Subject: Re: [PATCH v2 1/7] powerpc: properly negate error in syscall_set_return_value()

From: Michael Ellerman <[email-masked]>
Date: Sat, 25 Jan 2025 23:17:58 +1100

"Dmitry V. Levin" <[email-masked]> writes:
> On Thu, Jan 23, 2025 at 08:28:15PM +0200, Dmitry V. Levin wrote:
...
>> After looking at system_call_exception() I doubt this inconsistency can be
>> easily avoided, so I don't see how this patch could be enhanced further,
>> and what else could I do with the patch besides dropping it and letting
>> !trap_is_scv case be unsupported by PTRACE_SET_SYSCALL_INFO API, which
>> would be unfortunate.
>
> If you say this would bring some consistency, I can extend the patch with
> something like this:

Yes that would improve things IMHO, with one caveat ....

> diff --git a/arch/powerpc/kernel/ptrace/ptrace.c b/arch/powerpc/kernel/ptrace/ptrace.c
> index 727ed4a14545..dda276a934fd 100644
> --- a/arch/powerpc/kernel/ptrace/ptrace.c
> +++ b/arch/powerpc/kernel/ptrace/ptrace.c
> @@ -207,7 +207,7 @@ static int do_seccomp(struct pt_regs *regs)
>  	 * syscall parameter. This is different to the ptrace ABI where
>  	 * both r3 and orig_gpr3 contain the first syscall parameter.
>  	 */
> -	regs->gpr[3] = -ENOSYS;
> +	syscall_set_return_value(current, regs, -ENOSYS, 0);
>  
>  	/*
>  	 * We use the __ version here because we have already checked
> @@ -225,7 +225,7 @@ static int do_seccomp(struct pt_regs *regs)
>  	 * modify the first syscall parameter (in orig_gpr3) and also
>  	 * allow the syscall to proceed.
>  	 */
> -	regs->gpr[3] = regs->orig_gpr3;
> +	syscall_set_return_value(current, regs, 0, regs->orig_gpr3);

This case should remain as-is. The orig_gpr3 value here is not a syscall
error code, it's the original r3 value, which is a syscall parameter.

If the tracer wants to fail the syscall it should have set something in
r3, not orig_gpr3.

>  	return 0;
>  }
> @@ -315,7 +315,7 @@ long do_syscall_trace_enter(struct pt_regs *regs)
>  	 * If we are aborting explicitly, or if the syscall number is
>  	 * now invalid, set the return value to -ENOSYS.
>  	 */
> -	regs->gpr[3] = -ENOSYS;
> +	syscall_set_return_value(current, regs, -ENOSYS, 0);
>  	return -1;
>  }
>  
> diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
> index aa17e62f3754..c921e0cb54b8 100644
> --- a/arch/powerpc/kernel/signal.c
> +++ b/arch/powerpc/kernel/signal.c
> @@ -229,14 +229,8 @@ static void check_syscall_restart(struct pt_regs *regs, struct k_sigaction *ka,
>  		regs_add_return_ip(regs, -4);
>  		regs->result = 0;
>  	} else {
> -		if (trap_is_scv(regs)) {
> -			regs->result = -EINTR;
> -			regs->gpr[3] = -EINTR;
> -		} else {
> -			regs->result = -EINTR;
> -			regs->gpr[3] = EINTR;
> -			regs->ccr |= 0x10000000;
> -		}
> +		regs->result = -EINTR;
> +		syscall_set_return_value(current, regs, -EINTR, 0);
>  	}
>  }

cheers