Subject: Re: [PATCH v2 1/3] powerpc: Don't force ENOSYS as error on syscall fail

From: Michael Ellerman <[email-masked]>
Date: Thu, 12 Feb 2015 16:24:39 +1100

On Wed, 2015-02-11 at 08:36 +0000, Bogdan Purcareata wrote:
> In certain scenarios - e.g. seccomp filtering with ERRNO as default action -
> the system call fails for other reasons than the syscall not being available.
> The seccomp filter can be configured to store a user-defined error code on
> return from a blacklisted syscall. Don't always set ENOSYS on
> do_syscall_trace_enter failure.
> 
> v2:
> - move setting ENOSYS as errno from the syscall entry assembly to
>   do_syscall_trace_enter, only in the specific case

> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 194e46d..0111e04 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -269,7 +269,6 @@ syscall_dotrace:
>  	b	.Lsyscall_dotrace_cont
>  
>  syscall_enosys:
> -	li	r3,-ENOSYS
>  	b	syscall_exit

This still looks wrong to me.

On 64 bit we do:

	CURRENT_THREAD_INFO(r11, r1)
	ld	r10,TI_FLAGS(r11)
	andi.	r11,r10,_TIF_SYSCALL_DOTRACE
	bne	syscall_dotrace
.Lsyscall_dotrace_cont:
	cmpldi	0,r0,NR_syscalls
	bge-	syscall_enosys
...

syscall_enosys:
	li	r3,-ENOSYS
	b	.Lsyscall_exit

Your patch removes the load of ENOSYS.

Which means if we're not doing syscall tracing, and we get an out-of-bounds
syscall number, we'll return with something random on r3. Won't we?

The 32-bit code looks more or less similar, although the label has a different
name.

cheers