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 // -*- mode: C++ -*-

 // Copyright (c) 2010 Google Inc. All Rights Reserved.

 //

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 #ifndef COMMON_DWARF_BYTEREADER_H__

 #define COMMON_DWARF_BYTEREADER_H__

 #include <string>

 #include "common/dwarf/types.h"

 #include "common/dwarf/dwarf2enums.h"

 namespace dwarf2reader {

 // We can't use the obvious name of LITTLE_ENDIAN and BIG_ENDIAN

 // because it conflicts with a macro

 enum Endianness {

   ENDIANNESS_BIG,

   ENDIANNESS_LITTLE

 };

 // A ByteReader knows how to read single- and multi-byte values of

 // various endiannesses, sizes, and encodings, as used in DWARF

 // debugging information and Linux C++ exception handling data.

 class ByteReader {

  public:

   // Construct a ByteReader capable of reading one-, two-, four-, and

   // eight-byte values according to ENDIANNESS, absolute machine-sized

   // addresses, DWARF-style "initial length" values, signed and

   // unsigned LEB128 numbers, and Linux C++ exception handling data's

   // encoded pointers.

   explicit ByteReader(enum Endianness endianness);

   virtual ~ByteReader();

   // Read a single byte from BUFFER and return it as an unsigned 8 bit

   // number.

   uint8 ReadOneByte(const char* buffer) const;

   // Read two bytes from BUFFER and return them as an unsigned 16 bit

   // number, using this ByteReader's endianness.

   uint16 ReadTwoBytes(const char* buffer) const;

   // Read four bytes from BUFFER and return them as an unsigned 32 bit

   // number, using this ByteReader's endianness. This function returns

   // a uint64 so that it is compatible with ReadAddress and

   // ReadOffset. The number it returns will never be outside the range

   // of an unsigned 32 bit integer.

   uint64 ReadFourBytes(const char* buffer) const;

   // Read eight bytes from BUFFER and return them as an unsigned 64

   // bit number, using this ByteReader's endianness.

   uint64 ReadEightBytes(const char* buffer) const;

   // Read an unsigned LEB128 (Little Endian Base 128) number from

   // BUFFER and return it as an unsigned 64 bit integer. Set LEN to

   // the number of bytes read.

   //

   // The unsigned LEB128 representation of an integer N is a variable

   // number of bytes:

   //

   // - If N is between 0 and 0x7f, then its unsigned LEB128

   //   representation is a single byte whose value is N.

   //

   // - Otherwise, its unsigned LEB128 representation is (N & 0x7f) |

   //   0x80, followed by the unsigned LEB128 representation of N /

   //   128, rounded towards negative infinity.

   //

   // In other words, we break VALUE into groups of seven bits, put

   // them in little-endian order, and then write them as eight-bit

   // bytes with the high bit on all but the last.

   uint64 ReadUnsignedLEB128(const char* buffer, size_t* len) const;

   // Read a signed LEB128 number from BUFFER and return it as an

   // signed 64 bit integer. Set LEN to the number of bytes read.

   //

   // The signed LEB128 representation of an integer N is a variable

   // number of bytes:

   //

   // - If N is between -0x40 and 0x3f, then its signed LEB128

   //   representation is a single byte whose value is N in two's

   //   complement.

   //

   // - Otherwise, its signed LEB128 representation is (N & 0x7f) |

   //   0x80, followed by the signed LEB128 representation of N / 128,

   //   rounded towards negative infinity.

   //

   // In other words, we break VALUE into groups of seven bits, put

   // them in little-endian order, and then write them as eight-bit

   // bytes with the high bit on all but the last.

   int64 ReadSignedLEB128(const char* buffer, size_t* len) const;

   // Indicate that addresses on this architecture are SIZE bytes long. SIZE

   // must be either 4 or 8. (DWARF allows addresses to be any number of

   // bytes in length from 1 to 255, but we only support 32- and 64-bit

   // addresses at the moment.) You must call this before using the

   // ReadAddress member function.

