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 <[email protected]>
> >
> > 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 <[email protected]>
> > [...]
> > 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; \
