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From:   Arvind Sankar <nivedita@alum.mit.edu>
To:     x86@kernel.org
Cc:     linux-kernel@vger.kernel.org,
        Ross Philipson <ross.philipson@oracle.com>
Subject: [PATCH] x86/boot: Place kernel_info at a fixed offset
Date:   Tue, 29 Sep 2020 17:04:55 -0400
Message-Id: <20200929210455.2718827-1-nivedita@alum.mit.edu>
MIME-Version: 1.0
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There are use cases for storing the offset of a symbol in kernel_info.
For example, the trenchboot series [0] needs to store the offset of the
Measured Launch Environment header in kernel_info.

Since commit (note: commit ID from tip/master)

  527afc212231 ("x86/boot: Check that there are no run-time relocations")

run-time relocations are not allowed in the compressed kernel, so simply
using the symbol in kernel_info, as

	.long	symbol

will cause a linker error because this is not position-independent.

With kernel_info being a separate object file and in a different section
from startup_32, there is no way to calculate the offset of a symbol
from the start of the image in a position-independent way.

To enable such use cases, put kernel_info into its own section which is
placed at a predetermined offset (KERNEL_INFO_OFFSET) via the linker
script. This will allow calculating the symbol offset in a
position-independent way, by adding the offset from the start of
kernel_info to KERNEL_INFO_OFFSET.

Ensure that kernel_info is aligned, and use the SYM_DATA.* macros
instead of bare labels. This stores the size of the kernel_info
structure in the ELF symbol table.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Ross Philipson <ross.philipson@oracle.com>
[0] https://lore.kernel.org/lkml/1600959521-24158-8-git-send-email-ross.philipson@oracle.com/
---
 arch/x86/boot/compressed/kernel_info.S | 19 +++++++++++++++----
 arch/x86/boot/compressed/kernel_info.h | 12 ++++++++++++
 arch/x86/boot/compressed/vmlinux.lds.S |  6 ++++++
 3 files changed, 33 insertions(+), 4 deletions(-)
 create mode 100644 arch/x86/boot/compressed/kernel_info.h

diff --git a/arch/x86/boot/compressed/kernel_info.S b/arch/x86/boot/compressed/kernel_info.S
index f818ee8fba38..c18f07181dd5 100644
--- a/arch/x86/boot/compressed/kernel_info.S
+++ b/arch/x86/boot/compressed/kernel_info.S
@@ -1,12 +1,23 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 
+#include <linux/linkage.h>
 #include <asm/bootparam.h>
+#include "kernel_info.h"
 
-	.section ".rodata.kernel_info", "a"
+/*
+ * If a field needs to hold the offset of a symbol from the start
+ * of the image, use the macro below, eg
+ *	.long	rva(symbol)
+ * This will avoid creating run-time relocations, which are not
+ * allowed in the compressed kernel.
+ */
+
+#define rva(X) (((X) - kernel_info) + KERNEL_INFO_OFFSET)
 
-	.global kernel_info
+	.section ".rodata.kernel_info", "a"
 
-kernel_info:
+	.balign	16
+SYM_DATA_START(kernel_info)
 	/* Header, Linux top (structure). */
 	.ascii	"LToP"
 	/* Size. */
@@ -19,4 +30,4 @@ kernel_info:
 
 kernel_info_var_len_data:
 	/* Empty for time being... */
-kernel_info_end:
+SYM_DATA_END_LABEL(kernel_info, SYM_L_LOCAL, kernel_info_end)
diff --git a/arch/x86/boot/compressed/kernel_info.h b/arch/x86/boot/compressed/kernel_info.h
new file mode 100644
index 000000000000..c127f84aec63
--- /dev/null
+++ b/arch/x86/boot/compressed/kernel_info.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef BOOT_COMPRESSED_KERNEL_INFO_H
+#define BOOT_COMPRESSED_KERNEL_INFO_H
+
+#ifdef CONFIG_X86_64
+#define KERNEL_INFO_OFFSET 0x500
+#else /* 32-bit */
+#define KERNEL_INFO_OFFSET 0x100
+#endif
+
+#endif /* BOOT_COMPRESSED_KERNEL_INFO_H */
diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
index 112b2375d021..84c7b4de489e 100644
--- a/arch/x86/boot/compressed/vmlinux.lds.S
+++ b/arch/x86/boot/compressed/vmlinux.lds.S
@@ -7,6 +7,7 @@ OUTPUT_FORMAT(CONFIG_OUTPUT_FORMAT)
 
 #include <asm/cache.h>
 #include <asm/page_types.h>
+#include "kernel_info.h"
 
 #ifdef CONFIG_X86_64
 OUTPUT_ARCH(i386:x86-64)
@@ -27,6 +28,11 @@ SECTIONS
 		HEAD_TEXT
 		_ehead = . ;
 	}
+	.rodata.kernel_info KERNEL_INFO_OFFSET : {
+		*(.rodata.kernel_info)
+	}
+	ASSERT(ABSOLUTE(kernel_info) == KERNEL_INFO_OFFSET, "kernel_info at bad address!")
+
 	.rodata..compressed : {
 		*(.rodata..compressed)
 	}
-- 
2.26.2