linux-kernel - Re: struct fanotify_event

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Message-ID: <1282317570.15124.20.camel@dhcp231-77.rdu.redhat.com>
Date:	Fri, 20 Aug 2010 11:19:30 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Andreas Schwab <schwab@...hat.com>
Cc:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>,
	Andreas Gruenbacher <agruen@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: struct fanotify_event_metadata

On Fri, 2010-08-20 at 15:27 +0200, Andreas Schwab wrote:
> Eric Paris <eparis@...hat.com> writes:
> 
> > Can you help me understand the packed attribute and why it hurts in this
> > case?
> 
> It changes the alignment of all applicable objects to 1 which means that
> the compiler cannot assume _any_ aligment.  Thus on STRICT_ALIGNMENT
> targets it has to use more expensive access methods to avoid generating
> unaligned loads and stores (unless it can infer proper alignment from
> the context).

Andreas suggested (I accidentally dropped the list when I ask him) I use
natural alignment and explicit padding rather than ((packed))

I'm open to the idea but I want to make it idiot proof (aka I won't
screw it up later)  My best offhand idea would be to do something like
so:

Expose this to userspace:
struct fanotify_event_metadata {
        __u32 event_len;
        __u32 vers;
        __s32 fd;
        __u64 mask;
        __s64 pid;
};

Wrap this in #ifdef KERNEL
struct fanotify_event_metadata_packed {
        __u32 event_len;
        __u32 vers;
        __s32 fd;
        __u64 mask;
        __s64 pid;
} __attribute__ ((packed));

Then add:
BUILD_BUG_ON(sizeof(struct fanotify_event_metadata) != 
             sizeof(struct fanotify_event_metadata_packed);

Is that a good way to make the actual object non-packed but keep myself
from ever letting it fail alignment and padding requirements?

-Eric

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