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overlay/tio: log dropped tun reads with bad virtio headers

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decodeRead failures were silently swallowed; a kernel emitting an
unnegotiated GSO type would blackhole all tun traffic with nothing in
the logs. Debug-gated per the usual idiom so the happy path pays
nothing, which means plumbing the logger down through the offload
queueset.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ugV2edVqoz3tBvq9J6yWp
This commit is contained in:
JackDoan
2026-07-29 17:16:47 -05:00
parent e9870686c8
commit fbcdd83359
4 changed files with 29 additions and 6 deletions
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overlay/tio/tio_gso_linux.go
+20 -2
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@@ -4,8 +4,10 @@
package tio
import (
"context"
"fmt"
"io"
"log/slog"
"os"
"sync/atomic"
"syscall"
@@ -87,9 +89,13 @@ type Offload struct {
// iovec[0] points at readVnetScratch
// iovec[1].Base/Len is updated per read to address the current rxBuf slot.
readIovs [2]unix.Iovec
// l is only consulted on the rare bad-vnet-header drop path; it lives
// after the hot state on purpose. May be nil (tests); drops go unlogged then.
l *slog.Logger
}
func newOffload(fd int, shutdownFd int, usoEnabled bool) (*Offload, error) {
func newOffload(fd int, shutdownFd int, usoEnabled bool, l *slog.Logger) (*Offload, error) {
if err := unix.SetNonblock(fd, true); err != nil {
return nil, fmt.Errorf("failed to set tun fd non-blocking: %w", err)
}
@@ -99,6 +105,7 @@ func newOffload(fd int, shutdownFd int, usoEnabled bool) (*Offload, error) {
shutdownFd: shutdownFd,
usoEnabled: usoEnabled,
closed: atomic.Bool{},
l: l,
rxBuf: make([]byte, tunRxBufCap),
gsoIovs: make([]unix.Iovec, 2, gsoMaxIovs),
@@ -178,7 +185,9 @@ func (r *Offload) Read() ([]Packet, error) {
return nil, err
}
if err := r.decodeRead(n); err != nil {
// Drop and read again. A bad packet should not kill the reader.
// Drop and read again. A bad packet should not kill the reader,
// but a systematic decode failure must not be invisible either.
r.logDroppedRead(err)
continue
}
break
@@ -200,6 +209,7 @@ func (r *Offload) Read() ([]Packet, error) {
if err := r.decodeRead(n); err != nil {
// Drop this packet and stop the drain; we'd rather hand off
// what we have than keep spinning here.
r.logDroppedRead(err)
break
}
}
@@ -207,6 +217,14 @@ func (r *Offload) Read() ([]Packet, error) {
return r.pending, nil
}
// logDroppedRead reports a tun packet dropped for a bad/unsupported virtio
// header. Debug-gated so the happy path never pays for attribute assembly.
func (r *Offload) logDroppedRead(err error) {
if r.l != nil && r.l.Enabled(context.Background(), slog.LevelDebug) {
r.l.Debug("dropping tun packet with bad virtio header", "error", err)
}
}
// decodeRead processes the packet sitting in rxBuf at rxOff (length pktLen).
// The bytes stay in rxBuf:
// - for GSO_NONE we slice them as a regular IP datagram (running finishChecksum if NEEDS_CSUM is set);