From 2c33c27fd6033ced942c9a591b8ac15c07c57d70 Mon Sep 17 00:00:00 2001 From: Daniel Kiper Date: Tue, 12 Nov 2019 14:46:38 +0100 Subject: x86/boot: Introduce kernel_info The relationships between the headers are analogous to the various data sections: setup_header = .data boot_params/setup_data = .bss What is missing from the above list? That's right: kernel_info = .rodata We have been (ab)using .data for things that could go into .rodata or .bss for a long time, for lack of alternatives and -- especially early on -- inertia. Also, the BIOS stub is responsible for creating boot_params, so it isn't available to a BIOS-based loader (setup_data is, though). setup_header is permanently limited to 144 bytes due to the reach of the 2-byte jump field, which doubles as a length field for the structure, combined with the size of the "hole" in struct boot_params that a protected-mode loader or the BIOS stub has to copy it into. It is currently 119 bytes long, which leaves us with 25 very precious bytes. This isn't something that can be fixed without revising the boot protocol entirely, breaking backwards compatibility. boot_params proper is limited to 4096 bytes, but can be arbitrarily extended by adding setup_data entries. It cannot be used to communicate properties of the kernel image, because it is .bss and has no image-provided content. kernel_info solves this by providing an extensible place for information about the kernel image. It is readonly, because the kernel cannot rely on a bootloader copying its contents anywhere, but that is OK; if it becomes necessary it can still contain data items that an enabled bootloader would be expected to copy into a setup_data chunk. Do not bump setup_header version in arch/x86/boot/header.S because it will be followed by additional changes coming into the Linux/x86 boot protocol. Suggested-by: H. Peter Anvin (Intel) Signed-off-by: Daniel Kiper Signed-off-by: Borislav Petkov Reviewed-by: Konrad Rzeszutek Wilk Reviewed-by: Ross Philipson Reviewed-by: H. Peter Anvin (Intel) Cc: Andy Lutomirski Cc: ard.biesheuvel@linaro.org Cc: Boris Ostrovsky Cc: dave.hansen@linux.intel.com Cc: eric.snowberg@oracle.com Cc: Ingo Molnar Cc: Jonathan Corbet Cc: Juergen Gross Cc: kanth.ghatraju@oracle.com Cc: linux-doc@vger.kernel.org Cc: linux-efi Cc: Peter Zijlstra Cc: rdunlap@infradead.org Cc: ross.philipson@oracle.com Cc: Thomas Gleixner Cc: x86-ml Cc: xen-devel@lists.xenproject.org Link: https://lkml.kernel.org/r/20191112134640.16035-2-daniel.kiper@oracle.com --- arch/x86/include/uapi/asm/bootparam.h | 1 + 1 file changed, 1 insertion(+) (limited to 'arch/x86/include/uapi') diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h index c895df5482c5..a1ebcd7a991c 100644 --- a/arch/x86/include/uapi/asm/bootparam.h +++ b/arch/x86/include/uapi/asm/bootparam.h @@ -88,6 +88,7 @@ struct setup_header { __u64 pref_address; __u32 init_size; __u32 handover_offset; + __u32 kernel_info_offset; } __attribute__((packed)); struct sys_desc_table { -- cgit v1.2.3