linux-kernel - Re: A bug about system call on ARM

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Message-ID: <20130603101809.GK18614@n2100.arm.linux.org.uk>
Date:	Mon, 3 Jun 2013 11:18:09 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Will Deacon <will.deacon@....com>
Cc:	"Wang, Yalin" <Yalin.Wang@...ymobile.com>,
	'richard -rw- weinberger' <richard.weinberger@...il.com>,
	"'linux-arch@...r.kernel.org'" <linux-arch@...r.kernel.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'linux-arm-kernel@...ts.infradead.org'" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: A bug about system call on ARM

On Thu, May 30, 2013 at 12:41:12PM +0100, Will Deacon wrote:
> +#if defined(CONFIG_OABI_COMPAT) || !defined(CONFIG_AEABI)
> +	/*
> +	 * We may have faulted trying to load the SWI instruction due to
> +	 * concurrent page aging on another CPU. In this case, return
> +	 * back to the swi instruction and fault the page back.
> +	 */
> +9001:
> +	sub	lr, lr, #4
> +	str	lr, [sp, #S_PC]
> +	b	ret_fast_syscall
> +#endif

The comment is wrong.  If we get here, it means that the fault from
trying to loading the instruction can't be fixed up.  Arguably, that
should result in a SIGSEGV being sent immediately, but we'll get to
that when we then try to re-load the instruction.

What it means is that the page we were trying to execute has been
unmapped beneath us.

BTW, I notice that the kernel oops was never posted to the list, so it's
impossible for other people following this thread to see what the real
problem is...
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