Re: [PATCH bpf-next 1/3] s390/bpf: Add orig_gpr2 to user_pt_regs

[Vasily Gorbik <gor@linux.ibm.com>]

On Wed, Feb 02, 2022 at 12:41:58AM +0100, Ilya Leoshkevich wrote:
> user_pt_regs is used by eBPF in order to access userspace registers -
> see commit 466698e654e8 ("s390/bpf: correct broken uapi for
> BPF_PROG_TYPE_PERF_EVENT program type"). In order to access the first
> syscall argument from eBPF programs, we need to export orig_gpr2.

> diff --git a/arch/s390/include/asm/ptrace.h b/arch/s390/include/asm/ptrace.h
> index 4ffa8e7f0ed3..c8698e643904 100644
> --- a/arch/s390/include/asm/ptrace.h
> +++ b/arch/s390/include/asm/ptrace.h
> @@ -83,9 +83,9 @@ struct pt_regs {
>  			unsigned long args[1];
>  			psw_t psw;
>  			unsigned long gprs[NUM_GPRS];
> +			unsigned long orig_gpr2;
>  		};
>  	};
> -	unsigned long orig_gpr2;
>  	union {
>  		struct {
>  			unsigned int int_code;
> diff --git a/arch/s390/include/uapi/asm/ptrace.h b/arch/s390/include/uapi/asm/ptrace.h
> index ad64d673b5e6..b3dec603f507 100644
> --- a/arch/s390/include/uapi/asm/ptrace.h
> +++ b/arch/s390/include/uapi/asm/ptrace.h
> @@ -295,6 +295,7 @@ typedef struct {
>  	unsigned long args[1];
>  	psw_t psw;
>  	unsigned long gprs[NUM_GPRS];
> +	unsigned long orig_gpr2;
>  } user_pt_regs;

It could be a good opportunity to get rid of that "args[1]" which is not
used for syscall parameters handling since commit baa071588c3f ("[S390]
cleanup system call parameter setup") [v2.6.37], as well as completely
unused now, and shouldn't really be exported to eBPF. And luckily eBPF
never used it.

So, how about reusing "args[1]" slot for orig_gpr2 instead?