linux-kernel - Re: [PATCH 3/7] openrisc: restore call-saved regs on sigreturn

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Message-ID: <57DFF60E.5030106@southpole.se>
Date:   Mon, 19 Sep 2016 16:28:30 +0200
From:   Jonas Bonn <jonas@...thpole.se>
To:     Stafford Horne <shorne@...il.com>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] openrisc: restore call-saved regs on sigreturn

On 09/16/2016 04:43 PM, Stafford Horne wrote:
> From: Jonas Bonn <jonas@...thpole.se>
>
> Return to userspace via _resume_userspace instead of via syscall return
> path for the rt_sigreturn syscall.
>
> I'll rework this comment more later, but this patch needs testing.

This whole patch was reworked later... what happened to that work? I 
recall it being posted for testing without feedback...

In any case, the patch comment needs cleaning up.

/Jonas

>
> Old comment from previous patch:
>
> The sigreturn syscall is more like a context switch than a function call;
> it entails a return from one context (the signal handler) to another
> (the process in question).  For a context switch like this there are
> effectively no call-saved regs that remain constant across the transition.
>
> This patch restores the call-saved regs from pt_regs before returning from
> the syscall, effectively restoring the context that the process had before
> being interrupted by the signal handler.  Restoring the call-saved regs
> in this way allows us to return to userspace via the usual syscall fast
> path.
>
> Reported-by: Sebastian Macke <sebastian@...ke.de>
> Signed-off-by: Jonas Bonn <jonas@...thpole.se>
> Signed-off-by: Stafford Horne <shorne@...il.com>
> ---
>   arch/openrisc/kernel/entry.S | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/openrisc/kernel/entry.S b/arch/openrisc/kernel/entry.S
> index fec8bf9..572d223 100644
> --- a/arch/openrisc/kernel/entry.S
> +++ b/arch/openrisc/kernel/entry.S
> @@ -1101,8 +1101,16 @@ ENTRY(__sys_fork)
>   	 l.addi	r3,r1,0
>   
>   ENTRY(sys_rt_sigreturn)
> -	l.j	_sys_rt_sigreturn
> +	l.jal	_sys_rt_sigreturn
>   	 l.addi	r3,r1,0
> +	l.sfne	r30,r0
> +	l.bnf	_no_syscall_trace
> +	 l.nop
> +	l.jal	do_syscall_trace_leave
> +	 l.addi	r3,r1,0
> +_no_syscall_trace:
> +	l.j	_resume_userspace
> +	 l.nop
>   
>   /* This is a catch-all syscall for atomic instructions for the OpenRISC 1000.
>    * The functions takes a variable number of parameters depending on which