of userspace programs (in my case, accel-ppp
> (https://accel-ppp.org)).
> 
> This happens for userspace programs that end up including
> linux/if_ether.h, netinet/in.h and linux/in.h in this order:
> 
> # cat test_ifether.c
> #include <linux/if_ether.h>
> #include <netinet/in.h>
> #include <linux/in.h>
> 
> int main(void)
> {
> 	return 0;
> }
> 
> # gcc test_ifether.c
> In file included from test_ifether.c:2:0:
> /usr/include/linux/in.h:29:3: error: redeclaration of enumerator ‘IPPROTO_IP’
>    IPPROTO_IP = 0,  /* Dummy protocol for TCP  */
>    ^
> /usr/include/netinet/in.h:42:5: note: previous definition of ‘IPPROTO_IP’ was here
>      IPPROTO_IP = 0,    /* Dummy protocol for TCP.  */
>      ^~~~~~~~~~
> 
> 
> 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.
> 
> Of course the situation is a bit more complicated on real projects, as
> these files aren't included directly. For example, in accel-ppp, the
> PPPoE module (accel-ppp/accel-pppd/ctrl/pppoe/pppoe.c) uses
> #include <net/ethernet.h>   /* includes linux/if_ether.h */
> #include <arpa/inet.h>      /* includes netinet/in.h */
> #include <linux/if_pppox.h> /* (through pppoe.h), includes linux/in.h */
> 
> 
> I don't have a satisfying solution for now, but I'd really like it if
> we could avoid shipping a kernel which forces userspace to play with
> include files ordering to keep compiling.
> 
Hi,

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 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.

Hauke