Re: [PATCH 5/7] RISC-V: fix auipc-jalr addresses in patched alternatives

["Lad, Prabhakar" <prabhakar.csengg@gmail.com>]

Hi Heiko,

On Mon, Nov 21, 2022 at 10:17 PM Heiko Stübner <heiko@sntech.de> wrote:
>
> Am Montag, 21. November 2022, 22:31:36 CET schrieb Lad, Prabhakar:
> > Hi Heiko,
> >
> > On Mon, Nov 21, 2022 at 3:06 PM Lad, Prabhakar
> > <prabhakar.csengg@gmail.com> wrote:
> > >
<snip>
> > Some more information,
> >
> > - If I drop the riscv_alternative_fix_auipc_jalr() call after
> > patch_text_nosync() and then print the alt->old_ptr instructions
> > before patching I can see the instructions as 0x13 (nop) which is
> > correct.
> >
> > - if I call riscv_alternative_fix_auipc_jalr() call after
> > patch_text_nosync() and then print the alt->old_ptr instructions
> > before patching I dont see 0x13 (nop) consistently for old
> > instructions.
>
> which is to be expected I guess.
>
> alt->old_ptr points to the memory location where the live kernel code
> lives.
>
Agreed.

> I.e. the code at this location is the thing the kernel actually runs.
> The code at this location then gets overwritten by the alternative
> assembly.
>
But shouldn't the actual code be nops before patching?

>
> > - If I replace the nop's in the old instructions with my assembly code
> > of rz/five cmo and then just use patch_text_nosync() I can see the
> > correct actual instruction being printed apart from jalr (is some sort
> > of offset added to it as I see last 4 bits match?) and then is
> > replaced correctly by the same alt instructions apart from the jalr
> > (log [0]).
> >
> > - If I replace the nop's in the old instructions with my assembly code
> > of rz/five cmo and then use patch_text_nosync() and
> > riscv_alternative_fix_auipc_jalr() I can see the actual old
> > instructions differs a bit and again the jalr instruction differs too
> > in the patched code (log [1]).
> >
> > [0] https://paste.debian.net/1261412/
> > [1] https://paste.debian.net/1261413/
> >
> > Attached is the objump of dma-noncoherent.o for reference.
>
> I did read that objdumps are not really conclusive when looking
> at auipc + jalr instructions, hence the printing of the actual instructions.
>
> As either manually or with a helper like
>
>         https://luplab.gitlab.io/rvcodecjs/#q=0xf4c080e7
>
> you can then decode the actual instruction and compare.
>
OK, I will give it a try.


> In your log the two jalr instructions decode to different offsets,
>         jalr x1, x1, -180
> vs
>         jalr x1, x1, -834
>
> Can you check what the patch_offset value is in your case?
>
I'll check that and let you know.

> Interestingly the
>         auipc x1, 0
> is 0 for both cases.
>
> I'll try to build a real test-setup mimicing what you're doing
> tomorrow (european tomorrow).
>
Thank you!

Cheers,
Prabhakar

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