linux-kernel - Re: [PATCH v8 01/14] riscv: prevent null-pointer dereference with sbi_remote_fence

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Date:   Thu, 31 Mar 2022 14:28:06 +0200
From:   Heiko Stübner <heiko@...ech.de>
To:     Christoph Hellwig <hch@....de>
Cc:     palmer@...belt.com, paul.walmsley@...ive.com,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, wefu@...hat.com,
        liush@...winnertech.com, guoren@...nel.org, atishp@...shpatra.org,
        anup@...infault.org, drew@...gleboard.org, hch@....de,
        arnd@...db.de, wens@...e.org, maxime@...no.tech,
        gfavor@...tanamicro.com, andrea.mondelli@...wei.com,
        behrensj@....edu, xinhaoqu@...wei.com, mick@....forth.gr,
        allen.baum@...erantotech.com, jscheid@...tanamicro.com,
        rtrauben@...il.com, samuel@...lland.org, cmuellner@...ux.com,
        philipp.tomsich@...ll.eu, Atish Patra <atishp@...osinc.com>
Subject: Re: [PATCH v8 01/14] riscv: prevent null-pointer dereference with sbi_remote_fence_i

Hi,

Am Donnerstag, 31. März 2022, 11:51:55 CEST schrieb Christoph Hellwig:
> On Thu, Mar 24, 2022 at 01:06:57AM +0100, Heiko Stuebner wrote:
> > The callback used inside sbi_remote_fence_i is set at sbi probe time
> > to the needed variant. Before that it is a NULL pointer.
> > 
> > Some users like the flush_icache_*() functions suggest a generic
> > functionality, that doesn't depend on a specific boot-stage but
> > uses sbi_remote_fence_i as one option to flush other cpu cores.
> > 
> > So they definitely shouldn't run into null-pointer dereference
> > issues when called "too early" during boot.
> > 
> > So introduce an empty function to be the standard for the __sbi_rfence
> > function pointer until sbi_init has run.
> > 
> > Users of sbi_remote_fence_i will have separate code for the local
> > cpu and sbi_init() is called before other cpus are brought up.
> > So there are no other cpus present at the time when the issue
> > might happen.
> 
> I don't really understand this changelog.  If flush_icache_* or
> other routines using SBI calls are called too early they won't
> do what they are asked to do, which implies a bug in the code.
> 
> So crashing absolutely is the right thing to do here as we don't
> really have any other error reporting method available.
> 
> So unless I'm totally misunderstanding what you are saying here:
> 
> Nacked-by: Christoph Hellwig <hch@....de>

The function is defined as

void flush_icache_all(void)
{
	local_flush_icache_all();

	if (IS_ENABLED(CONFIG_RISCV_SBI))
		sbi_remote_fence_i(NULL);
	else
		on_each_cpu(ipi_remote_fence_i, NULL, 1);
}

so essentially flushes the _local_ icache first and then tries to flush
caches on other cores, either via an ipi or via sbi.

The remote-fence callback is set correctly during sbi_init().
The other cores are only brought up after sbi-init is done.

So it's not really about error reporting but making sure that flush_icache_all()
does something sane even when still running on the first core.
As I assume the "all" means on all available cores (which would be the
core the system booted on).

Does this make it clearer what this tries to solve?
Heiko