Re: [RFC PATCH 1/2] riscv: Expose orig_a0 in the user_regs_struct structure

[Charlie Jenkins <charlie@rivosinc.com>]

On Wed, Jun 19, 2024 at 10:01:27AM +0800, zhouquan@iscas.ac.cn wrote:
> From: Quan Zhou <zhouquan@iscas.ac.cn>
> 
> Expose orig_a0 to userspace to ensure that users can modify
> the actual value of `a0` in the traced process through the
> ptrace(PTRACE_SETREGSET, ...) path. Since user_regs_struct is
> a subset of pt_regs, we also need to adjust the position of
> the orig_a0 field in pt_regs to achieve the correct copy.
> 
> Signed-off-by: Quan Zhou <zhouquan@iscas.ac.cn>
> ---
>  arch/riscv/include/asm/ptrace.h      | 4 ++--
>  arch/riscv/include/uapi/asm/ptrace.h | 2 ++
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/ptrace.h b/arch/riscv/include/asm/ptrace.h
> index b5b0adcc85c1..37f48d40ae46 100644
> --- a/arch/riscv/include/asm/ptrace.h
> +++ b/arch/riscv/include/asm/ptrace.h
> @@ -45,12 +45,12 @@ struct pt_regs {
>  	unsigned long t4;
>  	unsigned long t5;
>  	unsigned long t6;
> +	/* a0 value before the syscall */
> +	unsigned long orig_a0;
>  	/* Supervisor/Machine CSRs */
>  	unsigned long status;
>  	unsigned long badaddr;
>  	unsigned long cause;
> -	/* a0 value before the syscall */
> -	unsigned long orig_a0;
>  };
>  
>  #define PTRACE_SYSEMU			0x1f
> diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h
> index a38268b19c3d..3e37f80cb3e8 100644
> --- a/arch/riscv/include/uapi/asm/ptrace.h
> +++ b/arch/riscv/include/uapi/asm/ptrace.h
> @@ -54,6 +54,8 @@ struct user_regs_struct {
>  	unsigned long t4;
>  	unsigned long t5;
>  	unsigned long t6;
> +	/* a0 value before the syscall */
> +	unsigned long orig_a0;
>  };
>  
>  struct __riscv_f_ext_state {
> -- 
> 2.34.1
> 

This is a good addition!

Since orig_a0 is no longer at the bottom of pt_regs, MAX_REG_OFFSET is
now incorrect.

Can you adjust the value of:

#define MAX_REG_OFFSET offsetof(struct pt_regs, orig_a0)

in arch/riscv/include/asm/ptrace.h to be:

#define MAX_REG_OFFSET offsetof(struct pt_regs, cause)

This is something that is very easy to miss. I think it would be
valuable to leave a comment at the top of struct pt_regs pointing out
that MAX_REG_OFFSET needs to be adjusted if struct pt_regs changes.

- Charlie