linux-kernel - Re: lguest: bug in lg_irq

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Date:	Mon, 02 Mar 2015 10:44:55 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Denys Vlasenko <vda.linux@...glemail.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: lguest: bug in lg_irq_enable?

Denys Vlasenko <vda.linux@...glemail.com> writes:
> ENTRY(lg_irq_enable)
>         /*
>          * The reverse of irq_disable, this sets lguest_data.irq_enabled to
>          * X86_EFLAGS_IF (ie. "Interrupts enabled").
>          */
>         movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled
>         /*
>          * But now we need to check if the Host wants to know: there might have
>          * been interrupts waiting to be delivered, in which case it will have
>          * set lguest_data.irq_pending to X86_EFLAGS_IF.  If it's not zero, we
>          * jump to send_interrupts, otherwise we're done.
>          */
>         testl $0, lguest_data+LGUEST_DATA_irq_pending
> ^^^^^^^^^^^^^^^^^??????????

Heh, that should be cmpl!  Nice catch!

Cheers,
Rusty.

Subject: lguest: fix pending interrupt test.

Denys says:
  TEST with zero will always set ZF. Thus, "jnz send_interrupts" never jumps.

We get interrupts regularly enough that this didn't cause immediate problems.

Reported-by: Denys Vlasenko <vda.linux@...glemail.com>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>

diff --git a/arch/x86/lguest/head_32.S b/arch/x86/lguest/head_32.S
index 6ddfe4fc23c3..05b0a85507ce 100644
--- a/arch/x86/lguest/head_32.S
+++ b/arch/x86/lguest/head_32.S
@@ -84,7 +84,7 @@ ENTRY(lg_irq_enable)
 	 * set lguest_data.irq_pending to X86_EFLAGS_IF.  If it's not zero, we
 	 * jump to send_interrupts, otherwise we're done.
 	 */
-	testl $0, lguest_data+LGUEST_DATA_irq_pending
+	cmpl $0, lguest_data+LGUEST_DATA_irq_pending
 	jnz send_interrupts
 	/*
 	 * One cool thing about x86 is that you can do many things without using

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