linux-kernel - Re: [PATCH v10 04/10] kexec_file: Add support for purgatory built as PIE.

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Date:   Sun, 20 Nov 2016 10:45:46 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>
Cc:     kexec@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Eric Biederman <ebiederm@...ssion.com>,
        Vivek Goyal <vgoyal@...hat.com>, Baoquan He <bhe@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Stewart Smith <stewart@...ux.vnet.ibm.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH v10 04/10] kexec_file: Add support for purgatory built as
 PIE.

On 11/10/16 at 01:27am, Thiago Jung Bauermann wrote:
> powerpc's purgatory.ro has 12 relocation types when built as
> a relocatable object. To implement support for them requires
> arch_kexec_apply_relocations_add to duplicate a lot of code with
> module_64.c:apply_relocate_add.
> 
> When built as a Position Independent Executable there are only 4
> relocation types in purgatory.ro, so it becomes practical for the powerpc
> implementation of kexec_file to have its own relocation implementation.
> 
> Also, the purgatory is an executable and not an intermediary output from
> the compiler so it makes sense conceptually that it is easier to build
> it as a PIE than as a partially linked object.
> 
> Apart from the greatly reduced number of relocations, there are two
> differences between a relocatable object and a PIE:
> 
> 1. __kexec_load_purgatory needs to use the program headers rather than the
>    section headers to figure out how to load the binary.
> 2. Symbol values are absolute addresses instead of relative to the
>    start of the section.
> 
> This patch adds the support needed in generic code for the differences
> above and allows powerpc to load and relocate a position independent
> purgatory.
> 

[snip]

The kexec-tools machine_apply_elf_rel is pretty simple for ppc64, it is
not that complex. So could you look into simplify your kexec_file
implementation?

kernel/kexec_file.c kexec_apply_relocations only do limited things
and some of the logic is in arch/x86, so move general code out of arch
code, then I guess the arch code will be simpler and then we probably
do not need this PIE stuff anymore.

BTW, __kexec_really_load_purgatory looks worse than
___kexec_load_purgatory ;)

Thanks
Dave