Re: [PATCHv2 05/11] arm64: entry: prepare ret_to_user for function call

From: Guenter Roeck <linux@roeck-us.net>
To: Mark Rutland <mark.rutland@arm.com>
Cc: elver@google.com, paulmck@kernel.org, peterz@infradead.org,
	catalin.marinas@arm.com, james.morse@arm.com, dvyukov@google.com,
	will@kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCHv2 05/11] arm64: entry: prepare ret_to_user for function call
Date: Thu, 17 Dec 2020 09:57:40 -0800	[thread overview]
Message-ID: <20201217175740.GA90220@roeck-us.net> (raw)
In-Reply-To: <20201130115950.22492-6-mark.rutland@arm.com>

Hi,

On Mon, Nov 30, 2020 at 11:59:44AM +0000, Mark Rutland wrote:
> In a subsequent patch ret_to_user will need to make a C function call
> (in some configurations) which may clobber x0-x18 at the start of the
> finish_ret_to_user block, before enable_step_tsk consumes the flags
> loaded into x1.
> 
> In preparation for this, let's load the flags into x19, which is
> preserved across C function calls. This avoids a redundant reload of the
> flags and ensures we operate on a consistent shapshot regardless.
> 
> There should be no functional change as a result of this patch. At this
> point of the entry/exit paths we only need to preserve x28 (tsk) and the
> sp, and x19 is free for this use.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: James Morse <james.morse@arm.com>
> Cc: Will Deacon <will@kernel.org>

This patch results in:

arch/arm64/kernel/entry.S: Assembler messages:
arch/arm64/kernel/entry.S:774: Error: immediate out of range at operand 3 -- `and x2,x19,#((1<<1)|(1<<0)|(1<<2)|(1<<3)|(1<<4)|(1<<5)|(1<<7))'

This is with gcc 9.3.0 and binutils 2.34. Do I need a special compiler
and/or binutils version to make it compile ?

Thanks,
Guenter

> ---
>  arch/arm64/kernel/entry.S | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index b295fb912b12..84aec600eeed 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -774,13 +774,13 @@ SYM_CODE_END(el0_error)
>  SYM_CODE_START_LOCAL(ret_to_user)
>  	disable_daif
>  	gic_prio_kentry_setup tmp=x3
> -	ldr	x1, [tsk, #TSK_TI_FLAGS]
> -	and	x2, x1, #_TIF_WORK_MASK
> +	ldr	x19, [tsk, #TSK_TI_FLAGS]
> +	and	x2, x19, #_TIF_WORK_MASK
>  	cbnz	x2, work_pending
>  finish_ret_to_user:
>  	/* Ignore asynchronous tag check faults in the uaccess routines */
>  	clear_mte_async_tcf
> -	enable_step_tsk x1, x2
> +	enable_step_tsk x19, x2
>  #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>  	bl	stackleak_erase
>  #endif
> @@ -791,11 +791,12 @@ finish_ret_to_user:
>   */
>  work_pending:
>  	mov	x0, sp				// 'regs'
> +	mov	x1, x19
>  	bl	do_notify_resume
>  #ifdef CONFIG_TRACE_IRQFLAGS
>  	bl	trace_hardirqs_on		// enabled while in userspace
>  #endif
> -	ldr	x1, [tsk, #TSK_TI_FLAGS]	// re-check for single-step
> +	ldr	x19, [tsk, #TSK_TI_FLAGS]	// re-check for single-step
>  	b	finish_ret_to_user
>  SYM_CODE_END(ret_to_user)
>  
> -- 
> 2.11.0
> 
> 
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> linux-arm-kernel@lists.infradead.org
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