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

[Quan Zhou]

On 2024/6/20 09:05, Charlie Jenkins wrote:
> On Wed, Jun 19, 2024 at 10:01:27AM +0800, zhouquan at iscas.ac.cn wrote:
>> From: Quan Zhou <zhouquan at 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 at 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
>
Oh, good idea!  I'm sorry for missing such an important point earlier.

Once enough information is gathered from this RFC, I will include what 
you mentioned in the V1 series.

Thanks,
Quan
>
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