debug/elf: transparently decompress compressed sections

This adds support for compressed ELF sections. This compression is
treated as a framing issue and hence the package APIs all
transparently decompress compressed sections. This requires some
subtlety for (*Section).Open, which returns an io.ReadSeeker: since
the decompressed data comes from an io.Reader, this commit introduces
a Reader-to-ReadSeeker adapter that is efficient for common uses of
Seek and does what it can otherwise.

Fixes #11773.

Change-Id: Ic0cb7255a85cadf4c1d15fb563d5a2e89dbd3c36
Reviewed-on: https://go-review.googlesource.com/17341
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Austin Clements <austin@google.com>

6 files changed, 424 insertions, 77 deletions

diff --git a/src/debug/elf/elf.go b/src/debug/elf/elf.go
index 1c9dd7bc4a..0c79b7a02f 100644
--- a/src/debug/elf/elf.go
+++ b/src/debug/elf/elf.go
@@ -411,6 +411,7 @@ const (
 	SHF_OS_NONCONFORMING SectionFlag = 0x100      /* OS-specific processing required. */
 	SHF_GROUP            SectionFlag = 0x200      /* Member of section group. */
 	SHF_TLS              SectionFlag = 0x400      /* Section contains TLS data. */
+	SHF_COMPRESSED       SectionFlag = 0x800      /* Section is compressed. */
 	SHF_MASKOS           SectionFlag = 0x0ff00000 /* OS-specific semantics. */
 	SHF_MASKPROC         SectionFlag = 0xf0000000 /* Processor-specific semantics. */
 )
@@ -426,11 +427,34 @@ var shfStrings = []intName{
 	{0x100, "SHF_OS_NONCONFORMING"},
 	{0x200, "SHF_GROUP"},
 	{0x400, "SHF_TLS"},
+	{0x800, "SHF_COMPRESSED"},
 }
 
 func (i SectionFlag) String() string   { return flagName(uint32(i), shfStrings, false) }
 func (i SectionFlag) GoString() string { return flagName(uint32(i), shfStrings, true) }
 
+// Section compression type.
+type CompressionType int
+
+const (
+	COMPRESS_ZLIB   CompressionType = 1          /* ZLIB compression. */
+	COMPRESS_LOOS   CompressionType = 0x60000000 /* First OS-specific. */
+	COMPRESS_HIOS   CompressionType = 0x6fffffff /* Last OS-specific. */
+	COMPRESS_LOPROC CompressionType = 0x70000000 /* First processor-specific type. */
+	COMPRESS_HIPROC CompressionType = 0x7fffffff /* Last processor-specific type. */
+)
+
+var compressionStrings = []intName{
+	{0, "COMPRESS_ZLIB"},
+	{0x60000000, "COMPRESS_LOOS"},
+	{0x6fffffff, "COMPRESS_HIOS"},
+	{0x70000000, "COMPRESS_LOPROC"},
+	{0x7fffffff, "COMPRESS_HIPROC"},
+}
+
+func (i CompressionType) String() string   { return stringName(uint32(i), compressionStrings, false) }
+func (i CompressionType) GoString() string { return stringName(uint32(i), compressionStrings, true) }
+
 // Prog.Type
 type ProgType int
 
@@ -1878,6 +1902,13 @@ type Dyn32 struct {
 	Val uint32 /* Integer/Address value. */
 }
 
+// ELF32 Compression header.
+type Chdr32 struct {
+	Type      uint32
+	Size      uint32
+	Addralign uint32
+}
+
 /*
  * Relocation entries.
  */
@@ -1972,6 +2003,14 @@ type Dyn64 struct {
 	Val uint64 /* Integer/address value */
 }
 
+// ELF64 Compression header.
+type Chdr64 struct {
+	Type      uint32
+	Reserved  uint32
+	Size      uint64
+	Addralign uint64
+}
+
 /*
  * Relocation entries.
  */
diff --git a/src/debug/elf/file.go b/src/debug/elf/file.go
index a42bde94f8..199aa4ec57 100644
--- a/src/debug/elf/file.go
+++ b/src/debug/elf/file.go
@@ -58,6 +58,12 @@ type SectionHeader struct {
 	Info      uint32
 	Addralign uint64
 	Entsize   uint64
+
+	// FileSize is the size of this section in the file in bytes.
+	// If a section is compressed, FileSize is the size of the
+	// compressed data, while Size (above) is the size of the
+	// uncompressed data.
+	FileSize uint64
 }
 
 // A Section represents a single section in an ELF file.
@@ -70,17 +76,23 @@ type Section struct {
 	// If a client wants Read and Seek it must use
 	// Open() to avoid fighting over the seek offset
 	// with other clients.
+	//
+	// ReaderAt may be nil if the section is not easily available
+	// in a random-access form. For example, a compressed section
+	// may have a nil ReaderAt.
 	io.ReaderAt
 	sr *io.SectionReader
+
+	compressionType   CompressionType
+	compressionOffset int64
 }
 
 // Data reads and returns the contents of the ELF section.
+// Even if the section is stored compressed in the ELF file,
+// Data returns uncompressed data.
 func (s *Section) Data() ([]byte, error) {
-	dat := make([]byte, s.sr.Size())
-	n, err := s.sr.ReadAt(dat, 0)
-	if n == len(dat) {
-		err = nil
-	}
+	dat := make([]byte, s.Size)
+	n, err := io.ReadFull(s.Open(), dat)
 	return dat[0:n], err
 }
 
