linux-kernel - Re: [Patch] arm/arm64: Thumb=>ARM signal handling setup skips the a few instructions on Snapdragon S4/Krait

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Date:	Mon, 4 Nov 2013 12:11:40 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	"T.J. Purtell" <tj@...isocial.us>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rodrigo Kumpera <kumpera@...arin.com>,
	Mark Probst <mark@...arin.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Patch] arm/arm64: Thumb=>ARM signal handling setup skips the a
 few instructions on Snapdragon S4/Krait

(cc'ing the Linux ARM kernel list)

On 25 October 2013 05:54, T.J. Purtell <tj@...isocial.us> wrote:
> The ARM architecture reference specifies that the IT state bits in the
> PSR must be all zeros in ARM mode or behavior is unspecified.  On the
> Qualcomm Snapdragon S4/Krait architecture CPUs the processor continues
> to consider the IT state bits while in ARM mode.  This makes it so
> that some instructions are skipped by the CPU.

Good catch.

> diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
> index e393174..8f6c3be 100644
> --- a/arch/arm64/kernel/signal32.c
> +++ b/arch/arm64/kernel/signal32.c
> @@ -474,9 +474,11 @@ static void compat_setup_return(struct pt_regs
> *regs, struct k_sigaction *ka,
>   /* Check if the handler is written for ARM or Thumb */
>   thumb = handler & 1;
>
> + /* ARM spec requires this to be all 000s in ARM mode */
> + spsr &= ~COMPAT_PSR_IT_MASK;
> +
>   if (thumb) {
>   spsr |= COMPAT_PSR_T_BIT;
> - spsr &= ~COMPAT_PSR_IT_MASK;
>   } else {
>   spsr &= ~COMPAT_PSR_T_BIT;
>   }

Please send me a separate arm64 patch, I'll push it via my tree (and
cc stable as well).

Catalin
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