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

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Message-ID: <j3ops2mlylgtb5ybkht75gu3dljlee6omu6zu4hsmnojssziyo@gnpsrrvh7f7l>
Date: Tue, 26 Dec 2023 21:22:03 +0800
From: Shung-Hsi Yu <shung-hsi.yu@...e.com>
To: Maxim Mikityanskiy <maxtram95@...il.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, Dec 26, 2023 at 12:38:06PM +0200, Maxim Mikityanskiy wrote:
> 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.

Thank for the thorough explanation. Now I can see and agree that the
u16->u64 change should be made. Digging back a big, the change also
aligns with what's said in commit 0be2516f865f5 ("selftests/bpf: Tests
for state pruning with u32 spill/fill") that introduced the check:

  ... checks that a filled u64 register is marked unknown if the
  register spilled in the same slack slot was less than 8B.

Side note: the r4 value in comment is still "R4=umax=65535", that
probably should be updated as well now that r4 is unbounded.

> [...]
> > > -	r4 = *(u16*)(r10 - 8);				\
> > > +	r4 = *(u64*)(r10 - 8);				\
> > >  	r0 = r2;					\
> > >  	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=umax=65535 */\
> > >  	r0 += r4;					\