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Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility w

To: Arnd Bergmann <arnd@xxxxxxxx>
Subject: Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Fri, 30 Jan 2009 17:14:23 -0800
Cc: Ankit Jain <me@xxxxxxxxxxxxx>, viro@xxxxxxxxxxxxxxxxxx, hch@xxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, mfasheh@xxxxxxxx, joel.becker@xxxxxxxxxx, ocfs2-devel@xxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs-masters@xxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <200901310138.34164.arnd@xxxxxxxx>
References: <4980C71F.1010804@xxxxxxxxxxxxx> <20090130162252.7bf9c1f4.akpm@xxxxxxxxxxxxxxxxxxxx> <200901310138.34164.arnd@xxxxxxxx>
On Sat, 31 Jan 2009 01:38:32 +0100 Arnd Bergmann <arnd@xxxxxxxx> wrote:

> On Saturday 31 January 2009, Andrew Morton wrote:
> > On Thu, 29 Jan 2009 02:29:11 +0530 Ankit Jain <me@xxxxxxxxxxxxx> wrote:
> > > +struct space_resv {
> > > + __s16           l_type;
> > > + __s16           l_whence;
> > > + __s64           l_start;
> > > + __s64           l_len;          /* len == 0 means until end of file */
> > > + __s32           l_sysid;
> > > + __u32           l_pid;
> > > + __s32           l_pad[4];       /* reserve area                     */
> > > +};
> > > +
> > > +#define F_IOC_RESVSP             _IOW('X', 40, struct space_resv)
> > > +#define F_IOC_RESVSP64           _IOW('X', 42, struct space_resv)
> > 
> > Are we sure that the aligment of l_start will be reliably the same
> > across all compilers and versions thereof for all time?
> 
> On x86, the alignment differs between 32 and 64 bit, otherwise it's ok.

Is this written in a standard somewhere?  Is it guaranteed?

If some (perhaps non-gcc) compiler were to lay this out differently
(perhaps with suitable command-line options) then that's liveable
with - as long as the kernel never changes the layout.  Of course
it would be better to avoid this if poss.

The other potential issue with a structure like this is that there's a
risk that it will lead us to copy four bytes of uninitialised kernel
memory out to userspace.

IOW, it seems a generally bad idea to rely upon compiler-added padding
for this sort of thing.

> XFS handles the conversion for compat_ioctl in
> fs/xfs/linux-2.6/xfs_ioctl32.c. If this becomes a generic file ioctl,
> the conversion code should be moved to fs/compat_ioctl.c.

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