netdev - Re: [PATCH v2 20/20] ppc: propagate the calling conventions change down to csum_partial_copy

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Message-ID: <20201014231210.GM3576660@ZenIV.linux.org.uk>
Date:   Thu, 15 Oct 2020 00:12:10 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 20/20] ppc: propagate the calling conventions change
 down to csum_partial_copy_generic()

On Wed, Oct 14, 2020 at 03:51:00PM -0700, Linus Torvalds wrote:
> On Wed, Oct 14, 2020 at 3:27 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
> >
> > This patch is causing crashes in WireGuard's CI over at
> > https://www.wireguard.com/build-status/ . Apparently sending a simple
> > network packet winds up triggering refcount_t's warn-on-saturate code. I
> 
> Ouch.
> 
> The C parts look fairly straightforward, and I don't see how they
> could cause that odd refcount issue.
> 
> So I assume it's the low-level asm code conversion that is buggy. And
> it's apparently the 32-bit conversion, since your ppc64 status looks
> fine.
> 
> I think it's this instruction:
> 
>         addi    r1,r1,16
> 
> that should be removed from the function exit, because Al removed the
> 
> -       stwu    r1,-16(r1)
> 
> on function entry.
> 
> So I think you end up with a corrupt stack pointer and basically
> random behavior.

Gyahh...  ACK, and I really wonder how the hell has it managed to avoid
crashing on testing.

Mea culpa, folks.