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From: Philipp Rudo <prudo@redhat.com>
To: Alexander Egorenkov <egorenar@linux.ibm.com>
Cc: ltao@redhat.com, hca@linux.ibm.com, linux-s390@vger.kernel.org
Subject: Re: [PATCH 1/1] s390/kexec: handle R_390_PLT32DBL reloc entries in arch_kexec_do_relocs()
Date: Thu, 9 Dec 2021 12:00:37 +0100	[thread overview]
Message-ID: <20211209120037.050e3db1@rhtmp> (raw)
In-Reply-To: <8735n32dua.fsf@oc8242746057.ibm.com>

Hi Alexander,

On Wed, 08 Dec 2021 18:42:05 +0100
Alexander Egorenkov <egorenar@linux.ibm.com> wrote:

> Hi Philipp,
> 
> Philipp Rudo <prudo@redhat.com> writes:
> 
> > Hi Alexander,
> >
> > thanks for taking care of this so fast!
> >
> > My personal approach was slightly different (see below). The idea
> > behind this was that arch_kexec_do_relocs is also used by others which
> > might have a PLT. For them your approach would mean an ABI breakage. On
> > the other hand in case the other users have the same problem they would
> > need to handle it for themselves. Not sure what's the better approach.
> >  
> 
> I'm also fine with your proposal for the fix.
> 
> If i'm not mistaken, arch_kexec_do_relocs() is used in decompressor and
> for purgatory at the moment, and both should have all relocs resolved.
> Do we have more users of arch_kexec_do_relocs() ? That was the reason i
> added the fix to arch_kexec_do_relocs().

no, there are no other users of arch_kexec_do_relocs other than
kexec_file and kaslr at the moment.

From a technical perspective your patch was totally fine for todays
use cases. My concern was more for a potential future user. But maybe I
was a little bit overcautious... 

> Now i'm actually wondering why we don't have any issues with
> decompressor, probably because we do the final link where all
> PLT-relative addresses are fixed. Because i see plenty of R_390_PLT32DBL
> in startup.o e.g.

IIUC, the culprit is the '-r' option in the LDFLAGS. With this option
the output can be used again as input _for ld_. The way I interpret
this is that ld can (and apparently does) use any internal convention
without complying with the ABI when this option is given.

kaslr doesn't use '-r' but a combination of -fPIE (CFLAGS) and -pie
(LDFLAGS) so they are not affected by ld's internal behavior. Maybe
it's worth investigating in the long run if the purgatory could be build
with -fPIE/-pie as well.

Thanks
Philipp


      reply	other threads:[~2021-12-09 11:00 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-08 10:58 Alexander Egorenkov
2021-12-08 13:59 ` Philipp Rudo
2021-12-08 17:42   ` Alexander Egorenkov
2021-12-09 11:00     ` Philipp Rudo [this message]

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