linux-kernel - Re: [PATCH v06 18/36] uapi linux/errqueue.h: include linux/time.h in user space

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Date:   Sun, 6 Aug 2017 17:24:20 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Mikko Rapeli <mikko.rapeli@....fi>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v06 18/36] uapi linux/errqueue.h: include linux/time.h in
 user space

>> > +#ifdef __KERNEL__
>> > +#include <linux/time.h>
>> > +#else
>> > +#include <time.h>
>> > +#endif /* __KERNEL__ */
>>
>> This will break applications that include <linux/time.h> manually.
>> I previously sent a patch to use libc-compat to make compilation succeed
>> when both are included in the case where <linux/time.h> is included after
>> <time.h>.
>>
>>   https://lkml.org/lkml/2016/9/12/872
>>
>> The inverse will require changes to the libc header to avoid redefining
>> symbols already defined by <linux/time.h>
>>
>> The second patch in that 2-patch set included <linux/time.h>
>> unconditionally after the fix. This broke builds that also included
>> <time.h> in the wrong order. I did not resubmit the first patch as a
>> stand-alone, as it is not sufficient to avoid breakage.
>
> I wasn't aware of your change, but I was about to send this to fix the
> case when glibc <time.h> is included before <linux/time.h>:
>
> https://github.com/mcfrisk/linux/commit/f3952a27b8a21c6478d26e6246055383483f6a66

There are a few differences between the two. Including <time.h> does not
unconditionally define all the symbols. Some are conditional on additional
state, such as __timespec_defined.

> but you also ran into problems where <linux/time.h> is included before
> <time.h> which need fixes in libc header side.
>
> So how to proceed with these?

The libc-compat change is a good fix that can be submitted on its own.

> I don't like leaving a few dozen non-compiling header files into uapi.

I agree, but I do not see a simple solution.

Unless libc has the analogous change, including either <time.h> or
<linux/time.h> in userspace can unfortunately cause breakage.

The added include if __KERNEL__ is defined should be safe, though.