/
alignchecker.go
140 lines (122 loc) · 3.83 KB
/
alignchecker.go
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// Copyright 2018-2019 Authors of Cilium
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package alignchecker
import (
"debug/dwarf"
"debug/elf"
"fmt"
"reflect"
)
// CheckStructAlignments checks whether size and offsets match of the given
// C and Go structs which are listed in the given toCheck map (C struct name =>
// Go struct []reflect.Type).
//
// C struct size info is extracted from the given ELF object file debug section
// encoded in DWARF.
//
// To find a matching C struct field, a Go field has to be tagged with
// `align:"field_name_in_c_struct". In the case of unnamed union field, such
// union fields can be referred with special tags - `align:"$union0"`,
// `align:"$union1"`, etc.
func CheckStructAlignments(pathToObj string, toCheck map[string][]reflect.Type) error {
f, err := elf.Open(pathToObj)
if err != nil {
return fmt.Errorf("elf failed to open %s: %s", pathToObj, err)
}
defer f.Close()
d, err := getDWARFFromELF(f)
if err != nil {
return fmt.Errorf("cannot parse DWARF debug info %s: %s", pathToObj, err)
}
structInfo, err := getStructInfosFromDWARF(d, toCheck)
if err != nil {
return fmt.Errorf("cannot extract struct info from DWARF %s: %s", pathToObj, err)
}
for cName, goStructs := range toCheck {
if err := check(cName, goStructs, structInfo); err != nil {
return err
}
}
return nil
}
// structInfo contains C struct info
type structInfo struct {
size int64
fieldOffsets map[string]int64
}
func getStructInfosFromDWARF(d *dwarf.Data, toCheck map[string][]reflect.Type) (map[string]structInfo, error) {
structs := make(map[string]structInfo)
r := d.Reader()
for entry, err := r.Next(); entry != nil && err == nil; entry, err = r.Next() {
// Read only DWARF struct entries
if entry.Tag != dwarf.TagStructType {
continue
}
t, err := d.Type(entry.Offset)
if err != nil {
return nil, fmt.Errorf("cannot read DWARF info section at offset %d: %s",
entry.Offset, err)
}
st := t.(*dwarf.StructType)
if _, found := toCheck[st.StructName]; found {
unionCount := 0
offsets := make(map[string]int64)
for _, field := range st.Field {
n := field.Name
// Create surrogate names ($union0, $union1, etc) for unnamed
// union members
if n == "" {
if t, ok := field.Type.(*dwarf.StructType); ok {
if t.Kind == "union" {
n = fmt.Sprintf("$union%d", unionCount)
unionCount++
}
}
}
offsets[n] = field.ByteOffset
}
structs[st.StructName] = structInfo{
size: st.ByteSize,
fieldOffsets: offsets,
}
}
}
return structs, nil
}
func check(name string, toCheck []reflect.Type, structs map[string]structInfo) error {
for _, g := range toCheck {
c, found := structs[name]
if !found {
return fmt.Errorf("could not find C struct %s", name)
}
if c.size != int64(g.Size()) {
return fmt.Errorf("%s(%d) size does not match %s(%d)", g, g.Size(),
name, c.size)
}
for i := 0; i < g.NumField(); i++ {
fieldName := g.Field(i).Tag.Get("align")
// Ignore fields without `align` struct tag
if fieldName == "" {
continue
}
goOffset := int64(g.Field(i).Offset)
cOffset := structs[name].fieldOffsets[fieldName]
if goOffset != cOffset {
return fmt.Errorf("%s.%s offset(%d) does not match %s.%s(%d)",
g, g.Field(i).Name, goOffset, name, fieldName, cOffset)
}
}
}
return nil
}