linux-kernel - [PATCH v3 3/8] x86/boot: Define setup size in linker script

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Date:   Fri, 15 Sep 2023 17:16:27 +0000
From:   Ard Biesheuvel <ardb@...gle.com>
To:     linux-efi@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Evgeniy Baskov <baskov@...ras.ru>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Jones <pjones@...hat.com>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH v3 3/8] x86/boot: Define setup size in linker script

From: Ard Biesheuvel <ardb@...nel.org>

The setup block contains the real mode startup code that is used when
booting from a legacy BIOS, along with the boot_params/setup_data that
is used by legacy x86 bootloaders to pass the command line and initial
ramdisk parameters, among other things.

The setup block also contains the PE/COFF header of the entire combined
image, which includes the compressed kernel image, the decompressor and
the EFI stub.

This PE header describes the layout of the executable image in memory,
and currently, the fact that the setup block precedes it makes it rather
fiddly to get the right values into the right place in the final image.

Let's make things a bit easier by defining the setup_size in the linker
script so it can be referenced from the asm code directly, rather than
having to rely on the build tool to calculate it. For the time being,
add 64 bytes of fixed padding for the .reloc and .compat sections - this
will be removed in a subsequent patch after the PE/COFF header has been
reorganized.

This change has no impact on the resulting bzImage binary when
configured with CONFIG_EFI_MIXED=y.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/boot/header.S      | 2 +-
 arch/x86/boot/setup.ld      | 4 ++++
 arch/x86/boot/tools/build.c | 6 ------
 3 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 72744ba440f6..06bd72a324c1 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -231,7 +231,7 @@ sentinel:	.byte 0xff, 0xff        /* Used to detect broken loaders */
 
 	.globl	hdr
 hdr:
-setup_sects:	.byte 0			/* Filled in by build.c */
+		.byte setup_sects - 1
 root_flags:	.word ROOT_RDONLY
 syssize:	.long 0			/* Filled in by build.c */
 ram_size:	.word 0			/* Obsolete */
diff --git a/arch/x86/boot/setup.ld b/arch/x86/boot/setup.ld
index b11c45b9e51e..9bd5c1ada599 100644
--- a/arch/x86/boot/setup.ld
+++ b/arch/x86/boot/setup.ld
@@ -39,6 +39,10 @@ SECTIONS
 	.signature	: {
 		setup_sig = .;
 		LONG(0x5a5aaa55)
+
+		/* Reserve some extra space for the reloc and compat sections */
+		setup_size = ALIGN(ABSOLUTE(.) + 64, 512);
+		setup_sects = ABSOLUTE(setup_size / 512);
 	}
 
 
diff --git a/arch/x86/boot/tools/build.c b/arch/x86/boot/tools/build.c
index 069497543164..745d64b6d930 100644
--- a/arch/x86/boot/tools/build.c
+++ b/arch/x86/boot/tools/build.c
@@ -48,12 +48,7 @@ typedef unsigned int   u32;
 u8 buf[SETUP_SECT_MAX*512];
 
 #define PECOFF_RELOC_RESERVE 0x20
-
-#ifdef CONFIG_EFI_MIXED
 #define PECOFF_COMPAT_RESERVE 0x20
-#else
-#define PECOFF_COMPAT_RESERVE 0x0
-#endif
 
 static unsigned long efi_pe_entry;
 static unsigned long efi32_pe_entry;
@@ -388,7 +383,6 @@ int main(int argc, char ** argv)
 #endif
 
 	/* Patch the setup code with the appropriate size parameters */
-	buf[0x1f1] = setup_sectors-1;
 	put_unaligned_le32(sys_size, &buf[0x1f4]);
 
 	update_pecoff_text(setup_sectors * 512, i + (sys_size * 16));
-- 
2.42.0.459.ge4e396fd5e-goog