   //

   // For data in a .debug_info section, or something that .debug_info

   // refers to like line number or macro data, the compilation unit

   // header's address_size field indicates the address size to use. Call

   // frame information doesn't indicate its address size (a shortcoming of

   // the spec); you must supply the appropriate size based on the

   // architecture of the target machine.

   void SetAddressSize(uint8 size);

   // Return the current address size, in bytes. This is either 4,

   // indicating 32-bit addresses, or 8, indicating 64-bit addresses.

   uint8 AddressSize() const { return address_size_; }

   // Read an address from BUFFER and return it as an unsigned 64 bit

   // integer, respecting this ByteReader's endianness and address size. You

   // must call SetAddressSize before calling this function.

   uint64 ReadAddress(const char* buffer) const;

   // DWARF actually defines two slightly different formats: 32-bit DWARF

   // and 64-bit DWARF. This is *not* related to the size of registers or

   // addresses on the target machine; it refers only to the size of section

   // offsets and data lengths appearing in the DWARF data. One only needs

   // 64-bit DWARF when the debugging data itself is larger than 4GiB.

   // 32-bit DWARF can handle x86_64 or PPC64 code just fine, unless the

   // debugging data itself is very large.

   //

   // DWARF information identifies itself as 32-bit or 64-bit DWARF: each

   // compilation unit and call frame information entry begins with an

   // "initial length" field, which, in addition to giving the length of the

   // data, also indicates the size of section offsets and lengths appearing

   // in that data. The ReadInitialLength member function, below, reads an

   // initial length and sets the ByteReader's offset size as a side effect.

   // Thus, in the normal process of reading DWARF data, the appropriate

   // offset size is set automatically. So, you should only need to call

   // SetOffsetSize if you are using the same ByteReader to jump from the

   // midst of one block of DWARF data into another.

   // Read a DWARF "initial length" field from START, and return it as

   // an unsigned 64 bit integer, respecting this ByteReader's

   // endianness. Set *LEN to the length of the initial length in

   // bytes, either four or twelve. As a side effect, set this

   // ByteReader's offset size to either 4 (if we see a 32-bit DWARF

   // initial length) or 8 (if we see a 64-bit DWARF initial length).

   //

   // A DWARF initial length is either:

   //

   // - a byte count stored as an unsigned 32-bit value less than

   //   0xffffff00, indicating that the data whose length is being

   //   measured uses the 32-bit DWARF format, or

   //

   // - The 32-bit value 0xffffffff, followed by a 64-bit byte count,

   //   indicating that the data whose length is being measured uses

   //   the 64-bit DWARF format.

   uint64 ReadInitialLength(const char* start, size_t* len);

   // Read an offset from BUFFER and return it as an unsigned 64 bit

   // integer, respecting the ByteReader's endianness. In 32-bit DWARF, the

   // offset is 4 bytes long; in 64-bit DWARF, the offset is eight bytes

   // long. You must call ReadInitialLength or SetOffsetSize before calling

   // this function; see the comments above for details.

   uint64 ReadOffset(const char* buffer) const;

   // Return the current offset size, in bytes.

   // A return value of 4 indicates that we are reading 32-bit DWARF.

   // A return value of 8 indicates that we are reading 64-bit DWARF.

   uint8 OffsetSize() const { return offset_size_; }

   // Indicate that section offsets and lengths are SIZE bytes long. SIZE

   // must be either 4 (meaning 32-bit DWARF) or 8 (meaning 64-bit DWARF).

   // Usually, you should not call this function yourself; instead, let a

   // call to ReadInitialLength establish the data's offset size

   // automatically.

   void SetOffsetSize(uint8 size);

   // The Linux C++ ABI uses a variant of DWARF call frame information

   // for exception handling. This data is included in the program's

   // address space as the ".eh_frame" section, and intepreted at

   // runtime to walk the stack, find exception handlers, and run

   // cleanup code. The format is mostly the same as DWARF CFI, with

   // some adjustments made to provide the additional

   // exception-handling data, and to make the data easier to work with

   // in memory --- for example, to allow it to be placed in read-only

   // memory even when describing position-independent code.

   //

   // In particular, exception handling data can select a number of

   // different encodings for pointers that appear in the data, as

   // described by the DwarfPointerEncoding enum. There are actually

   // four axes(!) to the encoding:

   //

   // - The pointer size: pointers can be 2, 4, or 8 bytes long, or use

   //   the DWARF LEB128 encoding.

