Member sizes read from btf are always zero

[jschwinger233]

Describe the bug

I tried to read __sk_buff->gso_size's offset and size from BTF, found zero size as a result.

How to reproduce

package main

import (
	"fmt"

	"github.com/cilium/ebpf/btf"
)

func main() {
	btfSpec, err := btf.LoadKernelSpec()
	if err != nil {
		panic(fmt.Sprintf("Failed to load BTF spec: %v", err))
	}

	iter := btfSpec.Iterate()
	for iter.Next() {
		if strct, ok := iter.Type.(*btf.Struct); ok && strct.Name == "__sk_buff" {
			for _, member := range strct.Members {
				offsetInBytes := member.Offset.Bytes()
				SizeInBytes := member.BitfieldSize.Bytes()
				println("Member:", member.Name, "Offset:", offsetInBytes, "Size:", SizeInBytes)
			}
		}
	}

}

The output is:

Member: len Offset: 0 Size: 0
Member: pkt_type Offset: 4 Size: 0
Member: mark Offset: 8 Size: 0
Member: queue_mapping Offset: 12 Size: 0
Member: protocol Offset: 16 Size: 0
Member: vlan_present Offset: 20 Size: 0
Member: vlan_tci Offset: 24 Size: 0
Member: vlan_proto Offset: 28 Size: 0
Member: priority Offset: 32 Size: 0
Member: ingress_ifindex Offset: 36 Size: 0
Member: ifindex Offset: 40 Size: 0
Member: tc_index Offset: 44 Size: 0
Member: cb Offset: 48 Size: 0
Member: hash Offset: 68 Size: 0
Member: tc_classid Offset: 72 Size: 0
Member: data Offset: 76 Size: 0
Member: data_end Offset: 80 Size: 0
Member: napi_id Offset: 84 Size: 0
Member: family Offset: 88 Size: 0
Member: remote_ip4 Offset: 92 Size: 0
Member: local_ip4 Offset: 96 Size: 0
Member: remote_ip6 Offset: 100 Size: 0
Member: local_ip6 Offset: 116 Size: 0
Member: remote_port Offset: 132 Size: 0
Member: local_port Offset: 136 Size: 0
Member: data_meta Offset: 140 Size: 0
Member:  Offset: 144 Size: 0
Member: tstamp Offset: 152 Size: 0
Member: wire_len Offset: 160 Size: 0
Member: gso_segs Offset: 164 Size: 0
Member:  Offset: 168 Size: 0
Member: gso_size Offset: 176 Size: 0
Member: tstamp_type Offset: 180 Size: 0
Member: hwtstamp Offset: 184 Size: 0

Version information

github.com/cilium/ebpf v0.18.0

[florianl]

Hi 👋
Can you please also share the architecture and Kernel version on which you are running this experiment?

[jschwinger233]

$ uname -a
Linux graymon-Latitude-5530 6.11.0-26-generic #26~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Thu Apr 17 19:20:47 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

pahole and bpftool are both working properly with btf:

$ pahole -C __sk_buff | grep gso_size
               __u32                      gso_size;             /*   176     4 */

$ sudo bpftool btf d id 1 | grep __sk_buff -A50 | grep gso_size
               'gso_size' type_id=14 bits_offset=1408

$ sudo bpftool btf d id 1 f c | grep 'struct __sk_buff' -A50 | grep gso_size
	__u32 gso_size;

[Ghostbaby]

Hi @jschwinger233 .

1. BitfieldSize is only meaningful for bitfield members (when BitfieldSize > 0)
2. For regular struct members, BitfieldSize is typically 0
3. To get the actual size of a member, you need to use btf.Sizeof(member.Type)

From the debugger screenshot, we can see that the __u32 type has an actual size of 4 bytes, but BitfieldSize shows 0, causing all members to display a size of 0.

The code needs to distinguish between bitfield members and regular members, then use the appropriate method to calculate the size:

// ... existing code ...
iter := btfSpec.Iterate()
for iter.Next() {
    if strct, ok := iter.Type.(*btf.Struct); ok && strct.Name == "__sk_buff" {
        for _, member := range strct.Members {
            offsetInBytes := member.Offset.Bytes()
            
            // Check if this is a bitfield member
            var sizeInBytes uint32
            if member.BitfieldSize > 0 {
                // For bitfield members, use BitfieldSize
                sizeInBytes = member.BitfieldSize.Bytes()
            } else {
                // For regular members, calculate size from type
                size, err := btf.Sizeof(member.Type)
                if err != nil {
                    fmt.Printf("Failed to get size for member %s: %v\n", member.Name, err)
                    continue
                }
                sizeInBytes = uint32(size)
            }
            
            println("Member:", member.Name, "Offset:", offsetInBytes, "Size:", sizeInBytes)
        }
    }
}
// ... existing code ...

[jschwinger233]

Closed as resolved. Thank you @Ghostbaby 🙏