@@ -94,7 +106,24 @@ func (f *File) stringTable(link uint32) ([]byte, error) {
 }
 
 // Open returns a new ReadSeeker reading the ELF section.
-func (s *Section) Open() io.ReadSeeker { return io.NewSectionReader(s.sr, 0, 1<<63-1) }
+// Even if the section is stored compressed in the ELF file,
+// the ReadSeeker reads uncompressed data.
+func (s *Section) Open() io.ReadSeeker {
+	if s.Flags&SHF_COMPRESSED == 0 {
+		return io.NewSectionReader(s.sr, 0, 1<<63-1)
+	}
+	if s.compressionType == COMPRESS_ZLIB {
+		return &readSeekerFromReader{
+			reset: func() (io.Reader, error) {
+				fr := io.NewSectionReader(s.sr, s.compressionOffset, int64(s.FileSize)-s.compressionOffset)
+				return zlib.NewReader(fr)
+			},
+			size: int64(s.Size),
+		}
+	}
+	err := &FormatError{int64(s.Offset), "unknown compression type", s.compressionType}
+	return errorReader{err}
+}
 
 // A ProgHeader represents a single ELF program header.
 type ProgHeader struct {
@@ -344,7 +373,7 @@ func NewFile(r io.ReaderAt) (*File, error) {
 				Flags:     SectionFlag(sh.Flags),
 				Addr:      uint64(sh.Addr),
 				Offset:    uint64(sh.Off),
-				Size:      uint64(sh.Size),
+				FileSize:  uint64(sh.Size),
 				Link:      uint32(sh.Link),
 				Info:      uint32(sh.Info),
 				Addralign: uint64(sh.Addralign),
@@ -360,7 +389,7 @@ func NewFile(r io.ReaderAt) (*File, error) {
 				Type:      SectionType(sh.Type),
 				Flags:     SectionFlag(sh.Flags),
 				Offset:    uint64(sh.Off),
-				Size:      uint64(sh.Size),
+				FileSize:  uint64(sh.Size),
 				Addr:      uint64(sh.Addr),
 				Link:      uint32(sh.Link),
 				Info:      uint32(sh.Info),
@@ -368,8 +397,35 @@ func NewFile(r io.ReaderAt) (*File, error) {
 				Entsize:   uint64(sh.Entsize),
 			}
 		}
-		s.sr = io.NewSectionReader(r, int64(s.Offset), int64(s.Size))
-		s.ReaderAt = s.sr
+		s.sr = io.NewSectionReader(r, int64(s.Offset), int64(s.FileSize))
+
+		if s.Flags&SHF_COMPRESSED == 0 {
+			s.ReaderAt = s.sr
+			s.Size = s.FileSize
+		} else {
+			// Read the compression header.
+			switch f.Class {
+			case ELFCLASS32:
+				ch := new(Chdr32)
+				if err := binary.Read(s.sr, f.ByteOrder, ch); err != nil {
+					return nil, err
+				}
+				s.compressionType = CompressionType(ch.Type)
+				s.Size = uint64(ch.Size)
+				s.Addralign = uint64(ch.Addralign)
+				s.compressionOffset = int64(binary.Size(ch))
+			case ELFCLASS64:
+				ch := new(Chdr64)
+				if err := binary.Read(s.sr, f.ByteOrder, ch); err != nil {
+					return nil, err
+				}
+				s.compressionType = CompressionType(ch.Type)
+				s.Size = ch.Size
+				s.Addralign = ch.Addralign
+				s.compressionOffset = int64(binary.Size(ch))
+			}
+		}
+
 		f.Sections[i] = s
 	}
 
