linux-kernel - Re: [PATCH -fixes] riscv: Fix relocatable kernels with early alternatives using -fno-pie

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Message-ID: <20230528-darkness-grandly-6cb9e014391d@spud>
Date:   Sun, 28 May 2023 14:12:24 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Alexandre Ghiti <alexghiti@...osinc.com>
Cc:     Alexandre Ghiti <alex@...ti.fr>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andreas Schwab <schwab@...ux-m68k.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -fixes] riscv: Fix relocatable kernels with early
 alternatives using -fno-pie

On Sun, May 28, 2023 at 03:00:57PM +0200, Alexandre Ghiti wrote:
> On Sat, May 27, 2023 at 12:02 PM Conor Dooley <conor@...nel.org> wrote:
> >
> > On Sat, May 27, 2023 at 11:13:18AM +0200, Alexandre Ghiti wrote:
> > >
> > > On 26/05/2023 18:35, Conor Dooley wrote:
> > > > On Fri, May 26, 2023 at 05:24:41PM +0100, Conor Dooley wrote:
> > > > > On Fri, May 26, 2023 at 05:46:30PM +0200, Alexandre Ghiti wrote:
> > > > > > Early alternatives are called with the mmu disabled, and then should not
> > > > > > access any global symbols through the GOT since it requires relocations,
> > > > > > relocations that we do before but *virtually*. So only use medany code
> > > > > > model for this early code.
> > > > > >
> > > > > > Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
> > > > > > ---
> > > > > >
> > > > > > Note that I'm not very happy with this fix, I think we need to put more
> > > > > > effort into "harmonizing" this very early code (ie before the mmu is
> > > > > > enabled) as it is spread between different locations and compiled
> > > > > > differently.
> > > > > Totally & I'll happily spend the time trying to review that work.
> > > > >
> > > > > > I'll work on that later, but for now, this fix does what is
> > > > > > needed to work (from my testing at least). Any Tested-by on the Unmatched
> > > > > > and T-head boards is welcome!
> > > > > On 6.4-rc1 & v6.4-rc1 + this patch, with CONFIG_RELOCATABLE added to my
> > > > > config, my Nezha fails to boot. There is no output whatsoever from the
> > > > > kernel. Turning off CONFIG_RELOCATABLE boots again.
> > > > I don't know if this is better or worse news, but same thing happens on
> > > > an icicle kit. What systems, other than QEMU, has the relocatable
> > > > eries been tested with, btw?
> > >
> > >
> > > I tested it on the Unmatched (Andreas did too).
> >
> > Cool. I cracked out my unmatched and it has the same issue as the
> > icicle. Ditto my Visionfive v2. Here's my config.
> > https://raw.githubusercontent.com/ConchuOD/riscv-env/dev/conf/defconfig
> >
> > A ~default qemu virt doesn't work either. (-m 2G -smp 5)
> 
> I can boot with this config using:
> 
> $ sudo ~/qemu/build/qemu-system-riscv64 -machine virt -cpu
> rv64,sv48=off -nographic -m 2G -smp 5 -kernel
> build_conor/arch/riscv/boot/Image -s

Just in case, that is my normal config that I use for testing random
stuff on LKML, I added CONFIG_RELOCATABLE in addition to that.

> I noticed when trying to add this to our internal CI that I had local
> failures that did not happen in the CI because the CI was not using
> the same toolchain: can you give me the full .config? So that I can
> see if the compiler added stack guards or some other things I did not
> think of.

https://gist.githubusercontent.com/ConchuOD/655f9cc19fb3be63f1c9da7e7e3ab717/raw/a1aad3c0d307609b2062fd3a66705166aede9f9f/.config

90% of what I test for upstream stuff uses clang, since clang appears to
be a minority choice - but I could reproduce this with gcc-12 as well,
using the same defconfig as linked above + CONFIG_RELOCATABLE.

Cheers,
Conor.

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