linux-kernel - Re: [PATCH v2] powerpc: Fix irq_soft_mask_set() and irq_soft_mask

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Date:   Fri, 2 Sep 2022 12:10:51 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Peter Bergner <bergner@...ux.ibm.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Zhouyi Zhou <zhouzhouyi@...il.com>
Subject: Re: [PATCH v2] powerpc: Fix irq_soft_mask_set() and irq_soft_mask_return() with sanitizer

On Fri, Sep 02, 2022 at 10:57:27AM -0500, Peter Bergner wrote:
> On 8/31/22 5:45 PM, Segher Boessenkool wrote:
> > Yes, this is guaranteed.
> 
> Agree with Segher here.  That said, there was a gcc bug a looooong time
> ago where gcc copied r13 into a temporary register and used it from there.

r13 is a fixed register on most of our ABIs (everything that is not AIX
or Darwin, even), so this can never happen.  Except if there are bugs,
of course ;-)

> That's ok (correctness wise, but not ideal) from user land standpoint,
> but we took a context switch after the reg copy and it was restarted on
> a different cpu, so differnt local_paca and r13 value.  We went boom
> because the copy wasn't pointing to the correct local_paca anymore.
> So it is very important the compiler always use r13 when accessing
> the local_paca.

Yes.  So we either whould use -ffixed-r13, or just not use unsupported
compilers.  powerpc*-linux and powerpc*-elf work fine for example :-)


Segher