diff --git a/src/debug/elf/file_test.go b/src/debug/elf/file_test.go
index 6864b6df79..5c0df0f1e9 100644
--- a/src/debug/elf/file_test.go
+++ b/src/debug/elf/file_test.go
@@ -10,6 +10,7 @@ import (
 	"debug/dwarf"
 	"encoding/binary"
 	"io"
+	"math/rand"
 	"net"
 	"os"
 	"path"
@@ -31,36 +32,36 @@ var fileTests = []fileTest{
 		"testdata/gcc-386-freebsd-exec",
 		FileHeader{ELFCLASS32, ELFDATA2LSB, EV_CURRENT, ELFOSABI_FREEBSD, 0, binary.LittleEndian, ET_EXEC, EM_386, 0x80483cc},
 		[]SectionHeader{
-			{"", SHT_NULL, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
-			{".interp", SHT_PROGBITS, SHF_ALLOC, 0x80480d4, 0xd4, 0x15, 0x0, 0x0, 0x1, 0x0},
-			{".hash", SHT_HASH, SHF_ALLOC, 0x80480ec, 0xec, 0x90, 0x3, 0x0, 0x4, 0x4},
-			{".dynsym", SHT_DYNSYM, SHF_ALLOC, 0x804817c, 0x17c, 0x110, 0x4, 0x1, 0x4, 0x10},
-			{".dynstr", SHT_STRTAB, SHF_ALLOC, 0x804828c, 0x28c, 0xbb, 0x0, 0x0, 0x1, 0x0},
-			{".rel.plt", SHT_REL, SHF_ALLOC, 0x8048348, 0x348, 0x20, 0x3, 0x7, 0x4, 0x8},
-			{".init", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x8048368, 0x368, 0x11, 0x0, 0x0, 0x4, 0x0},
-			{".plt", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x804837c, 0x37c, 0x50, 0x0, 0x0, 0x4, 0x4},
-			{".text", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x80483cc, 0x3cc, 0x180, 0x0, 0x0, 0x4, 0x0},
-			{".fini", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x804854c, 0x54c, 0xc, 0x0, 0x0, 0x4, 0x0},
-			{".rodata", SHT_PROGBITS, SHF_ALLOC, 0x8048558, 0x558, 0xa3, 0x0, 0x0, 0x1, 0x0},
-			{".data", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80495fc, 0x5fc, 0xc, 0x0, 0x0, 0x4, 0x0},
-			{".eh_frame", SHT_PROGBITS, SHF_ALLOC, 0x8049608, 0x608, 0x4, 0x0, 0x0, 0x4, 0x0},
-			{".dynamic", SHT_DYNAMIC, SHF_WRITE + SHF_ALLOC, 0x804960c, 0x60c, 0x98, 0x4, 0x0, 0x4, 0x8},
-			{".ctors", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80496a4, 0x6a4, 0x8, 0x0, 0x0, 0x4, 0x0},
-			{".dtors", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80496ac, 0x6ac, 0x8, 0x0, 0x0, 0x4, 0x0},
-			{".jcr", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80496b4, 0x6b4, 0x4, 0x0, 0x0, 0x4, 0x0},
-			{".got", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80496b8, 0x6b8, 0x1c, 0x0, 0x0, 0x4, 0x4},
-			{".bss", SHT_NOBITS, SHF_WRITE + SHF_ALLOC, 0x80496d4, 0x6d4, 0x20, 0x0, 0x0, 0x4, 0x0},
-			{".comment", SHT_PROGBITS, 0x0, 0x0, 0x6d4, 0x12d, 0x0, 0x0, 0x1, 0x0},
-			{".debug_aranges", SHT_PROGBITS, 0x0, 0x0, 0x801, 0x20, 0x0, 0x0, 0x1, 0x0},
-			{".debug_pubnames", SHT_PROGBITS, 0x0, 0x0, 0x821, 0x1b, 0x0, 0x0, 0x1, 0x0},
-			{".debug_info", SHT_PROGBITS, 0x0, 0x0, 0x83c, 0x11d, 0x0, 0x0, 0x1, 0x0},
-			{".debug_abbrev", SHT_PROGBITS, 0x0, 0x0, 0x959, 0x41, 0x0, 0x0, 0x1, 0x0},
-			{".debug_line", SHT_PROGBITS, 0x0, 0x0, 0x99a, 0x35, 0x0, 0x0, 0x1, 0x0},
-			{".debug_frame", SHT_PROGBITS, 0x0, 0x0, 0x9d0, 0x30, 0x0, 0x0, 0x4, 0x0},
-			{".debug_str", SHT_PROGBITS, 0x0, 0x0, 0xa00, 0xd, 0x0, 0x0, 0x1, 0x0},
-			{".shstrtab", SHT_STRTAB, 0x0, 0x0, 0xa0d, 0xf8, 0x0, 0x0, 0x1, 0x0},
-			{".symtab", SHT_SYMTAB, 0x0, 0x0, 0xfb8, 0x4b0, 0x1d, 0x38, 0x4, 0x10},
-			{".strtab", SHT_STRTAB, 0x0, 0x0, 0x1468, 0x206, 0x0, 0x0, 0x1, 0x0},
+			{"", SHT_NULL, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
+			{".interp", SHT_PROGBITS, SHF_ALLOC, 0x80480d4, 0xd4, 0x15, 0x0, 0x0, 0x1, 0x0, 0x15},
+			{".hash", SHT_HASH, SHF_ALLOC, 0x80480ec, 0xec, 0x90, 0x3, 0x0, 0x4, 0x4, 0x90},
+			{".dynsym", SHT_DYNSYM, SHF_ALLOC, 0x804817c, 0x17c, 0x110, 0x4, 0x1, 0x4, 0x10, 0x110},
