internal/elfexec/elfexec.go

// Copyright 2014 Google Inc. All Rights Reserved.
//
// 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 elfexec provides utility routines to examine ELF binaries.
package elfexec

import (
	"bufio"
	"debug/elf"
	"encoding/binary"
	"fmt"
	"io"
)

const (
	maxNoteSize        = 1 << 20 // in bytes
	noteTypeGNUBuildID = 3
)

// elfNote is the payload of a Note Section in an ELF file.
type elfNote struct {
	Name string // Contents of the "name" field, omitting the trailing zero byte.
	Desc []byte // Contents of the "desc" field.
	Type uint32 // Contents of the "type" field.
}

// parseNotes returns the notes from a SHT_NOTE section or PT_NOTE segment.
func parseNotes(reader io.Reader, alignment int, order binary.ByteOrder) ([]elfNote, error) {
	r := bufio.NewReader(reader)

	// padding returns the number of bytes required to pad the given size to an
	// alignment boundary.
	padding := func(size int) int {
		return ((size + (alignment - 1)) &^ (alignment - 1)) - size
	}

	var notes []elfNote
	for {
		noteHeader := make([]byte, 12) // 3 4-byte words
		if _, err := io.ReadFull(r, noteHeader); err == io.EOF {
			break
		} else if err != nil {
			return nil, err
		}
		namesz := order.Uint32(noteHeader[0:4])
		descsz := order.Uint32(noteHeader[4:8])
		typ := order.Uint32(noteHeader[8:12])

		if uint64(namesz) > uint64(maxNoteSize) {
			return nil, fmt.Errorf("note name too long (%d bytes)", namesz)
		}
		var name string
		if namesz > 0 {
			// Documentation differs as to whether namesz is meant to include the
			// trailing zero, but everyone agrees that name is null-terminated.
			// So we'll just determine the actual length after the fact.
			var err error
			name, err = r.ReadString('\x00')
			if err == io.EOF {
				return nil, fmt.Errorf("missing note name (want %d bytes)", namesz)
			} else if err != nil {
				return nil, err
			}
			namesz = uint32(len(name))
			name = name[:len(name)-1]
		}

		// Drop padding bytes until the desc field.
		for n := padding(len(noteHeader) + int(namesz)); n > 0; n-- {
			if _, err := r.ReadByte(); err == io.EOF {
				return nil, fmt.Errorf(
					"missing %d bytes of padding after note name", n)
			} else if err != nil {
				return nil, err
			}
		}

		if uint64(descsz) > uint64(maxNoteSize) {
			return nil, fmt.Errorf("note desc too long (%d bytes)", descsz)
		}
		desc := make([]byte, int(descsz))
		if _, err := io.ReadFull(r, desc); err == io.EOF {
			return nil, fmt.Errorf("missing desc (want %d bytes)", len(desc))
		} else if err != nil {
			return nil, err
		}

		notes = append(notes, elfNote{Name: name, Desc: desc, Type: typ})

		// Drop padding bytes until the next note or the end of the section,
		// whichever comes first.
		for n := padding(len(desc)); n > 0; n-- {
			if _, err := r.ReadByte(); err == io.EOF {
				// We hit the end of the section before an alignment boundary.
				// This can happen if this section is at the end of the file or the next
				// section has a smaller alignment requirement.
				break
			} else if err != nil {
				return nil, err
			}
		}
	}
	return notes, nil
}

// GetBuildID returns the GNU build-ID for an ELF binary.
//
// If no build-ID was found but the binary was read without error, it returns
// (nil, nil).
func GetBuildID(binary io.ReaderAt) ([]byte, error) {
	f, err := elf.NewFile(binary)
	if err != nil {
		return nil, err
	}

	findBuildID := func(notes []elfNote) ([]byte, error) {
		var buildID []byte
		for _, note := range notes {
			if note.Name == "GNU" && note.Type == noteTypeGNUBuildID {
				if buildID == nil {
					buildID = note.Desc
				} else {
					return nil, fmt.Errorf("multiple build ids found, don't know which to use")
				}
			}
		}
		return buildID, nil
	}

	for _, p := range f.Progs {
		if p.Type != elf.PT_NOTE {
			continue
		}
		notes, err := parseNotes(p.Open(), int(p.Align), f.ByteOrder)
		if err != nil {
			return nil, err
		}
		if b, err := findBuildID(notes); b != nil || err != nil {
			return b, err
		}
	}
	for _, s := range f.Sections {
		if s.Type != elf.SHT_NOTE {
			continue
		}
		notes, err := parseNotes(s.Open(), int(s.Addralign), f.ByteOrder)
		if err != nil {
			return nil, err
		}
		if b, err := findBuildID(notes); b != nil || err != nil {
			return b, err
		}
	}
	return nil, nil
}

