linux-kernel - Re: [PATCH v2 3/3] arm64: stacktrace: Convert to ARCH

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Date:   Wed, 2 Sep 2020 19:50:27 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Miroslav Benes <mbenes@...e.cz>, Mark Brown <broonie@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] arm64: stacktrace: Convert to ARCH_STACKWALK

On Wed, Sep 02, 2020 at 11:32:13AM +0200, Miroslav Benes wrote:
> Hi,
> 
> it could be a silly question, but better to ask...
> 
> > +	if (regs)
> > +		start_backtrace(&frame, regs->regs[29], regs->pc);
> > +	else
> > +		start_backtrace(&frame, thread_saved_fp(task),
> > +				thread_saved_pc(task));
> 
> Would this also work for task == current? Given that the original code had
> 
> > -		start_backtrace(&frame,
> > -				(unsigned long)__builtin_frame_address(0),
> > -				(unsigned long)__save_stack_trace);

Oh whoops; I'm annoyed I didn't spot that.

With that gone this cannot work for (task == current && regs == NULL), as
we'll erroneously use stale values from the task struct.

It looks like the LKDTM tests only trigger cases with non-NULL regs, but
IIUC this should show up with show_stack(NULL, NULL, KERN_INFO), as
drivers/tty/sysrq.c does for other cpus.

Thanks,
Mark.