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References: <20230201064608.3486136-1-guoren@kernel.org> <87tu05pvur.fsf@all.your.base.are.belong.to.us>
In-Reply-To: <87tu05pvur.fsf@all.your.base.are.belong.to.us>
From:   Guo Ren <guoren@kernel.org>
Date:   Thu, 2 Feb 2023 14:17:10 +0800
Message-ID: <CAJF2gTSj12E9+peMF0rNY_psGDyEG5BbMY6V-s4dK0FpkCC9Yw@mail.gmail.com>
Subject: Re: [PATCH] riscv: kprobe: Fixup misaligned load text
To:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
Cc:     palmer@rivosinc.com, conor.dooley@microchip.com,
        liaochang1@huawei.com, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
        Guo Ren <guoren@linux.alibaba.com>,
        Bjorn Topel <bjorn.topel@gmail.com>
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On Wed, Feb 1, 2023 at 5:40 PM Bj=C3=B6rn T=C3=B6pel <bjorn@kernel.org> wro=
te:
>
> guoren@kernel.org writes:
>
> > From: Guo Ren <guoren@linux.alibaba.com>
> >
> > 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@linux.alibaba.com>
> > Signed-off-by: Guo Ren <guoren@kernel.org>
> > Link: https://lore.kernel.org/linux-riscv/878rhig9zj.fsf@all.your.base.=
are.belong.to.us/
> > Reported-by: Bjorn Topel <bjorn.topel@gmail.com>
> > ---
> >  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/pro=
bes/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 =3D *p->addr;
> > +     p->opcode =3D (kprobe_opcode_t)(*(u16 *)probe_addr);
> > +     if (GET_INSN_LENGTH(p->opcode) =3D=3D 4)
> > +             p->opcode |=3D (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.

--=20
Best Regards
 Guo Ren