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Subject: Re: [PATCH] x86/boot: Place kernel_info at a fixed offset
To:     Arvind Sankar <nivedita@alum.mit.edu>, x86@kernel.org
Cc:     linux-kernel@vger.kernel.org
References: <20200929210455.2718827-1-nivedita@alum.mit.edu>
From:   Ross Philipson <ross.philipson@oracle.com>
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Date:   Tue, 29 Sep 2020 18:13:13 -0400
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On 9/29/20 5:04 PM, Arvind Sankar wrote:
> 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.

That is great. It looks like just what we will need. Thank you very much.

Acked-by: Ross Philipson <ross.philipson@oracle.com>

> 
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> Cc: Ross Philipson <ross.philipson@oracle.com>
> [0] https://urldefense.com/v3/__https://lore.kernel.org/lkml/1600959521-24158-8-git-send-email-ross.philipson@oracle.com/__;!!GqivPVa7Brio!JiEGSToX4gewsr8DY0iWrRR7oTzMZl1b-WBsZ1U6M7dCJXuGzIrF-5qVqN4v8LEys8M$ 
> ---
>  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)
>  	}
>