linux-kernel - Re: [PATCH] drm: Only use DRM_IOCTL_UPDATE

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Message-Id: <200902191519.03399.arnd@arndb.de>
Date:	Thu, 19 Feb 2009 15:19:01 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	David Miller <davem@...emloft.net>
Cc:	airlied@...ux.ie, benh@...nel.crashing.org,
	dri-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: Only use DRM_IOCTL_UPDATE_DRAW compat wrapper for compat X86.

On Wednesday 18 February 2009, David Miller wrote:
> drm: Only use DRM_IOCTL_UPDATE_DRAW compat wrapper for compat X86.
> 
> Only X86 32-bit uses a different alignment for "unsigned long long"
> than it's 64-bit counterpart.
> 
> Therefore this compat translation is only correct, and only needed,
> when either CONFIG_X86 or CONFIG_IA64.
> 
> Signed-off-by: David S. Miller <davem@...emloft.net>

The patch is correct AFAICT, but I'd like to point out that the
problem could have been avoided (besides using a non-padded layout)
by using a compat_u64 member in the struct definition instead of
the packed attribute:

typedef struct drm_update_draw32 {
	drm_drawable_t handle;
	unsigned int type;
	unsigned int num;
	compat_u64 data;	/**< Pointer */
} drm_update_draw32_t;

I find that too often __attribute__((packed)) is used on whole
structures where some other much more gentle solution can be
applied. In fact, there are very few files (e.g.
linux/unaligned/packed_struct.h) that look like they want all
of the implied meanings (pack members, drop alignment on whole
structure, access members as unaligned).

A grep for "packed" in compat_ioctl definitions revealed the
same bug as in drm_update_draw32 to be present in
raw32_config_request, and I'm rather sure that there are more
of these.

	Arnd <><
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