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@isovalent.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:
- Both tests would essentially test the same thing: a 16-bit fill after
a 32-bit spill.
- 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; \