Re: [PATCH] riscv: kprobe: Fixup misaligned load text

[Guo Ren]

On Wed, Feb 1, 2023 at 5:40 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>
> guoren@xxxxxxxxxx writes:
>
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > The current kprobe would cause a misaligned load for the probe point.
> > This patch fixup it with two half-word loads instead.
> >
> > Fixes: c22b0bcb1dd0 ("riscv: Add kprobes supported")
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > Link: https://lore.kernel.org/linux-riscv/878rhig9zj.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > Reported-by: Bjorn Topel <bjorn.topel@xxxxxxxxx>
> > ---
> >  arch/riscv/kernel/probes/kprobes.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
> > index 41c7481afde3..c1160629cef4 100644
> > --- a/arch/riscv/kernel/probes/kprobes.c
> > +++ b/arch/riscv/kernel/probes/kprobes.c
> > @@ -74,7 +74,9 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
> >               return -EILSEQ;
> >
> >       /* copy instruction */
> > -     p->opcode = *p->addr;
> > +     p->opcode = (kprobe_opcode_t)(*(u16 *)probe_addr);
> > +     if (GET_INSN_LENGTH(p->opcode) == 4)
> > +             p->opcode |= (kprobe_opcode_t)(*(u16 *)(probe_addr + 2))
> >       << 16;
>
> Ugh, those casts. :-( What about the memcpy variant you had in the other
> thread?
The memcpy version would force load probe_addr + 2. This one would
save an lh operation. The code text guarantees half-word alignment. No
misaligned load happened. Second, kprobe wouldn't write the last half
of 32b instruction.

-- 
Best Regards
 Guo Ren