linux-kernel - Re: [RFC][PATCH 1/3] powerpc: Don't force ENOSYS as error on syscall fail

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Date:	Wed, 11 Feb 2015 14:04:05 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Bogdan Purcareata <bogdan.purcareata@...escale.com>
Cc:	benh@...nel.crashing.org, paulus@...ba.org,
	linuxppc-dev@...ts.ozlabs.org, pmoore@...hat.com,
	linux-kernel@...r.kernel.org, strosake@...ux.vnet.ibm.com
Subject: Re: [RFC][PATCH 1/3] powerpc: Don't force ENOSYS as error on
 syscall fail

On Mon, 2015-02-09 at 07:55 +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.
> 
> The RFC is this: are there currently any user-space scenarios where it is
> required that the system call return ENOSYS as error code on failure, no matter
> the circumstances? I don't want to break userspace requirements. I have not
> added code to force this error code in situations different than
> secure_computing failure, in order to keep overhead at a minimum.
> 
> Signed-off-by: Bogdan Purcareata <bogdan.purcareata@...escale.com>
> ---
>  arch/powerpc/kernel/entry_32.S | 3 ++-
>  arch/powerpc/kernel/entry_64.S | 2 +-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
> index 59848e5..52e48dd 100644
> --- a/arch/powerpc/kernel/entry_32.S
> +++ b/arch/powerpc/kernel/entry_32.S
> @@ -425,7 +425,8 @@ END_FTR_SECTION_IFSET(CPU_FTR_NEED_PAIRED_STWCX)
>  	b	1b
>  #endif  /* CONFIG_44x */
>  
> -66:	li	r3,-ENOSYS
> +66:
> +#	li	r3,-ENOSYS
>  	b	ret_from_syscall
>  
>  	.globl	ret_from_fork
> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index e6bfe8e..80db02e 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -269,7 +269,7 @@ syscall_dotrace:
>  	b	.Lsyscall_dotrace_cont
>  
>  syscall_enosys:
> -	li	r3,-ENOSYS
> +#	li	r3,-ENOSYS
>  	b	syscall_exit

So what happens if you call this with a syscall number that's out of bounds?

cheers


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