   //

   // - The pointer's signedness: pointers can be signed or unsigned.

   //

   // - The pointer's base address: the data stored in the exception

   //   handling data can be the actual address (that is, an absolute

   //   pointer), or relative to one of a number of different base

   //   addreses --- including that of the encoded pointer itself, for

   //   a form of "pc-relative" addressing.

   //

   // - The pointer may be indirect: it may be the address where the

   //   true pointer is stored. (This is used to refer to things via

   //   global offset table entries, program linkage table entries, or

   //   other tricks used in position-independent code.)

   //

   // There are also two options that fall outside that matrix

   // altogether: the pointer may be omitted, or it may have padding to

   // align it on an appropriate address boundary. (That last option

   // may seem like it should be just another axis, but it is not.)

   // Indicate that the exception handling data is loaded starting at

   // SECTION_BASE, and that the start of its buffer in our own memory

   // is BUFFER_BASE. This allows us to find the address that a given

   // byte in our buffer would have when loaded into the program the

   // data describes. We need this to resolve DW_EH_PE_pcrel pointers.

   void SetCFIDataBase(uint64 section_base, const char *buffer_base);

   // Indicate that the base address of the program's ".text" section

   // is TEXT_BASE. We need this to resolve DW_EH_PE_textrel pointers.

   void SetTextBase(uint64 text_base);

   // Indicate that the base address for DW_EH_PE_datarel pointers is

   // DATA_BASE. The proper value depends on the ABI; it is usually the

   // address of the global offset table, held in a designated register in

   // position-independent code. You will need to look at the startup code

   // for the target system to be sure. I tried; my eyes bled.

   void SetDataBase(uint64 data_base);

   // Indicate that the base address for the FDE we are processing is

   // FUNCTION_BASE. This is the start address of DW_EH_PE_funcrel

   // pointers. (This encoding does not seem to be used by the GNU

   // toolchain.)

   void SetFunctionBase(uint64 function_base);

   // Indicate that we are no longer processing any FDE, so any use of

   // a DW_EH_PE_funcrel encoding is an error.

   void ClearFunctionBase();

   // Return true if ENCODING is a valid pointer encoding.

   bool ValidEncoding(DwarfPointerEncoding encoding) const;

   // Return true if we have all the information we need to read a

   // pointer that uses ENCODING. This checks that the appropriate

   // SetFooBase function for ENCODING has been called.

   bool UsableEncoding(DwarfPointerEncoding encoding) const;

   // Read an encoded pointer from BUFFER using ENCODING; return the

   // absolute address it represents, and set *LEN to the pointer's

   // length in bytes, including any padding for aligned pointers.

   //

   // This function calls 'abort' if ENCODING is invalid or refers to a

   // base address this reader hasn't been given, so you should check

   // with ValidEncoding and UsableEncoding first if you would rather

   // die in a more helpful way.

   uint64 ReadEncodedPointer(const char *buffer, DwarfPointerEncoding encoding,

                             size_t *len) const;

  private:

   // Function pointer type for our address and offset readers.

   typedef uint64 (ByteReader::*AddressReader)(const char*) const;

   // Read an offset from BUFFER and return it as an unsigned 64 bit

   // integer.  DWARF2/3 define offsets as either 4 or 8 bytes,

   // generally depending on the amount of DWARF2/3 info present.

   // This function pointer gets set by SetOffsetSize.

   AddressReader offset_reader_;

   // Read an address from BUFFER and return it as an unsigned 64 bit

   // integer.  DWARF2/3 allow addresses to be any size from 0-255

   // bytes currently.  Internally we support 4 and 8 byte addresses,

   // and will CHECK on anything else.

   // This function pointer gets set by SetAddressSize.

   AddressReader address_reader_;

   Endianness endian_;

   uint8 address_size_;

   uint8 offset_size_;

   // Base addresses for Linux C++ exception handling data's encoded pointers.

   bool have_section_base_, have_text_base_, have_data_base_;

   bool have_function_base_;

   uint64 section_base_, text_base_, data_base_, function_base_;

   const char *buffer_base_;

 };

 }  // namespace dwarf2reader

 #endif  // COMMON_DWARF_BYTEREADER_H__