whatcanGOwrong

go/pkg/mod/github.com/cilium/ebpf@v0.11.0/btf/strings.go

@@ -0,0 +1,214 @@
+package btf
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+
+	"golang.org/x/exp/maps"
+)
+
+type stringTable struct {
+	base    *stringTable
+	offsets []uint32
+	strings []string
+}
+
+// sizedReader is implemented by bytes.Reader, io.SectionReader, strings.Reader, etc.
+type sizedReader interface {
+	io.Reader
+	Size() int64
+}
+
+func readStringTable(r sizedReader, base *stringTable) (*stringTable, error) {
+	// When parsing split BTF's string table, the first entry offset is derived
+	// from the last entry offset of the base BTF.
+	firstStringOffset := uint32(0)
+	if base != nil {
+		idx := len(base.offsets) - 1
+		firstStringOffset = base.offsets[idx] + uint32(len(base.strings[idx])) + 1
+	}
+
+	// Derived from vmlinux BTF.
+	const averageStringLength = 16
+
+	n := int(r.Size() / averageStringLength)
+	offsets := make([]uint32, 0, n)
+	strings := make([]string, 0, n)
+
+	offset := firstStringOffset
+	scanner := bufio.NewScanner(r)
+	scanner.Split(splitNull)
+	for scanner.Scan() {
+		str := scanner.Text()
+		offsets = append(offsets, offset)
+		strings = append(strings, str)
+		offset += uint32(len(str)) + 1
+	}
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	if len(strings) == 0 {
+		return nil, errors.New("string table is empty")
+	}
+
+	if firstStringOffset == 0 && strings[0] != "" {
+		return nil, errors.New("first item in string table is non-empty")
+	}
+
+	return &stringTable{base, offsets, strings}, nil
+}
+
+func splitNull(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	i := bytes.IndexByte(data, 0)
+	if i == -1 {
+		if atEOF && len(data) > 0 {
+			return 0, nil, errors.New("string table isn't null terminated")
+		}
+		return 0, nil, nil
+	}
+
+	return i + 1, data[:i], nil
+}
+
+func (st *stringTable) Lookup(offset uint32) (string, error) {
+	if st.base != nil && offset <= st.base.offsets[len(st.base.offsets)-1] {
+		return st.base.lookup(offset)
+	}
+	return st.lookup(offset)
+}
+
+func (st *stringTable) lookup(offset uint32) (string, error) {
+	i := search(st.offsets, offset)
+	if i == len(st.offsets) || st.offsets[i] != offset {
+		return "", fmt.Errorf("offset %d isn't start of a string", offset)
+	}
+
+	return st.strings[i], nil
+}
+
+func (st *stringTable) Marshal(w io.Writer) error {
+	for _, str := range st.strings {
+		_, err := io.WriteString(w, str)
+		if err != nil {
+			return err
+		}
+		_, err = w.Write([]byte{0})
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Num returns the number of strings in the table.
+func (st *stringTable) Num() int {
+	return len(st.strings)
+}
+
+// search is a copy of sort.Search specialised for uint32.
+//
+// Licensed under https://go.dev/LICENSE
+func search(ints []uint32, needle uint32) int {
+	// Define f(-1) == false and f(n) == true.
+	// Invariant: f(i-1) == false, f(j) == true.
+	i, j := 0, len(ints)
+	for i < j {
+		h := int(uint(i+j) >> 1) // avoid overflow when computing h
+		// i ≤ h < j
+		if !(ints[h] >= needle) {
+			i = h + 1 // preserves f(i-1) == false
+		} else {
+			j = h // preserves f(j) == true
+		}
+	}
+	// i == j, f(i-1) == false, and f(j) (= f(i)) == true  =>  answer is i.
+	return i
+}
+
+// stringTableBuilder builds BTF string tables.
+type stringTableBuilder struct {
+	length  uint32
+	strings map[string]uint32
+}
+
+// newStringTableBuilder creates a builder with the given capacity.
+//
+// capacity may be zero.
+func newStringTableBuilder(capacity int) *stringTableBuilder {
+	var stb stringTableBuilder
+
+	if capacity == 0 {
+		// Use the runtime's small default size.
+		stb.strings = make(map[string]uint32)
+	} else {
+		stb.strings = make(map[string]uint32, capacity)
+	}
+
+	// Ensure that the empty string is at index 0.
+	stb.append("")
+	return &stb
+}
+
+// Add a string to the table.
+//
+// Adding the same string multiple times will only store it once.
+func (stb *stringTableBuilder) Add(str string) (uint32, error) {
+	if strings.IndexByte(str, 0) != -1 {
+		return 0, fmt.Errorf("string contains null: %q", str)
+	}
+
+	offset, ok := stb.strings[str]
+	if ok {
+		return offset, nil
+	}
+
+	return stb.append(str), nil
+}
+
+func (stb *stringTableBuilder) append(str string) uint32 {
+	offset := stb.length
+	stb.length += uint32(len(str)) + 1
+	stb.strings[str] = offset
+	return offset
+}
+
+// Lookup finds the offset of a string in the table.
+//
+// Returns an error if str hasn't been added yet.
+func (stb *stringTableBuilder) Lookup(str string) (uint32, error) {
+	offset, ok := stb.strings[str]
+	if !ok {
+		return 0, fmt.Errorf("string %q is not in table", str)
+	}
+
+	return offset, nil
+}
+
+// Length returns the length in bytes.
+func (stb *stringTableBuilder) Length() int {
+	return int(stb.length)
+}
+
+// AppendEncoded appends the string table to the end of the provided buffer.
+func (stb *stringTableBuilder) AppendEncoded(buf []byte) []byte {
+	n := len(buf)
+	buf = append(buf, make([]byte, stb.Length())...)
+	strings := buf[n:]
+	for str, offset := range stb.strings {
+		copy(strings[offset:], str)
+	}
+	return buf
+}
+
+// Copy the string table builder.
+func (stb *stringTableBuilder) Copy() *stringTableBuilder {
+	return &stringTableBuilder{
+		stb.length,
+		maps.Clone(stb.strings),
+	}
+}