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

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Date:   Mon, 7 Aug 2017 09:45:05 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Arnd Bergmann <arnd@...db.de>
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 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:
> >> Fixes user space compilation error:
> >>
> >> linux/fsmap.h:71:19: error: unknown type name ‘size_t’
> >>  static __inline__ size_t
> >>                    ^~~~~~
> >>
> >> Signed-off-by: Mikko Rapeli <mikko.rapeli@....fi>
> >> Cc: Darrick J. Wong <darrick.wong@...cle.com>
> >> ---
> >>  include/uapi/linux/fsmap.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/include/uapi/linux/fsmap.h b/include/uapi/linux/fsmap.h
> >> index 7e8e5f0bd6d2..99600bbed6b6 100644
> >> --- a/include/uapi/linux/fsmap.h
> >> +++ b/include/uapi/linux/fsmap.h
> >> @@ -68,7 +68,7 @@ struct fsmap_head {
> >>  };
> >>
> >>  /* Size of an fsmap_head with room for nr records. */
> >> -static inline size_t
> >> +static inline __kernel_size_t
> >
> > This is a static inline helper to assist in malloc calls.  We don't pass
> > size_t to the kernel, so why is this necessary over, say,
> >
> > #include <sys/types.h>
> >
> 
> 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.  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.

--D

> 
>          Arnd
> --
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