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@xxxxxxxxxxxxx>
> > >
> > > 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; \
