Use seqfile.SeqHandles correctly in VFS1 /proc/net/.

Before this change:

```
$ docker run --runtime=runsc --rm -it -v ~/tmp:/hosttmp ubuntu:focal /hosttmp/issue5732 --bytes1=128 --bytes2=1024
#1: read(128) = 128
#2: read(1024) = EOF
$ docker run --runtime=runsc-vfs2 --rm -it -v ~/tmp:/hosttmp ubuntu:focal /hosttmp/issue5732 --bytes1=128 --bytes2=1024
#1: read(128) = 128
#2: read(1024) = 256
```

After this change:

```
$ docker run --runtime=runsc --rm -it -v ~/tmp:/hosttmp ubuntu:focal /hosttmp/issue5732 --bytes1=128 --bytes2=1024
#1: read(128) = 128
#2: read(1024) = 256
$ docker run --runtime=runsc-vfs2 --rm -it -v ~/tmp:/hosttmp ubuntu:focal /hosttmp/issue5732 --bytes1=128 --bytes2=1024
#1: read(128) = 128
#2: read(1024) = 256
```

Fixes #5732

PiperOrigin-RevId: 365178386

1 files changed, 77 insertions, 55 deletions

diff --git a/pkg/sentry/fs/proc/net.go b/pkg/sentry/fs/proc/net.go
index 03127f816..203cfa061 100644
--- a/pkg/sentry/fs/proc/net.go
+++ b/pkg/sentry/fs/proc/net.go
@@ -128,13 +128,17 @@ func (*ifinet6) NeedsUpdate(generation int64) bool {
 
 // ReadSeqFileData implements seqfile.SeqSource.ReadSeqFileData.
 func (n *ifinet6) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]seqfile.SeqData, int64) {
+	contents := n.contents()
+	minI := 0
 	if h != nil {
-		return nil, 0
+		minI = h.(int) + 1
+		if minI > len(contents) {
+			minI = len(contents)
+		}
 	}
-
 	var data []seqfile.SeqData
-	for _, l := range n.contents() {
-		data = append(data, seqfile.SeqData{Buf: []byte(l), Handle: (*ifinet6)(nil)})
+	for i, l := range contents[minI:] {
+		data = append(data, seqfile.SeqData{Buf: []byte(l), Handle: i + minI})
 	}
 
 	return data, 0
@@ -155,10 +159,6 @@ func (n *netDev) NeedsUpdate(generation int64) bool {
 // ReadSeqFileData implements seqfile.SeqSource.ReadSeqFileData. See Linux's
 // net/core/net-procfs.c:dev_seq_show.
 func (n *netDev) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]seqfile.SeqData, int64) {
-	if h != nil {
-		return nil, 0
-	}
-
 	interfaces := n.s.Interfaces()
 	contents := make([]string, 2, 2+len(interfaces))
 	// Add the table header. From net/core/net-procfs.c:dev_seq_show.
@@ -197,9 +197,16 @@ func (n *netDev) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]se
 		contents = append(contents, l)
 	}
 
+	minI := 0
+	if h != nil {
+		minI = h.(int) + 1
+		if minI > len(contents) {
+			minI = len(contents)
+		}
+	}
 	var data []seqfile.SeqData
-	for _, l := range contents {
-		data = append(data, seqfile.SeqData{Buf: []byte(l), Handle: (*netDev)(nil)})
+	for i, l := range contents[minI:] {
+		data = append(data, seqfile.SeqData{Buf: []byte(l), Handle: i + minI})
 	}
 
 	return data, 0
@@ -264,10 +271,6 @@ func sprintSlice(s []uint64) string {
 // ReadSeqFileData implements seqfile.SeqSource.ReadSeqFileData. See Linux's
 // net/core/net-procfs.c:dev_seq_show.
 func (n *netSnmp) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]seqfile.SeqData, int64) {
-	if h != nil {
-		return nil, 0
-	}
-
 	contents := make([]string, 0, len(snmp)*2)
 	types := []interface{}{
 		&inet.StatSNMPIP{},
@@ -309,9 +312,16 @@ func (n *netSnmp) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]s
 		)
 	}
 
+	minI := 0
+	if h != nil {
+		minI = h.(int) + 1
+		if minI > len(contents) {
+			minI = len(contents)
+		}
+	}
 	data := make([]seqfile.SeqData, 0, len(snmp)*2)
-	for _, l := range contents {
-		data = append(data, seqfile.SeqData{Buf: []byte(l), Handle: (*netSnmp)(nil)})
+	for i, l := range contents[minI:] {
+		data = append(data, seqfile.SeqData{Buf: []byte(l), Handle: i + minI})
 	}
 
 	return data, 0
@@ -332,10 +342,6 @@ func (n *netRoute) NeedsUpdate(generation int64) bool {
 // ReadSeqFileData implements seqfile.SeqSource.ReadSeqFileData.
 // See Linux's net/ipv4/fib_trie.c:fib_route_seq_show.
 func (n *netRoute) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]seqfile.SeqData, int64) {
-	if h != nil {
-		return nil, 0
-	}
-
 	interfaces := n.s.Interfaces()
 	contents := []string{"Iface\tDestination\tGateway\tFlags\tRefCnt\tUse\tMetric\tMask\tMTU\tWindow\tIRTT"}
 	for _, rt := range n.s.RouteTable() {
@@ -383,10 +389,17 @@ func (n *netRoute) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]
 		contents = append(contents, l)
 	}
 
