[RFC PATCH 2/3] arm64: add C struct definition for Image header

[Geoff Levand]

Hi Ard,

On Tue, 2014-07-08 at 14:50 +0200, Ard Biesheuvel wrote:
> In order to be able to interpret the Image header from C code, we need a
> struct definition that reflects the specification for Image headers as laid
> out in Documentation/arm64/booting.txt.

I think the duplication of the structure definition in booting.txt
and in image_hdr.h will not be maintained, so I recommend we remove
the definition in booting.txt and replace it with something like:

-The decompressed kernel image contains a 64-byte header as follows:
...
+The decompressed kernel image contains a 64-byte header as described in
+arch/arm64/include/asm/image_hdr.h.


> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  arch/arm64/include/asm/image_hdr.h | 53 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 53 insertions(+)
>  create mode 100644 arch/arm64/include/asm/image_hdr.h
> 
> diff --git a/arch/arm64/include/asm/image_hdr.h b/arch/arm64/include/asm/image_hdr.h
> new file mode 100644
> index 000000000000..16d32600fb18
> --- /dev/null
> +++ b/arch/arm64/include/asm/image_hdr.h
> @@ -0,0 +1,53 @@
> +/*
> + * image_hdr.h - C struct definition of the Image header format
> + *
> + * Copyright (C) 2014 Linaro Ltd  <ard.biesheuvel at linaro.org>

Are you really a copyright holder of this code?

> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + */
> +
> +#ifndef __ASM_IMAGE_HDR_H
> +#define __ASM_IMAGE_HDR_H
> +
> +#include <linux/bug.h>
> +#include <linux/byteorder/generic.h>
> +#include <linux/types.h>
> +
> +/*
> + * As defined in Documentation/arm64/booting.txt
> + */
> +#define IMAGE_HDR_SIZE		64

I can't see any use for this IMAGE_HDR_SIZE.  We have the
structure def, so use sizeof.


Please add a comment here to document this structure and the
members, not in the structure itself.  Something like:

+/**
+ * struct arm64_image_header - arm64 kernel image header.
+ *
+ * @code0: ...

> +
> +struct image_hdr {

Since this is a global def, and possibly exported to user space,
I think arm64_image_hdr would be more appropriate.

> +	u32	code0;		/* Executable code */
> +	u32	code1;		/* Executable code */
> +	__le64	text_offset;	/* Image load offset */
> +	u64	res0;		/* Reserved, must be 0 */
> +	u64	res1;		/* Reserved, must be 0 */
> +	u64	res2;		/* Reserved, must be 0 */
> +	u64	res3;		/* Reserved, must be 0 */
> +	u64	res4;		/* Reserved, must be 0 */
> +	__le32	magic;		/* Magic number, little endian, "ARM\x64" */
> +	__le32	pehdr_offset;	/* PE header offset, only used by EFI */
> +};

These are kernel types.  If we used standard C types then this header
could also be exported for user space programs, so replace u32 with
uint32_t, etc.  We can use helper routines to like image_hdr_text_offset()
to wrap the little endian members.

To export it we would need to add this to arch/arm64/include/asm/Kbuild:

+header-y += image_hrd.h

> +
> +/*
> + * bool image_hdr_check() - checks the Image header for inconsistencies.
> + */
> +static inline bool image_hdr_check(struct image_hdr const *hdr)

> +{
> +	BUILD_BUG_ON(sizeof(struct image_hdr) != IMAGE_HDR_SIZE);
> +
> +	if (hdr->res0 | hdr->res1 | hdr->res2 | hdr->res3 | hdr->res4)
> +		return false;

I don't think we should check these reserved members, as it will limit
forward compatibility of this routine.  The magic check should be a
sufficient check.  If it is not, then we need a better magic value.
 
> +	return hdr->magic == cpu_to_le32(0x644d5241);
> +}
> +
> +static inline u64 image_hdr_text_offset(struct image_hdr const *hdr)
> +{
> +	return le64_to_cpu(hdr->text_offset);
> +}

-Geoff