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