[PATCHv3 3/4] arm64: Update the Image header

Mark Rutland mark.rutland at arm.com
Thu Jun 19 03:49:22 PDT 2014

Currently the kernel Image is stripped of everything past the initial
stack, and at runtime the memory is initialised and used by the kernel.
This makes the effective minimum memory footprint of the kernel larger
than the size of the loaded binary, though bootloaders have no mechanism
to identify how large this minimum memory footprint is. This makes it
difficult to choose safe locations to place both the kernel and other
binaries required at boot (DTB, initrd, etc), such that the kernel won't
clobber said binaries or other reserved memory during initialisation.

Additionally when big endian support was added the image load offset was
overlooked, and is currently of an arbitrary endianness, which makes it
difficult for bootloaders to make use of it. It seems that bootloaders
aren't respecting the image load offset at present anyway, and are
assuming that offset 0x80000 will always be correct.

This patch adds an effective image size to the kernel header which
describes the amount of memory from the start of the kernel Image binary
which the kernel expects to use before detecting memory and handling any
memory reservations. This can be used by bootloaders to choose suitable
locations to load the kernel and/or other binaries such that the kernel
will not clobber any memory unexpectedly. As before, memory reservations
are required to prevent the kernel from clobbering these locations

Both the image load offset and the effective image size are forced to be
little-endian regardless of the native endianness of the kernel to
enable bootloaders to load a kernel of arbitrary endianness. Bootloaders
which wish to make use of the load offset can inspect the effective
image size field for a non-zero value to determine if the offset is of a
known endianness. To enable software to determine the endinanness of the
kernel as may be required for certain use-cases, a new flags field (also
little-endian) is added to the kernel header to export this information.

The documentation is updated to clarify these details. To discourage
future assumptions regarding the value of text_offset, the value at this
point in time is removed from the main flow of the documentation (though
kept as a compatibility note). Some minor formatting issues in the
documentation are also corrected.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Acked-by: Tom Rini <trini at ti.com>
Cc: Geoff Levand <geoff at infradead.org>
Cc: Kevin Hilman <kevin.hilman at linaro.org>
 Documentation/arm64/booting.txt | 43 ++++++++++++++++++++++------
 arch/arm64/include/asm/image.h  | 62 +++++++++++++++++++++++++++++++++++++++++
 arch/arm64/kernel/head.S        |  6 ++--
 arch/arm64/kernel/vmlinux.lds.S |  3 ++
 4 files changed, 103 insertions(+), 11 deletions(-)
 create mode 100644 arch/arm64/include/asm/image.h

diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
index 37fc4f6..85af34d 100644
--- a/Documentation/arm64/booting.txt
+++ b/Documentation/arm64/booting.txt
@@ -72,27 +72,54 @@ The decompressed kernel image contains a 64-byte header as follows:
   u32 code0;			/* Executable code */
   u32 code1;			/* Executable code */
-  u64 text_offset;		/* Image load offset */
-  u64 res0	= 0;		/* reserved */
-  u64 res1	= 0;		/* reserved */
+  u64 text_offset;		/* Image load offset, little endian */
+  u64 image_size;		/* Effective Image size, little endian */
+  u64 flags;			/* kernel flags, little endian */
   u64 res2	= 0;		/* reserved */
   u64 res3	= 0;		/* reserved */
   u64 res4	= 0;		/* reserved */
   u32 magic	= 0x644d5241;	/* Magic number, little endian, "ARM\x64" */
-  u32 res5 = 0;      		/* reserved */
+  u32 res5;      		/* reserved (used for PE COFF offset) */
 Header notes:
+- As of v3.17, all fields are little endian unless stated otherwise.
 - code0/code1 are responsible for branching to stext.
 - when booting through EFI, code0/code1 are initially skipped.
   res5 is an offset to the PE header and the PE header has the EFI
-  entry point (efi_stub_entry). When the stub has done its work, it
+  entry point (efi_stub_entry).  When the stub has done its work, it
   jumps to code0 to resume the normal boot process.
-The image must be placed at the specified offset (currently 0x80000)
-from the start of the system RAM and called there. The start of the
-system RAM must be aligned to 2MB.
+- Prior to v3.17, the endianness of text_offset was not specified.  In
+  these cases image_size is zero and text_offset is 0x80000 in the
+  endianness of the kernel.  Where image_size is non-zero image_size is
+  little-endian and must be respected.  Where image_size is zero,
+  text_offset can be assumed to be 0x80000.
+- The flags field (introduced in v3.17) is a little-endian 64-bit field
+  composed as follows:
+  Bit 0: 	Kernel endianness.  1 if BE, 0 if LE.
+  Bits 1-63:	Reserved.
+- When image_size is zero, a bootloader should attempt to keep as much
+  memory as possible free for use by the kernel immediately after the
+  end of the kernel image. The amount of space required will vary
+  depending on selected features, and is effectively unbound.
+The Image must be placed text_offset bytes from a 2MB aligned base
+address near the start of usable system RAM and called there. Memory
+below that base address is currently unusable by Linux, and therefore it
+is strongly recommended that this location is the start of system RAM.
+At least image_size bytes from the start of the image must be free for
+use by the kernel.
+Any memory described to the kernel (even that below the 2MB aligned base
+address) which is not marked as reserved from the kernel e.g. with a
+memreserve region in the device tree) will be considered as available to
+the kernel.
 Before jumping into the kernel, the following conditions must be met:
diff --git a/arch/arm64/include/asm/image.h b/arch/arm64/include/asm/image.h
new file mode 100644
index 0000000..8fae075
--- /dev/null
+++ b/arch/arm64/include/asm/image.h
@@ -0,0 +1,62 @@
+ * Linker script macros to generate Image header fields.
+ *
+ * Copyright (C) 2014 ARM Ltd.
+ *
+ * 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.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef __ASM_IMAGE_H
+#define __ASM_IMAGE_H
+#error This file should only be included in vmlinux.lds.S
+ * There aren't any ELF relocations we can use to endian-swap values known only
+ * at link time (e.g. the subtraction of two symbol addresses), so we must get
+ * the linker to endian-swap certain values before emitting them.
+ */
+#define DATA_LE64(data)					\
+	((((data) & 0x00000000000000ff) << 56) |	\
+	 (((data) & 0x000000000000ff00) << 40) |	\
+	 (((data) & 0x0000000000ff0000) << 24) |	\
+	 (((data) & 0x00000000ff000000) << 8)  |	\
+	 (((data) & 0x000000ff00000000) >> 8)  |	\
+	 (((data) & 0x0000ff0000000000) >> 24) |	\
+	 (((data) & 0x00ff000000000000) >> 40) |	\
+	 (((data) & 0xff00000000000000) >> 56))
+#define DATA_LE64(data) ((data) & 0xffffffffffffffff)
+#define __HEAD_FLAG_BE	1
+#define __HEAD_FLAG_BE	0
+#define __HEAD_FLAGS	(__HEAD_FLAG_BE << 0)
+ * These will output as part of the Image header, which should be little-endian
+ * regardless of the endianness of the kernel. While constant values could be
+ * endian swapped in head.S, all are done here for consistency.
+ */
+#define HEAD_SYMBOLS						\
+	_kernel_size_le		= DATA_LE64(_end - _text);	\
+	_kernel_offset_le	= DATA_LE64(TEXT_OFFSET);	\
+	_kernel_flags_le	= DATA_LE64(__HEAD_FLAGS);
+#endif /* __ASM_IMAGE_H */
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index e048f2b..7b59c3d 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -108,9 +108,9 @@ efi_head:
 	b	stext				// branch to kernel start, magic
 	.long	0				// reserved
-	.quad	TEXT_OFFSET			// Image load offset from start of RAM
-	.quad	0				// reserved
-	.quad	0				// reserved
+	.quad	_kernel_offset_le		// Image load offset from start of RAM, little-endian
+	.quad	_kernel_size_le			// Effective size of kernel image, little-endian
+	.quad	_kernel_flags_le		// Informative flags, little-endian
 	.quad	0				// reserved
 	.quad	0				// reserved
 	.quad	0				// reserved
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index c6648d3..9cbf77a 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -5,6 +5,7 @@
 #include <asm-generic/vmlinux.lds.h>
+#include <asm/image.h>
 #include <asm/thread_info.h>
 #include <asm/memory.h>
 #include <asm/page.h>
@@ -114,6 +115,8 @@ SECTIONS
 	_end = .;

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