+			{".dynstr", SHT_STRTAB, SHF_ALLOC, 0x804828c, 0x28c, 0xbb, 0x0, 0x0, 0x1, 0x0, 0xbb},
+			{".rel.plt", SHT_REL, SHF_ALLOC, 0x8048348, 0x348, 0x20, 0x3, 0x7, 0x4, 0x8, 0x20},
+			{".init", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x8048368, 0x368, 0x11, 0x0, 0x0, 0x4, 0x0, 0x11},
+			{".plt", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x804837c, 0x37c, 0x50, 0x0, 0x0, 0x4, 0x4, 0x50},
+			{".text", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x80483cc, 0x3cc, 0x180, 0x0, 0x0, 0x4, 0x0, 0x180},
+			{".fini", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x804854c, 0x54c, 0xc, 0x0, 0x0, 0x4, 0x0, 0xc},
+			{".rodata", SHT_PROGBITS, SHF_ALLOC, 0x8048558, 0x558, 0xa3, 0x0, 0x0, 0x1, 0x0, 0xa3},
+			{".data", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80495fc, 0x5fc, 0xc, 0x0, 0x0, 0x4, 0x0, 0xc},
+			{".eh_frame", SHT_PROGBITS, SHF_ALLOC, 0x8049608, 0x608, 0x4, 0x0, 0x0, 0x4, 0x0, 0x4},
+			{".dynamic", SHT_DYNAMIC, SHF_WRITE + SHF_ALLOC, 0x804960c, 0x60c, 0x98, 0x4, 0x0, 0x4, 0x8, 0x98},
+			{".ctors", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80496a4, 0x6a4, 0x8, 0x0, 0x0, 0x4, 0x0, 0x8},
+			{".dtors", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80496ac, 0x6ac, 0x8, 0x0, 0x0, 0x4, 0x0, 0x8},
+			{".jcr", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80496b4, 0x6b4, 0x4, 0x0, 0x0, 0x4, 0x0, 0x4},
+			{".got", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x80496b8, 0x6b8, 0x1c, 0x0, 0x0, 0x4, 0x4, 0x1c},
+			{".bss", SHT_NOBITS, SHF_WRITE + SHF_ALLOC, 0x80496d4, 0x6d4, 0x20, 0x0, 0x0, 0x4, 0x0, 0x20},
+			{".comment", SHT_PROGBITS, 0x0, 0x0, 0x6d4, 0x12d, 0x0, 0x0, 0x1, 0x0, 0x12d},
+			{".debug_aranges", SHT_PROGBITS, 0x0, 0x0, 0x801, 0x20, 0x0, 0x0, 0x1, 0x0, 0x20},
+			{".debug_pubnames", SHT_PROGBITS, 0x0, 0x0, 0x821, 0x1b, 0x0, 0x0, 0x1, 0x0, 0x1b},
+			{".debug_info", SHT_PROGBITS, 0x0, 0x0, 0x83c, 0x11d, 0x0, 0x0, 0x1, 0x0, 0x11d},
+			{".debug_abbrev", SHT_PROGBITS, 0x0, 0x0, 0x959, 0x41, 0x0, 0x0, 0x1, 0x0, 0x41},
+			{".debug_line", SHT_PROGBITS, 0x0, 0x0, 0x99a, 0x35, 0x0, 0x0, 0x1, 0x0, 0x35},
+			{".debug_frame", SHT_PROGBITS, 0x0, 0x0, 0x9d0, 0x30, 0x0, 0x0, 0x4, 0x0, 0x30},
+			{".debug_str", SHT_PROGBITS, 0x0, 0x0, 0xa00, 0xd, 0x0, 0x0, 0x1, 0x0, 0xd},
+			{".shstrtab", SHT_STRTAB, 0x0, 0x0, 0xa0d, 0xf8, 0x0, 0x0, 0x1, 0x0, 0xf8},
+			{".symtab", SHT_SYMTAB, 0x0, 0x0, 0xfb8, 0x4b0, 0x1d, 0x38, 0x4, 0x10, 0x4b0},
+			{".strtab", SHT_STRTAB, 0x0, 0x0, 0x1468, 0x206, 0x0, 0x0, 0x1, 0x0, 0x206},
 		},
 		[]ProgHeader{
 			{PT_PHDR, PF_R + PF_X, 0x34, 0x8048034, 0x8048034, 0xa0, 0xa0, 0x4},
@@ -75,43 +76,43 @@ var fileTests = []fileTest{
 		"testdata/gcc-amd64-linux-exec",
 		FileHeader{ELFCLASS64, ELFDATA2LSB, EV_CURRENT, ELFOSABI_NONE, 0, binary.LittleEndian, ET_EXEC, EM_X86_64, 0x4003e0},
 		[]SectionHeader{
-			{"", SHT_NULL, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
-			{".interp", SHT_PROGBITS, SHF_ALLOC, 0x400200, 0x200, 0x1c, 0x0, 0x0, 0x1, 0x0},
-			{".note.ABI-tag", SHT_NOTE, SHF_ALLOC, 0x40021c, 0x21c, 0x20, 0x0, 0x0, 0x4, 0x0},
-			{".hash", SHT_HASH, SHF_ALLOC, 0x400240, 0x240, 0x24, 0x5, 0x0, 0x8, 0x4},
-			{".gnu.hash", SHT_LOOS + 268435446, SHF_ALLOC, 0x400268, 0x268, 0x1c, 0x5, 0x0, 0x8, 0x0},
-			{".dynsym", SHT_DYNSYM, SHF_ALLOC, 0x400288, 0x288, 0x60, 0x6, 0x1, 0x8, 0x18},
-			{".dynstr", SHT_STRTAB, SHF_ALLOC, 0x4002e8, 0x2e8, 0x3d, 0x0, 0x0, 0x1, 0x0},
-			{".gnu.version", SHT_HIOS, SHF_ALLOC, 0x400326, 0x326, 0x8, 0x5, 0x0, 0x2, 0x2},
