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

["Björn Töpel" <bjorn@kernel.org>]

Guo Ren <guoren@kernel.org> writes:

> On Wed, Feb 1, 2023 at 5:40 PM Björn Töpel <bjorn@kernel.org> wrote:
>>
>> 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/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.

Ok, something more readable, like:

diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
index f21592d20306..3602352ba175 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -50,14 +50,16 @@ static void __kprobes arch_simulate_insn(struct kprobe *p, struct pt_regs *regs)
 
 int __kprobes arch_prepare_kprobe(struct kprobe *p)
 {
-	unsigned long probe_addr = (unsigned long)p->addr;
+	u16 *insn = (u16 *)p->addr;
 
-	if (probe_addr & 0x1)
+	if ((uintptr_t)insn & 0x1)
 		return -EILSEQ;
 
 	/* copy instruction */
-	p->opcode = *p->addr;
-
+	p->opcode = *insn++;
+	if (GET_INSN_LENGTH(p->opcode) == 4)
+		p->opcode |= *insn << 16;
+	
 	/* decode instruction */
 	switch (riscv_probe_decode_insn(p->addr, &p->ainsn.api)) {
 	case INSN_REJECTED:	/* insn not supported */


Björn