linux-kernel - Re: struct fanotify_event

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Date:	Fri, 20 Aug 2010 10:02:15 +0100
From:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
To:	Eric Paris <eparis@...hat.com>
CC:	Andreas Schwab <schwab@...ux-m68k.org>,
	Andreas Gruenbacher <agruen@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: struct fanotify_event_metadata

On Thursday 19 Aug 2010 18:53:46 Eric Paris wrote:
> On Thu, 2010-08-19 at 16:44 +0100, Tvrtko Ursulin wrote:
> > On Saturday 14 Aug 2010 18:44:38 Andreas Schwab wrote:
> > > The pid field of struct fanotify_event_metadata has 64 bits which looks
> > > excessive.  Wouldn't it make sense to make it 32 bits and swap it with
> > > the mask field?  That would avoid the unaligned mask field, and remove
> > > the need for the packed attribute.
>
> Wish this thought came up 2 weeks ago  :)  It's going to stay __packed__
> no matter what, even if the alignment works out nicely and it doesn't do
> anything.
>
> I'm certainly willing to shrink the pid and switch some locations if
> noone objects but it will definitely break userspace, in that it is
> going to require a recompile of anyone's userspace listener (the
> interface was only intended to grow, not get switched around) but it has
> only been in there about a week so I'm not seeing a huge harm.
>
> I would not be happy to see the mask shrink, we might not be there yet,
> we might not ever get there, but it was part of the future proofing of
> the interface.
>
> Would anyone like to send a patch? Tvrtko?

I think it is OK to break userspace while still in the merge window.
It is not even a big breakage but just a recompile.

So something like the below?
---
Shrink pid field in the fanotify_event_metadata to 32-bit to match
the kernel representation. Pull mask field up since it logically
comes before event auxiliary data and also makes for a nicer
alignment.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
---
 include/linux/fanotify.h |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
index f0949a5..0e7f1bb 100644
--- a/include/linux/fanotify.h
+++ b/include/linux/fanotify.h
@@ -70,9 +70,9 @@
 struct fanotify_event_metadata {
        __u32 event_len;
        __u32 vers;
-       __s32 fd;
        __u64 mask;
-       __s64 pid;
+       __s32 fd;
+       __s32 pid;
 } __attribute__ ((packed));

 struct fanotify_response {


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