linux-kernel - Re: kexec, x86/purgatory: Cleanup the unholy mess

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Date:   Fri, 10 Mar 2017 09:24:18 -0500
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Mike Galbraith <efault@....de>, Borislav Petkov <bp@...en8.de>,
        "Tobin C. Harding" <me@...in.cc>,
        LKML <linux-kernel@...r.kernel.org>,
        Nicholas Mc Guire <der.herr@...r.at>, x86@...nel.org,
        Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>
Subject: Re: kexec, x86/purgatory: Cleanup the unholy mess

On Fri, Mar 10, 2017 at 02:57:38PM +0100, Thomas Gleixner wrote:
> On Fri, 10 Mar 2017, Mike Galbraith wrote:
> > On Fri, 2017-03-10 at 13:17 +0100, Thomas Gleixner wrote:
> > > The purgatory code defines global variables which are referenced via a
> > > symbol lookup in the kexec code (core and arch).
> > > 
> > > A recent commit addressing sparse warning made these static and thereby
> > > broke kexec file.
> > > 
> > > Why did this happen? Simply because the whole machinery is undocumented and
> > > lacks any form of forward declarations. The variable names are unspecific
> > > and lack a prefix, so adding forward declarations creates shadow variables
> > > in the core code. Aside of that the code relies on magic constants and
> > > duplicate struct definitions with no way to ensure that these things stay
> > > in sync.
> > > 
> > > Unbreak kexec and cleanup the mess by:
> > > 
> > >  - Adding proper forward declarations and document the usage
> > >  - Use the proper common defines instead of magic constants
> > >  - Add a purgatory_ prefix to have a proper name space
> > >  - Use ARRAY_SIZE() instead of a homebrewn reimplementation
> > > 
> > > Fixes: 72042a8c7b01 ("x86/purgatory: Make functions and variables static")
> > 
> > Well, almost fixes.
> > 
> > [   15.118820] kexec: symbol 'purgatory_sha_regions' in common section
> > [   15.119187] kexec-bzImage64: Loading purgatory failed
> 
> Bah. /me goes to investigate.

I think we probably will have to initialize these global variables in
purgatory itself and that puts them in .data section and relocation
works.

That's how the code was intially. I initialized value of
purgatory_sha256_digest in the code and then did "readelf -a purgatory.o"
and symbol section index changed from COM to 3.

13: 0000000000000000    32 OBJECT  GLOBAL DEFAULT    3
purgatory_sha256_digest

[ 3] .data             PROGBITS         0000000000000000  00000120
         0000000000000020  0000000000000000  WA       0     0     32

Thanks
Vivek