linux-kernel - Re: [PATCH -v2] fanotify: drops the packed attribute from userspace event metadata

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Message-ID: <AANLkTimbQivEd7z1+sdLgChieGrY3c9iQWXVKgcYRAY=@mail.gmail.com>
Date:	Fri, 27 Aug 2010 19:51:53 -0400
From:	Eric Paris <eparis@...isplace.org>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Andreas Schwab <schwab@...hat.com>,
	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>,
	Andreas Gruenbacher <agruen@...e.de>
Subject: Re: [PATCH -v2] fanotify: drops the packed attribute from userspace
 event metadata

I liked this version until I realized that userspace doesn't have
aligned_u64 as a valid type.

I'm feeling more like my old version of the patch.  Anyone have
thoughts or comments?

-Eric

On Tue, Aug 24, 2010 at 4:43 PM, Eric Paris <eparis@...hat.com> wrote:
> The userspace event metadata structure was packed so when sent from a kernel
> with a certain set of alignment rules to a userspace listener with a different
> set of alignment rules the userspace process would be able to use the
> structure.  On some arches just using packed, even if it doesn't do anything
> to the alignment can cause a severe performance hit.  From now on we are
> not going to set the packed attribute and will just need to be very careful
> to make sure the structure is naturally aligned.
>
> Cc: Andreas Schwab <schwab@...hat.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
> Cc: Andreas Gruenbacher <agruen@...e.de>
> Signed-off-by: Eric Paris <eparis@...hat.com>
> ---
>  include/linux/fanotify.h |   13 ++++++++-----
>  1 files changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
> index 0535461..b892e46 100644
> --- a/include/linux/fanotify.h
> +++ b/include/linux/fanotify.h
> @@ -65,20 +65,23 @@
>                                 FAN_ALL_PERM_EVENTS |\
>                                 FAN_Q_OVERFLOW)
>
> -#define FANOTIFY_METADATA_VERSION      1
> -
> +#define FANOTIFY_METADATA_VERSION      2
> +/*
> + * These structures must be naturally aligned so that a 32 bit userspace process
> + * will find the offsets the same as a 64bit process.
> + */
>  struct fanotify_event_metadata {
>        __u32 event_len;
>        __u32 vers;
> -       __u64 mask;
> +       aligned_u64 mask;
>        __s32 fd;
>        __s32 pid;
> -} __attribute__ ((packed));
> +};
>
>  struct fanotify_response {
>        __s32 fd;
>        __u32 response;
> -} __attribute__ ((packed));
> +};
>
>  /* Legit userspace responses to a _PERM event */
>  #define FAN_ALLOW      0x01
> --
> 1.7.1
>
> --
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