+	minI := 0
+	if h != nil {
+		minI = h.(int) + 1
+		if minI > len(contents) {
+			minI = len(contents)
+		}
+	}
 	var data []seqfile.SeqData
-	for _, l := range contents {
+	for i, l := range contents[minI:] {
 		l = fmt.Sprintf("%-127s\n", l)
-		data = append(data, seqfile.SeqData{Buf: []byte(l), Handle: (*netRoute)(nil)})
+		data = append(data, seqfile.SeqData{Buf: []byte(l), Handle: i + minI})
 	}
 
 	return data, 0
@@ -406,10 +419,6 @@ func (*netUnix) NeedsUpdate(generation int64) bool {
 
 // ReadSeqFileData implements seqfile.SeqSource.ReadSeqFileData.
 func (n *netUnix) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]seqfile.SeqData, int64) {
-	if h != nil {
-		return []seqfile.SeqData{}, 0
-	}
-
 	var buf bytes.Buffer
 	for _, se := range n.k.ListSockets() {
 		s := se.Sock.Get()
@@ -482,15 +491,22 @@ func (n *netUnix) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]s
 		s.DecRef(ctx)
 	}
 
-	data := []seqfile.SeqData{
-		{
+	minI := 0
+	if h != nil {
+		minI = h.(int) + 1
+	}
+	var data []seqfile.SeqData
+	if minI <= 0 {
+		data = append(data, seqfile.SeqData{
 			Buf:    []byte("Num       RefCount Protocol Flags    Type St Inode Path\n"),
-			Handle: n,
-		},
-		{
+			Handle: 0,
+		})
+	}
+	if minI <= 1 {
+		data = append(data, seqfile.SeqData{
 			Buf:    buf.Bytes(),
-			Handle: n,
-		},
+			Handle: 1,
+		})
 	}
 	return data, 0
 }
@@ -557,10 +573,6 @@ func commonReadSeqFileDataTCP(ctx context.Context, n seqfile.SeqHandle, k *kerne
 	// degrade gracefully and retrieve what we can.
 	t := kernel.TaskFromContext(ctx)
 
-	if h != nil {
-		return nil, 0
-	}
-
 	var buf bytes.Buffer
 	for _, se := range k.ListSockets() {
 		s := se.Sock.Get()
@@ -667,15 +679,22 @@ func commonReadSeqFileDataTCP(ctx context.Context, n seqfile.SeqHandle, k *kerne
 		s.DecRef(ctx)
 	}
 
-	data := []seqfile.SeqData{
-		{
+	minI := 0
+	if h != nil {
+		minI = h.(int) + 1
+	}
+	var data []seqfile.SeqData
+	if minI <= 0 {
+		data = append(data, seqfile.SeqData{
 			Buf:    header,
-			Handle: n,
-		},
-		{
+			Handle: 0,
+		})
+	}
+	if minI <= 1 {
+		data = append(data, seqfile.SeqData{
 			Buf:    buf.Bytes(),
-			Handle: n,
-		},
+			Handle: 1,
+		})
 	}
 	return data, 0
 }
@@ -735,10 +754,6 @@ func (n *netUDP) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]se
 	// degrade gracefully and retrieve what we can.
 	t := kernel.TaskFromContext(ctx)
 
-	if h != nil {
-		return nil, 0
-	}
-
 	var buf bytes.Buffer
 	for _, se := range n.k.ListSockets() {
 		s := se.Sock.Get()
@@ -825,15 +840,22 @@ func (n *netUDP) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) ([]se
 		s.DecRef(ctx)
 	}
 
-	data := []seqfile.SeqData{
-		{
+	minI := 0
+	if h != nil {
+		minI = h.(int) + 1
+	}
+	var data []seqfile.SeqData
+	if minI <= 0 {
+		data = append(data, seqfile.SeqData{
 			Buf:    []byte("  sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode ref pointer drops             \n"),
-			Handle: n,
-		},
-		{
+			Handle: 0,
+		})
+	}
+	if minI <= 1 {
+		data = append(data, seqfile.SeqData{
 			Buf:    buf.Bytes(),
-			Handle: n,
-		},
+			Handle: 1,
+		})
 	}
 	return data, 0
 }