Re: [PATCH 1/3] riscv: kprobes: simulate c.j instruction

[Charlie Jenkins <charlie@rivosinc.com>]

On Sun, Jul 30, 2023 at 10:27:07AM +0200, Nam Cao wrote:
> kprobes currently rejects c.j instruction. Implement it.
> 
> Signed-off-by: Nam Cao <namcaov@gmail.com>
> ---
>  arch/riscv/kernel/probes/decode-insn.c   |  3 ++-
>  arch/riscv/kernel/probes/simulate-insn.c | 24 ++++++++++++++++++++++++
>  arch/riscv/kernel/probes/simulate-insn.h |  1 +
>  3 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/kernel/probes/decode-insn.c b/arch/riscv/kernel/probes/decode-insn.c
> index 64f6183b4717..39adb07a342d 100644
> --- a/arch/riscv/kernel/probes/decode-insn.c
> +++ b/arch/riscv/kernel/probes/decode-insn.c
> @@ -29,13 +29,14 @@ riscv_probe_decode_insn(probe_opcode_t *addr, struct arch_probe_insn *api)
>  	 * TODO: the REJECTED ones below need to be implemented
>  	 */
>  #ifdef CONFIG_RISCV_ISA_C
> -	RISCV_INSN_REJECTED(c_j,		insn);
>  	RISCV_INSN_REJECTED(c_jr,		insn);
>  	RISCV_INSN_REJECTED(c_jal,		insn);
>  	RISCV_INSN_REJECTED(c_jalr,		insn);
>  	RISCV_INSN_REJECTED(c_beqz,		insn);
>  	RISCV_INSN_REJECTED(c_bnez,		insn);
>  	RISCV_INSN_REJECTED(c_ebreak,		insn);
> +
> +	RISCV_INSN_SET_SIMULATE(c_j,		insn);
>  #endif
>  
>  	RISCV_INSN_SET_SIMULATE(jal,		insn);
> diff --git a/arch/riscv/kernel/probes/simulate-insn.c b/arch/riscv/kernel/probes/simulate-insn.c
> index 7441ac8a6843..3ba45c612cd8 100644
> --- a/arch/riscv/kernel/probes/simulate-insn.c
> +++ b/arch/riscv/kernel/probes/simulate-insn.c
> @@ -188,3 +188,27 @@ bool __kprobes simulate_branch(u32 opcode, unsigned long addr, struct pt_regs *r
>  
>  	return true;
>  }
> +
> +bool __kprobes simulate_c_j(u32 opcode, unsigned long addr, struct pt_regs *regs)
> +{
> +	/*
> +	 *  15    13 12                            2 1      0
> +	 * | funct3 | offset[11|4|9:8|10|6|7|3:1|5] | opcode |
> +	 *     3                   11                    2
> +	 */
> +
> +	s32 offset;
> +
> +	offset  = ((opcode >> 3)  & 0x7) << 1;
> +	offset |= ((opcode >> 11) & 0x1) << 4;
> +	offset |= ((opcode >> 2)  & 0x1) << 5;
> +	offset |= ((opcode >> 7)  & 0x1) << 6;
> +	offset |= ((opcode >> 6)  & 0x1) << 7;
> +	offset |= ((opcode >> 9)  & 0x3) << 8;
> +	offset |= ((opcode >> 8)  & 0x1) << 10;
> +	offset |= ((opcode >> 12) & 0x1) << 11;
> +
> +	instruction_pointer_set(regs, addr + sign_extend32(offset, 11));
Can you use riscv_insn_insert_jtype_imm() from insn.h since it is
already created? It will also sign extend for you. Don't worry about
creating a similar function for the branches, I am in the process of
refactoring the insn.h file.
> +
> +	return true;
> +}
> diff --git a/arch/riscv/kernel/probes/simulate-insn.h b/arch/riscv/kernel/probes/simulate-insn.h
> index 61e35db31001..4bd6c266e7d3 100644
> --- a/arch/riscv/kernel/probes/simulate-insn.h
> +++ b/arch/riscv/kernel/probes/simulate-insn.h
> @@ -24,5 +24,6 @@ bool simulate_auipc(u32 opcode, unsigned long addr, struct pt_regs *regs);
>  bool simulate_branch(u32 opcode, unsigned long addr, struct pt_regs *regs);
>  bool simulate_jal(u32 opcode, unsigned long addr, struct pt_regs *regs);
>  bool simulate_jalr(u32 opcode, unsigned long addr, struct pt_regs *regs);
> +bool simulate_c_j(u32 opcode, unsigned long addr, struct pt_regs *regs);
>  
>  #endif /* _RISCV_KERNEL_PROBES_SIMULATE_INSN_H */
> -- 
> 2.34.1
> 
> 
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Thanks,
Charlie