-			{".gnu.version_r", SHT_LOOS + 268435454, SHF_ALLOC, 0x400330, 0x330, 0x20, 0x6, 0x1, 0x8, 0x0},
-			{".rela.dyn", SHT_RELA, SHF_ALLOC, 0x400350, 0x350, 0x18, 0x5, 0x0, 0x8, 0x18},
-			{".rela.plt", SHT_RELA, SHF_ALLOC, 0x400368, 0x368, 0x30, 0x5, 0xc, 0x8, 0x18},
-			{".init", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x400398, 0x398, 0x18, 0x0, 0x0, 0x4, 0x0},
-			{".plt", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x4003b0, 0x3b0, 0x30, 0x0, 0x0, 0x4, 0x10},
-			{".text", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x4003e0, 0x3e0, 0x1b4, 0x0, 0x0, 0x10, 0x0},
-			{".fini", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x400594, 0x594, 0xe, 0x0, 0x0, 0x4, 0x0},
-			{".rodata", SHT_PROGBITS, SHF_ALLOC, 0x4005a4, 0x5a4, 0x11, 0x0, 0x0, 0x4, 0x0},
-			{".eh_frame_hdr", SHT_PROGBITS, SHF_ALLOC, 0x4005b8, 0x5b8, 0x24, 0x0, 0x0, 0x4, 0x0},
-			{".eh_frame", SHT_PROGBITS, SHF_ALLOC, 0x4005e0, 0x5e0, 0xa4, 0x0, 0x0, 0x8, 0x0},
-			{".ctors", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600688, 0x688, 0x10, 0x0, 0x0, 0x8, 0x0},
-			{".dtors", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600698, 0x698, 0x10, 0x0, 0x0, 0x8, 0x0},
-			{".jcr", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x6006a8, 0x6a8, 0x8, 0x0, 0x0, 0x8, 0x0},
-			{".dynamic", SHT_DYNAMIC, SHF_WRITE + SHF_ALLOC, 0x6006b0, 0x6b0, 0x1a0, 0x6, 0x0, 0x8, 0x10},
-			{".got", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600850, 0x850, 0x8, 0x0, 0x0, 0x8, 0x8},
-			{".got.plt", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600858, 0x858, 0x28, 0x0, 0x0, 0x8, 0x8},
-			{".data", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600880, 0x880, 0x18, 0x0, 0x0, 0x8, 0x0},
-			{".bss", SHT_NOBITS, SHF_WRITE + SHF_ALLOC, 0x600898, 0x898, 0x8, 0x0, 0x0, 0x4, 0x0},
-			{".comment", SHT_PROGBITS, 0x0, 0x0, 0x898, 0x126, 0x0, 0x0, 0x1, 0x0},
-			{".debug_aranges", SHT_PROGBITS, 0x0, 0x0, 0x9c0, 0x90, 0x0, 0x0, 0x10, 0x0},
-			{".debug_pubnames", SHT_PROGBITS, 0x0, 0x0, 0xa50, 0x25, 0x0, 0x0, 0x1, 0x0},
-			{".debug_info", SHT_PROGBITS, 0x0, 0x0, 0xa75, 0x1a7, 0x0, 0x0, 0x1, 0x0},
-			{".debug_abbrev", SHT_PROGBITS, 0x0, 0x0, 0xc1c, 0x6f, 0x0, 0x0, 0x1, 0x0},
-			{".debug_line", SHT_PROGBITS, 0x0, 0x0, 0xc8b, 0x13f, 0x0, 0x0, 0x1, 0x0},
-			{".debug_str", SHT_PROGBITS, SHF_MERGE + SHF_STRINGS, 0x0, 0xdca, 0xb1, 0x0, 0x0, 0x1, 0x1},
-			{".debug_ranges", SHT_PROGBITS, 0x0, 0x0, 0xe80, 0x90, 0x0, 0x0, 0x10, 0x0},
-			{".shstrtab", SHT_STRTAB, 0x0, 0x0, 0xf10, 0x149, 0x0, 0x0, 0x1, 0x0},
-			{".symtab", SHT_SYMTAB, 0x0, 0x0, 0x19a0, 0x6f0, 0x24, 0x39, 0x8, 0x18},
-			{".strtab", SHT_STRTAB, 0x0, 0x0, 0x2090, 0x1fc, 0x0, 0x0, 0x1, 0x0},
+			{"", SHT_NULL, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
+			{".interp", SHT_PROGBITS, SHF_ALLOC, 0x400200, 0x200, 0x1c, 0x0, 0x0, 0x1, 0x0, 0x1c},
+			{".note.ABI-tag", SHT_NOTE, SHF_ALLOC, 0x40021c, 0x21c, 0x20, 0x0, 0x0, 0x4, 0x0, 0x20},
+			{".hash", SHT_HASH, SHF_ALLOC, 0x400240, 0x240, 0x24, 0x5, 0x0, 0x8, 0x4, 0x24},
+			{".gnu.hash", SHT_LOOS + 268435446, SHF_ALLOC, 0x400268, 0x268, 0x1c, 0x5, 0x0, 0x8, 0x0, 0x1c},
+			{".dynsym", SHT_DYNSYM, SHF_ALLOC, 0x400288, 0x288, 0x60, 0x6, 0x1, 0x8, 0x18, 0x60},
+			{".dynstr", SHT_STRTAB, SHF_ALLOC, 0x4002e8, 0x2e8, 0x3d, 0x0, 0x0, 0x1, 0x0, 0x3d},
+			{".gnu.version", SHT_HIOS, SHF_ALLOC, 0x400326, 0x326, 0x8, 0x5, 0x0, 0x2, 0x2, 0x8},
+			{".gnu.version_r", SHT_LOOS + 268435454, SHF_ALLOC, 0x400330, 0x330, 0x20, 0x6, 0x1, 0x8, 0x0, 0x20},
+			{".rela.dyn", SHT_RELA, SHF_ALLOC, 0x400350, 0x350, 0x18, 0x5, 0x0, 0x8, 0x18, 0x18},
