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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:   Mon, 5 Oct 2020 17:51:36 -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.

I pulled this change in and made all the changes to remove other code
that was causing relocations in the compressed kernel. I tested it and
it is working well. Thanks for all the help.

Tested-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$ 
> ---

[snip]