netdev - Re: Regression bisected to "crypto: af_alg: Convert af_alg_sendpage() to use MSG_SPLICE

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Date: Tue, 04 Jul 2023 10:41:45 +0100
From: David Howells <dhowells@...hat.com>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: dhowells@...hat.com, Ondrej Mosnacek <omosnacek@...il.com>,
    Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
    Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
    Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
    regressions@...ts.linux.dev
Subject: Re: Regression bisected to "crypto: af_alg: Convert af_alg_sendpage() to use MSG_SPLICE_PAGES"

Herbert Xu <herbert@...dor.apana.org.au> wrote:

> All it's saying is that if you modify the data after sending it off
> via splice then the data that will be on the wire is undefined.

Er, no.  It can literally remove the page from the process's VM and paste it
somewhere else - though in this case, that shouldn't happen.  However, the
buffer passed to SPLICE_F_GIFT should also be page-aligned, which it might not
be because they used calloc().

There's no reason to use SPLICE_F_GIFT here.  vmsplice() still attaches the 

> There is no reason why this should crash.

Agreed.  I'm still looking at it.  Interestingly, the output comes out the
same, no matter whether vmsplice(), vmsplice() + SPLICE_F_GIFT or writev(), so
it looks like the buffers get to 

> If we can't fix this the patches should be reverted.

I didn't change vmsplice() or the way pages are stored in the pipe.

And, note, there are also a bunch of GUP changes that could have an effect.

David