Make TC BPF helpers preserve skb metadata #6302
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Lay groundwork for fixing BPF helpers available to TC(X) programs. When skb_push() or skb_pull() is called in a TC(X) ingress BPF program, the skb metadata must be kept in front of the MAC header. Otherwise, BPF programs using the __sk_buff->data_meta pseudo-pointer lose access to it. Introduce a helper that moves both metadata and a specified number of packet data bytes together, suitable as a drop-in replacement for memmove(). Signed-off-by: Jakub Sitnicki <[email protected]>
pskb_expand_head() copies headroom, including skb metadata, into the newly allocated head, but then clears the metadata. As a result, metadata is lost when BPF helpers trigger an skb head reallocation. Let the skb metadata remain in the newly created copy of head. Signed-off-by: Jakub Sitnicki <[email protected]>
Currently bpf_dynptr_from_skb_meta() marks the dynptr as read-only when the skb is cloned, preventing writes to metadata. Remove this restriction and unclone the skb head on bpf_dynptr_write() to metadata, now that the metadata is preserved during uncloning. This makes metadata dynptr consistent with skb dynptr, allowing writes regardless of whether the skb is cloned. Signed-off-by: Jakub Sitnicki <[email protected]>
All callers ignore the return value. Prepare to reorder memmove() after skb_pull() which is a common pattern. Signed-off-by: Jakub Sitnicki <[email protected]>
Use the metadata-aware helper to move packet bytes after skb_pull(), ensuring metadata remains valid after calling the BPF helper. Signed-off-by: Jakub Sitnicki <[email protected]>
Use the metadata-aware helper to move packet bytes after skb_push(), ensuring metadata remains valid after calling the BPF helper. Also, take care to reserve sufficient headroom for metadata to fit. Signed-off-by: Jakub Sitnicki <[email protected]>
bpf_skb_adjust_room() may push or pull bytes from skb->data. In both cases, skb metadata must be moved accordingly to stay accessible. Replace existing memmove() calls, which only move payload, with a helper that also handles metadata. Reserve enough space for metadata to fit after skb_push. Signed-off-by: Jakub Sitnicki <[email protected]>
bpf_skb_change_proto reuses the same headroom operations as bpf_skb_adjust_room, already updated to handle metadata safely. The remaining step is to ensure that there is sufficient headroom to accommodate metadata on skb_push(). Signed-off-by: Jakub Sitnicki <[email protected]>
Although bpf_skb_change_head() doesn't move packet data after skb_push(), skb metadata still needs to be relocated. Use the dedicated helper to handle it. Signed-off-by: Jakub Sitnicki <[email protected]>
Move metadata verification into the BPF TC programs. Previously, userspace read metadata from a map and verified it once at test end. Now TC programs compare metadata directly using __builtin_memcmp() and set a test_pass flag. This enables verification at multiple points during test execution rather than a single final check. Signed-off-by: Jakub Sitnicki <[email protected]>
Add diagnostic output when metadata verification fails to help with troubleshooting test failures. Introduce a check_metadata() helper that prints both expected and received metadata to the BPF program's stderr stream on mismatch. The userspace test reads and dumps this stream on failure. Signed-off-by: Jakub Sitnicki <[email protected]>
Since pskb_expand_head() no longer clears metadata on unclone, update tests
for cloned packets to expect metadata to remain intact.
Also simplify the clone_dynptr_kept_on_{data,meta}_slice_write tests.
Creating an r/w dynptr slice is sufficient to trigger an unclone in the
prologue, so remove the extraneous writes to the data/meta slice.
Signed-off-by: Jakub Sitnicki <[email protected]>
Add a test to verify that skb metadata remains accessible after calling bpf_skb_vlan_push() and bpf_skb_vlan_pop(), which modify the packet headroom. Signed-off-by: Jakub Sitnicki <[email protected]>
Add a test to verify that skb metadata remains accessible after calling bpf_skb_adjust_room(), which modifies the packet headroom and can trigger head reallocation. The helper expects an Ethernet frame carrying an IP packet so switch test packet identification by source MAC address since we can no longer rely on Ethernet proto being set to zero. Signed-off-by: Jakub Sitnicki <[email protected]>
Add a test to verify that skb metadata remains accessible after calling bpf_skb_change_head() and bpf_skb_change_tail(), which modify packet headroom/tailroom and can trigger head reallocation. Signed-off-by: Jakub Sitnicki <[email protected]>
Add a test to verify that skb metadata remains accessible after calling bpf_skb_change_proto(), which modifies packet headroom to accommodate different IP header sizes. Signed-off-by: Jakub Sitnicki <[email protected]>
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Pull request for series with
subject: Make TC BPF helpers preserve skb metadata
version: 4
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1020081