+			{".rela.plt", SHT_RELA, SHF_ALLOC, 0x400368, 0x368, 0x30, 0x5, 0xc, 0x8, 0x18, 0x30},
+			{".init", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x400398, 0x398, 0x18, 0x0, 0x0, 0x4, 0x0, 0x18},
+			{".plt", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x4003b0, 0x3b0, 0x30, 0x0, 0x0, 0x4, 0x10, 0x30},
+			{".text", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x4003e0, 0x3e0, 0x1b4, 0x0, 0x0, 0x10, 0x0, 0x1b4},
+			{".fini", SHT_PROGBITS, SHF_ALLOC + SHF_EXECINSTR, 0x400594, 0x594, 0xe, 0x0, 0x0, 0x4, 0x0, 0xe},
+			{".rodata", SHT_PROGBITS, SHF_ALLOC, 0x4005a4, 0x5a4, 0x11, 0x0, 0x0, 0x4, 0x0, 0x11},
+			{".eh_frame_hdr", SHT_PROGBITS, SHF_ALLOC, 0x4005b8, 0x5b8, 0x24, 0x0, 0x0, 0x4, 0x0, 0x24},
+			{".eh_frame", SHT_PROGBITS, SHF_ALLOC, 0x4005e0, 0x5e0, 0xa4, 0x0, 0x0, 0x8, 0x0, 0xa4},
+			{".ctors", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600688, 0x688, 0x10, 0x0, 0x0, 0x8, 0x0, 0x10},
+			{".dtors", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600698, 0x698, 0x10, 0x0, 0x0, 0x8, 0x0, 0x10},
+			{".jcr", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x6006a8, 0x6a8, 0x8, 0x0, 0x0, 0x8, 0x0, 0x8},
+			{".dynamic", SHT_DYNAMIC, SHF_WRITE + SHF_ALLOC, 0x6006b0, 0x6b0, 0x1a0, 0x6, 0x0, 0x8, 0x10, 0x1a0},
+			{".got", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600850, 0x850, 0x8, 0x0, 0x0, 0x8, 0x8, 0x8},
+			{".got.plt", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600858, 0x858, 0x28, 0x0, 0x0, 0x8, 0x8, 0x28},
+			{".data", SHT_PROGBITS, SHF_WRITE + SHF_ALLOC, 0x600880, 0x880, 0x18, 0x0, 0x0, 0x8, 0x0, 0x18},
+			{".bss", SHT_NOBITS, SHF_WRITE + SHF_ALLOC, 0x600898, 0x898, 0x8, 0x0, 0x0, 0x4, 0x0, 0x8},
+			{".comment", SHT_PROGBITS, 0x0, 0x0, 0x898, 0x126, 0x0, 0x0, 0x1, 0x0, 0x126},
+			{".debug_aranges", SHT_PROGBITS, 0x0, 0x0, 0x9c0, 0x90, 0x0, 0x0, 0x10, 0x0, 0x90},
+			{".debug_pubnames", SHT_PROGBITS, 0x0, 0x0, 0xa50, 0x25, 0x0, 0x0, 0x1, 0x0, 0x25},
+			{".debug_info", SHT_PROGBITS, 0x0, 0x0, 0xa75, 0x1a7, 0x0, 0x0, 0x1, 0x0, 0x1a7},
+			{".debug_abbrev", SHT_PROGBITS, 0x0, 0x0, 0xc1c, 0x6f, 0x0, 0x0, 0x1, 0x0, 0x6f},
+			{".debug_line", SHT_PROGBITS, 0x0, 0x0, 0xc8b, 0x13f, 0x0, 0x0, 0x1, 0x0, 0x13f},
+			{".debug_str", SHT_PROGBITS, SHF_MERGE + SHF_STRINGS, 0x0, 0xdca, 0xb1, 0x0, 0x0, 0x1, 0x1, 0xb1},
+			{".debug_ranges", SHT_PROGBITS, 0x0, 0x0, 0xe80, 0x90, 0x0, 0x0, 0x10, 0x0, 0x90},
+			{".shstrtab", SHT_STRTAB, 0x0, 0x0, 0xf10, 0x149, 0x0, 0x0, 0x1, 0x0, 0x149},
+			{".symtab", SHT_SYMTAB, 0x0, 0x0, 0x19a0, 0x6f0, 0x24, 0x39, 0x8, 0x18, 0x6f0},
+			{".strtab", SHT_STRTAB, 0x0, 0x0, 0x2090, 0x1fc, 0x0, 0x0, 0x1, 0x0, 0x1fc},
 		},
 		[]ProgHeader{
 			{PT_PHDR, PF_R + PF_X, 0x40, 0x400040, 0x400040, 0x1c0, 0x1c0, 0x8},
@@ -150,6 +151,64 @@ var fileTests = []fileTest{
 		},
 		nil,
 	},
+	{
+		"testdata/compressed-32.obj",
+		FileHeader{ELFCLASS32, ELFDATA2LSB, EV_CURRENT, ELFOSABI_NONE, 0x0, binary.LittleEndian, ET_REL, EM_386, 0x0},
+		[]SectionHeader{
+			{"", SHT_NULL, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
+			{".text", SHT_PROGBITS, SHF_ALLOC | SHF_EXECINSTR, 0x0, 0x34, 0x17, 0x0, 0x0, 0x1, 0x0, 0x17},
+			{".rel.text", SHT_REL, SHF_INFO_LINK, 0x0, 0x3dc, 0x10, 0x13, 0x1, 0x4, 0x8, 0x10},
+			{".data", SHT_PROGBITS, SHF_WRITE | SHF_ALLOC, 0x0, 0x4b, 0x0, 0x0, 0x0, 0x1, 0x0, 0x0},
+			{".bss", SHT_NOBITS, SHF_WRITE | SHF_ALLOC, 0x0, 0x4b, 0x0, 0x0, 0x0, 0x1, 0x0, 0x0},
+			{".rodata", SHT_PROGBITS, SHF_ALLOC, 0x0, 0x4b, 0xd, 0x0, 0x0, 0x1, 0x0, 0xd},
