linux-kernel - Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls

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Date:	Sun, 01 Feb 2009 11:48:22 +0200
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Ankit Jain <me@...itjain.org>, viro@...iv.linux.org.uk,
	hch@...radead.org, linux-fsdevel@...r.kernel.org, mfasheh@...e.com,
	joel.becker@...cle.com, ocfs2-devel@....oracle.com,
	linux-kernel@...r.kernel.org, xfs-masters@....sgi.com,
	xfs@....sgi.com
Subject: Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility
 with legacy xfs ioctls

Arnd Bergmann wrote:
> On Saturday 31 January 2009, Andrew Morton wrote:
>> Is this written in a standard somewhere?  Is it guaranteed?
> 
> Alignment is defined in the architecture psABI documents. 
> Unfortunately, many of them were written before the 'long long'
> type became part of the C standard, so it's not strictly guaranteed.
> AFAICT, the alignment of __u64 on x86 is the same as the alignment
> of 'double' by convention.
> 
> However, the problem is well-understood: x86 is the only one
> that has a problem in 32/64 bit compat mode. m68k has similar
> issues with 16/32 bit integers, but those don't apply here.
> 
>> 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.
> 
> If a compiler was using irregular structure alignment, all sorts of
> library interfaces would break. The kernel ABI is only a small part
> of the problem then.
> 
>> 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.
> 
> Agreed in general, but the whole point of this particular patch was to
> provide compatibility with an interface that has been part of XFS for
> many years.
> Linux already has a better interface for new users (sys_fallocate), so
> changing the patch would not be helpful and not provide any advantage.
> 
> There is also no leak of uninitialized data here, because this structure
> is only read, never written.
> 
> 	Arnd <><
Arnd Bergmann 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			    */
> +};

What about telling the compiler exactly what you said above, just
to be sure we all mean the same thing. (And as documentation for new
comers):

+struct space_resv_64 {
+	__s16		l_type;
+	__s16		l_whence;
+	__u32		reserved;
+	__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			    */
+} __packed;

And define another one for x86_32

Boaz
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