linux-kernel - Re: [PATCH] bpf: return EOPNOTSUPP when JIT is needed and not possible

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Message-ID: <YbNG5BliqnCyhs4J@mussarela>
Date:   Fri, 10 Dec 2021 09:24:04 -0300
From:   Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Ido Schimmel <idosch@...sch.org>,
        John Fastabend <john.fastabend@...il.com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: return EOPNOTSUPP when JIT is needed and not
 possible

On Thu, Dec 09, 2021 at 06:23:49PM -0800, Jakub Kicinski wrote:
> On Fri, 10 Dec 2021 00:03:40 +0100 Daniel Borkmann wrote:
> > > Similar issue was discussed in the past. See:
> > > https://lore.kernel.org/netdev/20191204.125135.750458923752225025.davem@davemloft.net/  
> > 
> > With regards to ENOTSUPP exposure, if the consensus is that we should fix all
> > occurences over to EOPNOTSUPP even if they've been exposed for quite some time
> > (Jakub?), 
> 
> Did you mean me? :) In case you did - I think we should avoid it 
> for new code but changing existing now seems risky. Alexei and Andrii
> would know best but quick search of code bases at work reveals some
> scripts looking for ENOTSUPP.
> 
> Thadeu, what motivated the change?
> 
> If we're getting those changes fixes based on checkpatch output maybe 
> there is a way to mute the checkpatch warnings when it's not run on a 
> diff?
> 

It was not checkpatch that motivated me.

I was looking into the following commits as we hit a failed test.

be08815c5d3b ("bpf: add also cbpf long jump test cases with heavy expansion")
050fad7c4534 ("bpf: fix truncated jump targets on heavy expansions") 

Then, I realized that if given the right number of BPF_LDX | BPF_B | BPF_MSH
instructions, it will pass the bpf_convert_filter stage, but fail at blinding.
And if you have CONFIG_BPF_JIT_ALWAYS_ON, setting the filter will fail with
ENOTSUPP, which should not be sent to userspace.

I noticed other ENOTSUPP, but they seemed to be returned by helpers, and I was
not sure this would be relayed to userspace. So, I went for fixing the observed
case.

I will see if any of the tests I can run is broken by this change and submit it
again with the tests fixed as well.

Cascardo.

> > we could give this patch a try maybe via bpf-next and see if anyone complains.
> > 
> > Thadeu, I think you also need to fix up BPF selftests as test_verifier, to mention
> > one example (there are also bunch of others under tools/testing/selftests/), is
> > checking for ENOTSUPP specifically..