+			{".debug_info", SHT_PROGBITS, SHF_COMPRESSED, 0x0, 0x58, 0xb4, 0x0, 0x0, 0x1, 0x0, 0x84},
+			{".rel.debug_info", SHT_REL, SHF_INFO_LINK, 0x0, 0x3ec, 0xa0, 0x13, 0x6, 0x4, 0x8, 0xa0},
+			{".debug_abbrev", SHT_PROGBITS, 0x0, 0x0, 0xdc, 0x5a, 0x0, 0x0, 0x1, 0x0, 0x5a},
+			{".debug_aranges", SHT_PROGBITS, 0x0, 0x0, 0x136, 0x20, 0x0, 0x0, 0x1, 0x0, 0x20},
+			{".rel.debug_aranges", SHT_REL, SHF_INFO_LINK, 0x0, 0x48c, 0x10, 0x13, 0x9, 0x4, 0x8, 0x10},
+			{".debug_line", SHT_PROGBITS, 0x0, 0x0, 0x156, 0x5c, 0x0, 0x0, 0x1, 0x0, 0x5c},
+			{".rel.debug_line", SHT_REL, SHF_INFO_LINK, 0x0, 0x49c, 0x8, 0x13, 0xb, 0x4, 0x8, 0x8},
+			{".debug_str", SHT_PROGBITS, SHF_MERGE | SHF_STRINGS | SHF_COMPRESSED, 0x0, 0x1b2, 0x10f, 0x0, 0x0, 0x1, 0x1, 0xb3},
+			{".comment", SHT_PROGBITS, SHF_MERGE | SHF_STRINGS, 0x0, 0x265, 0x2a, 0x0, 0x0, 0x1, 0x1, 0x2a},
+			{".note.GNU-stack", SHT_PROGBITS, 0x0, 0x0, 0x28f, 0x0, 0x0, 0x0, 0x1, 0x0, 0x0},
+			{".eh_frame", SHT_PROGBITS, SHF_ALLOC, 0x0, 0x290, 0x38, 0x0, 0x0, 0x4, 0x0, 0x38},
+			{".rel.eh_frame", SHT_REL, SHF_INFO_LINK, 0x0, 0x4a4, 0x8, 0x13, 0x10, 0x4, 0x8, 0x8},
+			{".shstrtab", SHT_STRTAB, 0x0, 0x0, 0x4ac, 0xab, 0x0, 0x0, 0x1, 0x0, 0xab},
+			{".symtab", SHT_SYMTAB, 0x0, 0x0, 0x2c8, 0x100, 0x14, 0xe, 0x4, 0x10, 0x100},
+			{".strtab", SHT_STRTAB, 0x0, 0x0, 0x3c8, 0x13, 0x0, 0x0, 0x1, 0x0, 0x13},
+		},
+		[]ProgHeader{},
+		nil,
+	},
+	{
+		"testdata/compressed-64.obj",
+		FileHeader{ELFCLASS64, ELFDATA2LSB, EV_CURRENT, ELFOSABI_NONE, 0x0, binary.LittleEndian, ET_REL, EM_X86_64, 0x0},
+		[]SectionHeader{
+			{"", SHT_NULL, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
+			{".text", SHT_PROGBITS, SHF_ALLOC | SHF_EXECINSTR, 0x0, 0x40, 0x1b, 0x0, 0x0, 0x1, 0x0, 0x1b},
+			{".rela.text", SHT_RELA, SHF_INFO_LINK, 0x0, 0x488, 0x30, 0x13, 0x1, 0x8, 0x18, 0x30},
+			{".data", SHT_PROGBITS, SHF_WRITE | SHF_ALLOC, 0x0, 0x5b, 0x0, 0x0, 0x0, 0x1, 0x0, 0x0},
+			{".bss", SHT_NOBITS, SHF_WRITE | SHF_ALLOC, 0x0, 0x5b, 0x0, 0x0, 0x0, 0x1, 0x0, 0x0},
+			{".rodata", SHT_PROGBITS, SHF_ALLOC, 0x0, 0x5b, 0xd, 0x0, 0x0, 0x1, 0x0, 0xd},
+			{".debug_info", SHT_PROGBITS, SHF_COMPRESSED, 0x0, 0x68, 0xba, 0x0, 0x0, 0x1, 0x0, 0x72},
+			{".rela.debug_info", SHT_RELA, SHF_INFO_LINK, 0x0, 0x4b8, 0x1c8, 0x13, 0x6, 0x8, 0x18, 0x1c8},
+			{".debug_abbrev", SHT_PROGBITS, 0x0, 0x0, 0xda, 0x5c, 0x0, 0x0, 0x1, 0x0, 0x5c},
+			{".debug_aranges", SHT_PROGBITS, SHF_COMPRESSED, 0x0, 0x136, 0x30, 0x0, 0x0, 0x1, 0x0, 0x2f},
+			{".rela.debug_aranges", SHT_RELA, SHF_INFO_LINK, 0x0, 0x680, 0x30, 0x13, 0x9, 0x8, 0x18, 0x30},
+			{".debug_line", SHT_PROGBITS, 0x0, 0x0, 0x165, 0x60, 0x0, 0x0, 0x1, 0x0, 0x60},
+			{".rela.debug_line", SHT_RELA, SHF_INFO_LINK, 0x0, 0x6b0, 0x18, 0x13, 0xb, 0x8, 0x18, 0x18},
+			{".debug_str", SHT_PROGBITS, SHF_MERGE | SHF_STRINGS | SHF_COMPRESSED, 0x0, 0x1c5, 0x104, 0x0, 0x0, 0x1, 0x1, 0xc3},
+			{".comment", SHT_PROGBITS, SHF_MERGE | SHF_STRINGS, 0x0, 0x288, 0x2a, 0x0, 0x0, 0x1, 0x1, 0x2a},
+			{".note.GNU-stack", SHT_PROGBITS, 0x0, 0x0, 0x2b2, 0x0, 0x0, 0x0, 0x1, 0x0, 0x0},
+			{".eh_frame", SHT_PROGBITS, SHF_ALLOC, 0x0, 0x2b8, 0x38, 0x0, 0x0, 0x8, 0x0, 0x38},
+			{".rela.eh_frame", SHT_RELA, SHF_INFO_LINK, 0x0, 0x6c8, 0x18, 0x13, 0x10, 0x8, 0x18, 0x18},
+			{".shstrtab", SHT_STRTAB, 0x0, 0x0, 0x6e0, 0xb0, 0x0, 0x0, 0x1, 0x0, 0xb0},
+			{".symtab", SHT_SYMTAB, 0x0, 0x0, 0x2f0, 0x180, 0x14, 0xe, 0x8, 0x18, 0x180},
+			{".strtab", SHT_STRTAB, 0x0, 0x0, 0x470, 0x13, 0x0, 0x0, 0x1, 0x0, 0x13},
+		},
+		[]ProgHeader{},
+		nil,
+	},
 }
 
