netdev - Re: [PATCH bpf-next 01/15] selftests/bpf: Fix the u64_offset_to_skb

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Date: Tue, 26 Dec 2023 12:38:06 +0200
From: Maxim Mikityanskiy <maxtram95@...il.com>
To: Shung-Hsi Yu <shung-hsi.yu@...e.com>
Cc: Eduard Zingerman <eddyz87@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>, bpf@...r.kernel.org,
	linux-kselftest@...r.kernel.org, netdev@...r.kernel.org,
	Maxim Mikityanskiy <maxim@...valent.com>
Subject: Re: [PATCH bpf-next 01/15] selftests/bpf: Fix the
 u64_offset_to_skb_data test

On Tue, 26 Dec 2023 at 17:52:56 +0800, Shung-Hsi Yu wrote:
> On Wed, Dec 20, 2023 at 11:39:59PM +0200, Maxim Mikityanskiy wrote:
> > From: Maxim Mikityanskiy <maxim@...valent.com>
> > 
> > The u64_offset_to_skb_data test is supposed to make a 64-bit fill, but
> > instead makes a 16-bit one. Fix the test according to its intention. The
> > 16-bit fill is covered by u16_offset_to_skb_data.
> 
> Cover letter mentioned
> 
>   Patch 1 (Maxim): Fix for an existing test, it will matter later in the
>   series.
> 
> However no subsequent patch touch upon u64_offset_to_skb_data(). Was the
> followup missing from this series?

Thanks for your vigilance, but it's actually correct, sorry for not
making it clear enough. In patch 11 ("bpf: Preserve boundaries and track
scalars on narrowing fill") I modify u16_offset_to_skb_data, because it
becomes a valid pattern after that change. If I didn't change and fix
u64_offset_to_skb_data here, I'd need to modify it in patch 11 as well
(that's what I meant when I said "it will matter later in the series",
it's indeed subtle and implicit, now that I look at it), because it
would also start passing, however, that's not what we want, because:

1. Both tests would essentially test the same thing: a 16-bit fill after
a 32-bit spill.

2. The description of u64_offset_to_skb_data clearly says: "Refill as
u64". It's a typo in the code, u16->u64 makes sense, because we spill
two u32s and fill them as a single u64.

So, this patch essentially prevents wrong changes in a further patch.

> > Signed-off-by: Maxim Mikityanskiy <maxim@...valent.com>
> > [...]
> >  SEC("tc")
> >  __description("Spill u32 const scalars.  Refill as u64.  Offset to skb->data")
> > -__failure __msg("invalid access to packet")
> > +__failure __msg("math between pkt pointer and register with unbounded min value is not allowed")
> >  __naked void u64_offset_to_skb_data(void)
> >  {
> >  	asm volatile ("					\
> > @@ -253,7 +253,7 @@ __naked void u64_offset_to_skb_data(void)
> >  	w7 = 20;					\
> >  	*(u32*)(r10 - 4) = r6;				\
> >  	*(u32*)(r10 - 8) = r7;				\
> > -	r4 = *(u16*)(r10 - 8);				\
> > +	r4 = *(u64*)(r10 - 8);				\
> >  	r0 = r2;					\
> >  	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=umax=65535 */\
> >  	r0 += r4;					\