Fix cgroup-skb-egress for new perf event array API

[Brskt]

PR aya-rs/aya#1554 replaced read_events with for_each; the
cgroup-skb-egress example no longer compiles. Migrate to the new
closure API and concat head + tail to handle samples that wrap
the ring boundary.

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[tamird]

@tamird reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on Brskt).

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examples/cgroup-skb-egress/cgroup-skb-egress/src/main.rs line 73 at r1 (raw file):

                    PerfEvent::Sample { head, tail } => {
                        // Samples can straddle the ring's wrap boundary; copy a contiguous window.
                        const N: usize = size_of::<PacketLog>();

PacketLog is Pod, right? Currently we are copying this twice: once from the sample to the stack buffer and then again into the PacketLog. Perhaps instead we could use MaybeUnint<PacketLog>? We would probably want https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.as_bytes_mut here but that's unstable so we can leave a TODO and use a handwritten implementation for now.

WDYT?

[Brskt]

@Brskt made 1 comment.
Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on tamird).

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examples/cgroup-skb-egress/cgroup-skb-egress/src/main.rs line 73 at r1 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

PacketLog is Pod, right? Currently we are copying this twice: once from the sample to the stack buffer and then again into the PacketLog. Perhaps instead we could use MaybeUnint<PacketLog>? We would probably want https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.as_bytes_mut here but that's unstable so we can leave a TODO and use a handwritten implementation for now.

WDYT?

Sure, looks good via Pod. Done with a small as_bytes_mut shim and a TODO.

[tamird]

@tamird reviewed all commit messages, made 1 comment, and resolved 1 discussion.
Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on Brskt).

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examples/cgroup-skb-egress/cgroup-skb-egress/src/main.rs line 103 at r2 (raw file):

                            );
                            data.assume_init()
                        };

Could we do something like

let bytes = as_bytes_mut(&mut data);
for (src, dst) in head.iter().chain(tail.iter()).zip(bytes) {
  dst.write(src);
}

otherwise if you want to copy through the pointer then you can just write

let mut data = MaybeUninit::<PacketLog>::uninit();
let dst = data.as_mut_ptr().cast::<u8>();
...

Code quote:

                        let bytes = as_bytes_mut(&mut data);
                        let head_len = head.len().min(bytes.len());
                        let dst = bytes.as_mut_ptr().cast::<u8>();
                        // SAFETY: source and destination are non-overlapping, and the writes
                        // cover all of `bytes`; PacketLog is Pod so any byte pattern is valid.
                        let data = unsafe {
                            std::ptr::copy_nonoverlapping(
                                head.as_ptr(),
                                dst,
                                head_len,
                            );
                            std::ptr::copy_nonoverlapping(
                                tail.as_ptr(),
                                dst.add(head_len),
                                bytes.len() - head_len,
                            );
                            data.assume_init()
                        };

[Brskt]

@Brskt made 1 comment.
Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on tamird).

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examples/cgroup-skb-egress/cgroup-skb-egress/src/main.rs line 103 at r2 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

Could we do something like

let bytes = as_bytes_mut(&mut data);
for (src, dst) in head.iter().chain(tail.iter()).zip(bytes) {
  dst.write(src);
}

otherwise if you want to copy through the pointer then you can just write

let mut data = MaybeUninit::<PacketLog>::uninit();
let dst = data.as_mut_ptr().cast::<u8>();
...

Yeah, the iterator path is cleaner. Done with a debug_assert_eq! for the kernel invariant.

[tamird]

Thanks!

@tamird reviewed 1 file and all commit messages, made 1 comment, and resolved 1 discussion.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on Brskt).