 func TestOpen(t *testing.T) {
@@ -542,6 +601,91 @@ func TestCompressedDWARF(t *testing.T) {
 	}
 }
 
+func TestCompressedSection(t *testing.T) {
+	// Test files built with gcc -g -S hello.c and assembled with
+	// --compress-debug-sections=zlib-gabi.
+	f, err := Open("testdata/compressed-64.obj")
+	if err != nil {
+		t.Fatal(err)
+	}
+	sec := f.Section(".debug_info")
+	wantData := []byte{
+		182, 0, 0, 0, 4, 0, 0, 0, 0, 0, 8, 1, 0, 0, 0, 0,
+		1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 27, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 8, 7,
+		0, 0, 0, 0, 2, 1, 8, 0, 0, 0, 0, 2, 2, 7, 0, 0,
+		0, 0, 2, 4, 7, 0, 0, 0, 0, 2, 1, 6, 0, 0, 0, 0,
+		2, 2, 5, 0, 0, 0, 0, 3, 4, 5, 105, 110, 116, 0, 2, 8,
+		5, 0, 0, 0, 0, 2, 8, 7, 0, 0, 0, 0, 4, 8, 114, 0,
+		0, 0, 2, 1, 6, 0, 0, 0, 0, 5, 0, 0, 0, 0, 1, 4,
+		0, 0, 0, 0, 0, 0, 0, 0, 27, 0, 0, 0, 0, 0, 0, 0,
+		1, 156, 179, 0, 0, 0, 6, 0, 0, 0, 0, 1, 4, 87, 0, 0,
+		0, 2, 145, 108, 6, 0, 0, 0, 0, 1, 4, 179, 0, 0, 0, 2,
+		145, 96, 0, 4, 8, 108, 0, 0, 0, 0,
+	}
+
+	// Test Data method.
+	b, err := sec.Data()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(wantData, b) {
+		t.Fatalf("want data %x, got %x", wantData, b)
+	}
+
+	// Test Open method and seeking.
+	buf, have, count := make([]byte, len(b)), make([]bool, len(b)), 0
+	sf := sec.Open()
+	if got, err := sf.Seek(0, 2); got != int64(len(b)) || err != nil {
+		t.Fatalf("want seek end %d, got %d error %v", len(b), got, err)
+	}
+	if n, err := sf.Read(buf); n != 0 || err != io.EOF {
+		t.Fatalf("want EOF with 0 bytes, got %v with %d bytes", err, n)
+	}
+	pos := int64(len(buf))
+	for count < len(buf) {
+		// Construct random seek arguments.
+		whence := rand.Intn(3)
+		target := rand.Int63n(int64(len(buf)))
+		var offset int64
+		switch whence {
+		case 0:
+			offset = target
+		case 1:
+			offset = target - pos
+		case 2:
+			offset = target - int64(len(buf))
+		}
+		pos, err = sf.Seek(offset, whence)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if pos != target {
+			t.Fatalf("want position %d, got %d", target, pos)
+		}
+
+		// Read data from the new position.
+		end := pos + 16
+		if end > int64(len(buf)) {
+			end = int64(len(buf))
+		}
+		n, err := sf.Read(buf[pos:end])
+		if err != nil {
+			t.Fatal(err)
+		}
+		for i := 0; i < n; i++ {
+			if !have[pos] {
+				have[pos] = true
+				count++
+			}
+			pos++
+		}
+	}
+	if !bytes.Equal(wantData, buf) {
+		t.Fatalf("want data %x, got %x", wantData, buf)
+	}
+}
+
 func TestNoSectionOverlaps(t *testing.T) {
 	// Ensure 6l outputs sections without overlaps.
 	if runtime.GOOS != "linux" && runtime.GOOS != "freebsd" {
diff --git a/src/debug/elf/reader.go b/src/debug/elf/reader.go
new file mode 100644
index 0000000000..17b57165bc
--- /dev/null
+++ b/src/debug/elf/reader.go
@@ -0,0 +1,108 @@
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package elf
+
+import (
+	"io"
+	"os"
+)
+
+// errorReader returns error from all operations.
+type errorReader struct {
+	error
+}
+
+func (r errorReader) Read(p []byte) (n int, err error) {
+	return 0, r.error
+}
+
+func (r errorReader) ReadAt(p []byte, off int64) (n int, err error) {
+	return 0, r.error
+}
+
+func (r errorReader) Seek(offset int64, whence int) (int64, error) {
+	return 0, r.error
+}
+
+func (r errorReader) Close() error {
+	return r.error
+}
+
+// readSeekerFromReader converts an io.Reader into an io.ReadSeeker.
+// In general Seek may not be efficient, but it is optimized for
+// common cases such as seeking to the end to find the length of the
+// data.
+type readSeekerFromReader struct {
+	reset  func() (io.Reader, error)
+	r      io.Reader
+	size   int64
+	offset int64
+}
+
+func (r *readSeekerFromReader) start() {
+	x, err := r.reset()
+	if err != nil {
+		r.r = errorReader{err}
+	} else {
+		r.r = x
+	}
+	r.offset = 0
+}
+
+func (r *readSeekerFromReader) Read(p []byte) (n int, err error) {
+	if r.r == nil {
+		r.start()
+	}
+	n, err = r.r.Read(p)
+	r.offset += int64(n)
+	return n, err
+}
+
+func (r *readSeekerFromReader) Seek(offset int64, whence int) (int64, error) {
+	var newOffset int64
+	switch whence {
+	case 0:
+		newOffset = offset
+	case 1:
+		newOffset = r.offset + offset
+	case 2:
+		newOffset = r.size + offset
+	default:
+		return 0, os.ErrInvalid
+	}
+
+	switch {
+	case newOffset == r.offset:
+		return newOffset, nil
+
+	case newOffset < 0, newOffset > r.size:
+		return 0, os.ErrInvalid
+
+	case newOffset == 0:
+		r.r = nil
+
+	case newOffset == r.size:
+		r.r = errorReader{io.EOF}
+
+	default:
+		if newOffset < r.offset {
+			// Restart at the beginning.
+			r.start()
+		}
+		// Read until we reach offset.
+		var buf [512]byte
+		for r.offset < newOffset {
+			b := buf[:]
+			if newOffset-r.offset < int64(len(buf)) {
+				b = buf[:newOffset-r.offset]
+			}
+			if _, err := r.Read(b); err != nil {
+				return 0, err
+			}
+		}
+	}
+	r.offset = newOffset
+	return r.offset, nil
+}
diff --git a/src/debug/elf/testdata/compressed-32.obj b/src/debug/elf/testdata/compressed-32.obj
new file mode 100644
index 0000000000..2bfdb44240
--- /dev/null
+++ b/src/debug/elf/testdata/compressed-32.obj
diff --git a/src/debug/elf/testdata/compressed-64.obj b/src/debug/elf/testdata/compressed-64.obj
new file mode 100644
index 0000000000..ffae56a931
--- /dev/null
+++ b/src/debug/elf/testdata/compressed-64.obj