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

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Message-ID: <CAHVXubgG31moSNOe3fRqFzUSJK9tRWKH1KPP_BO7wRNC4WzxTQ@mail.gmail.com>
Date:   Tue, 30 May 2023 20:04:17 +0200
From:   Alexandre Ghiti <alexghiti@...osinc.com>
To:     Conor Dooley <conor@...nel.org>
Cc:     Alexandre Ghiti <alex@...ti.fr>,
        Conor Dooley <conor.dooley@...rochip.com>,
        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 Tue, May 30, 2023 at 7:47 PM Conor Dooley <conor@...nel.org> wrote:
>
> On Tue, May 30, 2023 at 04:33:45PM +0200, Alexandre Ghiti wrote:
> >
> > On 30/05/2023 13:27, Conor Dooley wrote:
> > > On Mon, May 29, 2023 at 09:37:28PM +0200, Alexandre Ghiti wrote:
> > > > On 29/05/2023 21:06, Conor Dooley wrote:
> > > > > On Mon, May 29, 2023 at 08:51:57PM +0200, Alexandre Ghiti wrote:
> > > > > > On 28/05/2023 15:56, Conor Dooley wrote:
> > > > > > > On Sun, May 28, 2023 at 03:42:59PM +0200, Alexandre Ghiti wrote:
> > > > > > > > Hmmm, it still works for me with both clang and gcc-9.
> > > > > > > gcc-9 is a bit of a relic, do you have more recent compilers lying
> > > > > > > around? If not, I can try some older compilers at some point.
> > > > > > >
> > > > > > > > You don't have to do that now but is there a way I could get your compiled
> > > > > > > > image? With the sha1 used to build it? Sorry, I don't see what happens, I
> > > > > > > > need to get my hands dirty in some debug!
> > > > > > > What do you mean by "sha1"? It falls with v6.4-rc1 which is a stable
> > > > > > > hash, if that's what you're looking for.
> > > > > > >
> > > > > > > Otherwise,
> > > > > > > https://github.com/ConchuOD/riscv-env/releases/download/v2022.03/vmlinux.bin
> > > > > > > (ignore the release crap haha, too lazy to find a proper hosting
> > > > > > > mechanism)
> > > > > > Ok, I don't get much info without the symbols, can you also provide the
> > > > > > vmlinux please? But at least your image does not boot, not during the early
> > > > > > boot though because the mmu is enabled.
> > > > > Do you see anything print when you try it? Cos I do not. Iff I have time
> > > > > tomorrow, I'll go poking with gdb. I'm sorry I have not really done any
> > > > > investigating, I have been really busy this last week or so with
> > > > > dt-binding stuff but I should be freer again from tomorrow.
> > > > >
> > > > > https://github.com/ConchuOD/riscv-env/releases/download/v2022.03/vmlinux
> > > >
> > > > Better, the trap happens in kasan_early_init() when it tries to access a
> > > > global symbol using the GOT but ends up with a NULL pointer, which is weird.
> > > > So to me, this is not related to kasan, it happens that kasan_early_init()
> > > > is the first function called after enabling the mmu, I think you may have an
> > > > issue with the filling of the relocations.
> > > Yeah, it reproduces without KASAN.
> > >
> > > > Sorry to bother you again, but if
> > > > at some point you can recompile with DEBUG_INFO enabled, that would be
> > > > perfect! And also provide the vmlinux.relocs file. Sorry for all that, too
> > > > bad I can't reproduce it.
> > > New vmlinux & vmlinux.relocs here:
> > > https://microchiptechnology-my.sharepoint.com/:u:/g/personal/conor_dooley_microchip_com/EZpFNxYYrnNAh5Z3c-rf0pUBBpdPGTLafqdtfcXRUUBkXw?e=7KKMHX
> > > They're pretty massive unfortunately & hopefully that is not some
> > > garbage internal-only link.
> > > .config is a wee bit different, cos different build machine, but the
> > > problem still manifests on a icicle. I've added it to the tarball just
> > > in case.
> >
> >
> > Ok so I had to recreate the Image from the files you gave me and it boots
> > fine using qemu: is that expected? Because you only mention the icicle
> > above.
>
> Unfortunately you sent this one right as I left work..
> I ssh'ed in though and ran the vmlinux.bin & had the same issues.
> Silly question perhaps - is it just not possible to boot something that
> has been hit with `objcopy -O binary vmlinux vmlinux.bin` with
> CONFIG_RELOCATABLE? At this point that's the main thing that sticks out
> to me as being different. You couldn't boot the vmlinux.bin that I sent
> you either.

Ahah, I think we found the culprit!

With CONFIG_RELOCATABLE, vmlinux is actually stripped from all the
relocations (so that it can be shipped) and vmlinux.relocs is what you
should use instead, since it is just a copy of vmlinux before the
removal of the relocations!

>
> Cheers,
> Conor.