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 <[email protected]>
Run-TryBot: Austin Clements <[email protected]>

Author: aclements

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.
  */

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),
@@ -360,16 +389,43 @@ 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),
 				Addralign: uint64(sh.Addralign),
 				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
 	}