btf: size unit of Int btf type is bit?

[Asphaltt]

I think it's strange that the size unit of Int BTF type is bit, but size unit of Struct/Union BTF type is byte.

For example, here's some BTF info of struct sk_buff:

  : offset=1088
  : Union[fields=2]
    size: 4
    csum: offset=0
    csum: Typedef:"__wsum"[Typedef:"__u32"]
      __wsum: Typedef:"__u32"[Int:"unsigned int"]
        __u32: Int:"unsigned int"[unsigned size=32]
    : offset=0
    : Struct[fields=2]
  priority: offset=1120
  priority: Typedef:"__u32"[Int:"unsigned int"]
    __u32: Int:"unsigned int"[unsigned size=32]

But the __u32 in bpftool btf dump file /sys/kernel/btf/vmlinux format raw is:

[9] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
[23] TYPEDEF '__u32' type_id=9

Here's uname info:

# uname -a
Linux leon-bpf 6.6.0-rc2+ #45 SMP PREEMPT_DYNAMIC Wed Oct 11 10:52:00 +08 2023 x86_64 x86_64 x86_64 GNU/Linux

Should the size unit of Int BTF type be byte?

[Asphaltt]

The struct sk_buff BTF info is generated by:

package pwru

import (
	"fmt"
	"log"
	"os"
	"strings"

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

func main() {
	kernelSpec, err := btf.LoadKernelSpec()
	if err != nil {
		log.Fatalf("failed to load kernel spec: %v", err)
	}

	iter := kernelSpec.Iterate()
	for iter.Next() {
		s, ok := iter.Type.(*btf.Struct)
		if !ok {
			continue
		}

		if s.Name == "sk_buff" {
			printBtf(s, 0, s.Name)
			fmt.Println()
		}
	}

	os.Exit(0)
}

var seen = make(map[string]bool)

func init() {
	seen["net_device"] = true
	seen["sock"] = true
}

func indent(n int) string {
	return strings.Repeat("  ", n)
}

func printBtf(t btf.Type, depth int, name string) {
	// if depth > 10 {
	// 	return
	// }

	if name == "" {
		name = t.TypeName()
	}

	fmt.Printf("%s%s: %v\n", indent(depth), name, t)

	switch t := t.(type) {
	case *btf.Int:
		// nothing
	case *btf.Float:
		// nothing
	case *btf.Struct:
		if seen[t.TypeName()] {
			return
		}
		seen[t.TypeName()] = true

		fmt.Printf("%s  size: %d\n", indent(depth), t.Size)
		for _, m := range t.Members {
			if m.BitfieldSize != 0 {
				fmt.Printf("%s  %s: offset=%d bitfieldSize=%d\n", indent(depth), m.Name, m.Offset, m.BitfieldSize)
			} else {
				fmt.Printf("%s  %s: offset=%d\n", indent(depth), m.Name, m.Offset)
			}
			printBtf(m.Type, depth+1, m.Name)
		}
	case *btf.Pointer:
		printBtf(t.Target, depth+1, "")
	case *btf.Typedef:
		printBtf(t.Type, depth+1, t.Name)
	case *btf.Const:
		printBtf(t.Type, depth+1, "")
	case *btf.Volatile:
		printBtf(t.Type, depth+1, "")
	case *btf.Fwd:
		// nothing
	case *btf.Void:
		// nothing
	case *btf.Enum:
		for _, v := range t.Values {
			fmt.Printf("%s  %s: %d\n", indent(depth+1), v.Name, v.Value)
		}
	case *btf.Union:
		fmt.Printf("%s  size: %d\n", indent(depth), t.Size)
		for _, m := range t.Members {
			if m.BitfieldSize != 0 {
				fmt.Printf("%s  %s: offset=%d bitfieldSize=%d\n", indent(depth), m.Name, m.Offset, m.BitfieldSize)
			} else {
				fmt.Printf("%s  %s: offset=%d\n", indent(depth), m.Name, m.Offset)
			}
			printBtf(m.Type, depth+1, m.Name)
		}
	case *btf.Array:
		printBtf(t.Type, depth+1, "")
	}
}

[ti-mo]

Should the size unit of Int BTF type be byte?

[9] INT 'unsigned int' size=4 ...

Looks like the 'size' field in the BTF is in bytes as well. What are you suggesting?

[Asphaltt]

Thanks. I corrected it by the PR #1182