linux-kernel - Re: [PATCH v06 33/36] uapi linux/fsmap.h: use __kernel_size_t instead of size

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Message-ID: <CAK8P3a25BVQpqcB5c7nMHHaw383WVhir3rb+QovNeyaZtqmiug@mail.gmail.com>
Date:   Mon, 7 Aug 2017 22:20:58 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Mikko Rapeli <mikko.rapeli@....fi>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH v06 33/36] uapi linux/fsmap.h: use __kernel_size_t instead
 of size_t

On Mon, Aug 7, 2017 at 6:45 PM, Darrick J. Wong <darrick.wong@...cle.com> wrote:
> On Mon, Aug 07, 2017 at 06:01:43PM +0200, Arnd Bergmann wrote:
>> On Mon, Aug 7, 2017 at 5:54 PM, Darrick J. Wong <darrick.wong@...cle.com> wrote:
>> > On Sun, Aug 06, 2017 at 06:44:24PM +0200, Mikko Rapeli wrote:
>>
>> Either way works, but including a system header from a kernel header
>> requires an additional "#ifndef __KERNEL__" check, so I think Miko's
>> variant is a little nicer.
>>
>> Generally speaking, you also want to avoid including system headers
>> indirectly from kernel headers, as POSIX requires that including one
>> system header should not indirectly make symbols from other system
>> headers visible. I think this is not a problem here though, as no system
>> header should include linux/fsmap.h.
>
> Sorry, I guess I was a little unclear about what I was asking -- I was
> wondering why can't the userspace program include sys/types.h prior to
> linux/fsmap.h?  I wasn't proposing including C library headers in kernel
> headers.
>
> I think the patch author is pushing towards kernel headers never relying
> on /anything/ in the system headers.

Right, and I think that is a good thing to have, because it allows us to
do better compile-time testing of the exported kernel headers.

> For data structures being
> exchanged with the kernel I agree, but the fsmap_sizeof result is never
> passed to or received from the kernel; it exists purely for malloc
> convenience.

Would you prefer making fsmap_sizeof a macro? That would also
make it possible to do static checking on the header without having
to resort to odd types.

      Arnd