// GetBase determines the base address to subtract from virtual
// address to get symbol table address. For an executable, the base
// is 0. Otherwise, it's a shared library, and the base is the
// address where the mapping starts. The kernel is special, and may
// use the address of the _stext symbol as the mmap start. _stext
// offset can be obtained with `nm vmlinux | grep _stext`
func GetBase(fh *elf.FileHeader, loadSegment *elf.ProgHeader, stextOffset *uint64, start, limit, offset uint64) (uint64, error) {
	const (
		pageSize = 4096
		// PAGE_OFFSET for PowerPC64, see arch/powerpc/Kconfig in the kernel sources.
		pageOffsetPpc64 = 0xc000000000000000
	)

	if start == 0 && offset == 0 &&
		(limit == ^uint64(0) || limit == 0) {
		// Some tools may introduce a fake mapping that spans the entire
		// address space. Assume that the address has already been
		// adjusted, so no additional base adjustment is necessary.
		return 0, nil
	}

	switch fh.Type {
	case elf.ET_EXEC:
		if loadSegment == nil {
			// Fixed-address executable, no adjustment.
			return 0, nil
		}
		if start == 0 && limit != 0 {
			// ChromeOS remaps its kernel to 0. Nothing else should come
			// down this path. Empirical values:
			//       VADDR=0xffffffff80200000
			// stextOffset=0xffffffff80200198
			if stextOffset != nil {
				return -*stextOffset, nil
			}
			return -loadSegment.Vaddr, nil
		}
		if loadSegment.Vaddr-loadSegment.Off == start-offset {
			return offset, nil
		}
		if loadSegment.Vaddr == start-offset {
			return offset, nil
		}
		if start >= loadSegment.Vaddr && limit > start && (offset == 0 || offset == pageOffsetPpc64) {
			// Some kernels look like:
			//       VADDR=0xffffffff80200000
			// stextOffset=0xffffffff80200198
			//       Start=0xffffffff83200000
			//       Limit=0xffffffff84200000
			//      Offset=0 (0xc000000000000000 for PowerPC64)
			// So the base should be:
			if stextOffset != nil && (start%pageSize) == (*stextOffset%pageSize) {
				// perf uses the address of _stext as start. Some tools may
				// adjust for this before calling GetBase, in which case the the page
				// alignment should be different from that of stextOffset.
				return start - *stextOffset, nil
			}

			return start - loadSegment.Vaddr, nil
		} else if start%pageSize != 0 && stextOffset != nil && *stextOffset%pageSize == start%pageSize {
			// ChromeOS remaps its kernel to 0 + start%pageSize. Nothing
			// else should come down this path. Empirical values:
			//       start=0x198 limit=0x2f9fffff offset=0
			//       VADDR=0xffffffff81000000
			// stextOffset=0xffffffff81000198
			return -(*stextOffset - start), nil
		}

		return 0, fmt.Errorf("Don't know how to handle EXEC segment: %v start=0x%x limit=0x%x offset=0x%x", *loadSegment, start, limit, offset)
	case elf.ET_REL:
		if offset != 0 {
			return 0, fmt.Errorf("Don't know how to handle mapping.Offset")
		}
		return start, nil
	case elf.ET_DYN:
		// The process mapping information, start = start of virtual address range,
		// and offset = offset in the executable file of the start address, tells us
		// that a runtime virtual address x maps to a file offset
		// fx = x - start + offset.
		if loadSegment == nil {
			return start - offset, nil
		}
		// The program header, if not nil, indicates the offset in the file where
		// the executable segment is located (loadSegment.Off), and the base virtual
		// address where the first byte of the segment is loaded
		// (loadSegment.Vaddr). A file offset fx maps to a virtual (symbol) address
		// sx = fx - loadSegment.Off + loadSegment.Vaddr.
		//
		// Thus, a runtime virtual address x maps to a symbol address
		// sx = x - start + offset - loadSegment.Off + loadSegment.Vaddr.
		return start - offset + loadSegment.Off - loadSegment.Vaddr, nil
	}
	return 0, fmt.Errorf("Don't know how to handle FileHeader.Type %v", fh.Type)
}