3.3.2.1 File layout

The Coff backend is split into generic routines that are applicable to any Coff target and routines that are specific to a particular target. The target-specific routines are further split into ones which are basically the same for all Coff targets except that they use the external symbol format or use different values for certain constants.

The generic routines are in coffgen.c. These routines work for any Coff target. They use some hooks into the target specific code; the hooks are in a bfd_coff_backend_data structure, one of which exists for each target.

The essentially similar target-specific routines are in coffcode.h. This header file includes executable C code. The various Coff targets first include the appropriate Coff header file, make any special defines that are needed, and then include coffcode.h.

Some of the Coff targets then also have additional routines in the target source file itself.

3.3.2.2 Coff long section names

In the standard Coff object format, section names are limited to the eight bytes available in the s_name field of the SCNHDR section header structure. The format requires the field to be NUL-padded, but not necessarily NUL-terminated, so the longest section names permitted are a full eight characters.

The Microsoft PE variants of the Coff object file format add an extension to support the use of long section names. This extension is defined in section 4 of the Microsoft PE/COFF specification (rev 8.1). If a section name is too long to fit into the section header’s s_name field, it is instead placed into the string table, and the s_name field is filled with a slash ("/") followed by the ASCII decimal representation of the offset of the full name relative to the string table base.

Note that this implies that the extension can only be used in object files, as executables do not contain a string table. The standard specifies that long section names from objects emitted into executable images are to be truncated.

However, as a GNU extension, BFD can generate executable images that contain a string table and long section names. This would appear to be technically valid, as the standard only says that Coff debugging information is deprecated, not forbidden, and in practice it works, although some tools that parse PE files expecting the MS standard format may become confused; PEview is one known example.

The functionality is supported in BFD by code implemented under the control of the macro COFF_LONG_SECTION_NAMES. If not defined, the format does not support long section names in any way. If defined, it is used to initialise a flag, _bfd_coff_long_section_names, and a hook function pointer, _bfd_coff_set_long_section_names, in the Coff backend data structure. The flag controls the generation of long section names in output BFDs at runtime; if it is false, as it will be by default when generating an executable image, long section names are truncated; if true, the long section names extension is employed. The hook points to a function that allows the value of a copy of the flag in coff object tdata to be altered at runtime, on formats that support long section names at all; on other formats it points to a stub that returns an error indication.

With input BFDs, the flag is set according to whether any long section names are detected while reading the section headers. For a completely new BFD, the flag is set to the default for the target format. This information can be used by a client of the BFD library when deciding what output format to generate, and means that a BFD that is opened for read and subsequently converted to a writeable BFD and modified in-place will retain whatever format it had on input.

If COFF_LONG_SECTION_NAMES is simply defined (blank), or is defined to the value "1", then long section names are enabled by default; if it is defined to the value zero, they are disabled by default (but still accepted in input BFDs). The header coffcode.h defines a macro, COFF_DEFAULT_LONG_SECTION_NAMES, which is used in the backends to initialise the backend data structure fields appropriately; see the comments for further detail.

3.3.2.3 Bit twiddling

Each flavour of coff supported in BFD has its own header file describing the external layout of the structures. There is also an internal description of the coff layout, in coff/internal.h. A major function of the coff backend is swapping the bytes and twiddling the bits to translate the external form of the structures into the normal internal form. This is all performed in the bfd_swap_thing_direction routines. Some elements are different sizes between different versions of coff; it is the duty of the coff version specific include file to override the definitions of various packing routines in coffcode.h. E.g., the size of line number entry in coff is sometimes 16 bits, and sometimes 32 bits. #defineing PUT_LNSZ_LNNO and GET_LNSZ_LNNO will select the correct one. No doubt, some day someone will find a version of coff which has a varying field size not catered to at the moment. To port BFD, that person will have to add more #defines. Three of the bit twiddling routines are exported to gdb; coff_swap_aux_in, coff_swap_sym_in and coff_swap_lineno_in. GDB reads the symbol table on its own, but uses BFD to fix things up. More of the bit twiddlers are exported for gas; coff_swap_aux_out, coff_swap_sym_out, coff_swap_lineno_out, coff_swap_reloc_out, coff_swap_filehdr_out, coff_swap_aouthdr_out, coff_swap_scnhdr_out. Gas currently keeps track of all the symbol table and reloc drudgery itself, thereby saving the internal BFD overhead, but uses BFD to swap things on the way out, making cross ports much safer. Doing so also allows BFD (and thus the linker) to use the same header files as gas, which makes one avenue to disaster disappear.

3.3.2.4 Symbol reading

The simple canonical form for symbols used by BFD is not rich enough to keep all the information available in a coff symbol table. The back end gets around this problem by keeping the original symbol table around, "behind the scenes".

When a symbol table is requested (through a call to bfd_canonicalize_symtab), a request gets through to coff_get_normalized_symtab. This reads the symbol table from the coff file and swaps all the structures inside into the internal form. It also fixes up all the pointers in the table (represented in the file by offsets from the first symbol in the table) into physical pointers to elements in the new internal table. This involves some work since the meanings of fields change depending upon context: a field that is a pointer to another structure in the symbol table at one moment may be the size in bytes of a structure at the next. Another pass is made over the table. All symbols which mark file names (C_FILE symbols) are modified so that the internal string points to the value in the auxent (the real filename) rather than the normal text associated with the symbol (".file").

At this time the symbol names are moved around. Coff stores all symbols less than nine characters long physically within the symbol table; longer strings are kept at the end of the file in the string table. This pass moves all strings into memory and replaces them with pointers to the strings.

The symbol table is massaged once again, this time to create the canonical table used by the BFD application. Each symbol is inspected in turn, and a decision made (using the sclass field) about the various flags to set in the asymbol. See Symbols. The generated canonical table shares strings with the hidden internal symbol table.

Any linenumbers are read from the coff file too, and attached to the symbols which own the functions the linenumbers belong to.

3.3.2.5 Symbol writing

Writing a symbol to a coff file which didn’t come from a coff file will lose any debugging information. The asymbol structure remembers the BFD from which the symbol was taken, and on output the back end makes sure that the same destination target as source target is present.

When the symbols have come from a coff file then all the debugging information is preserved.

Symbol tables are provided for writing to the back end in a vector of pointers to pointers. This allows applications like the linker to accumulate and output large symbol tables without having to do too much byte copying.

This function runs through the provided symbol table and patches each symbol marked as a file place holder (C_FILE) to point to the next file place holder in the list. It also marks each offset field in the list with the offset from the first symbol of the current symbol.

Another function of this procedure is to turn the canonical value form of BFD into the form used by coff. Internally, BFD expects symbol values to be offsets from a section base; so a symbol physically at 0x120, but in a section starting at 0x100, would have the value 0x20. Coff expects symbols to contain their final value, so symbols have their values changed at this point to reflect their sum with their owning section. This transformation uses the output_section field of the asymbol’s asection See Sections.

• coff_mangle_symbols

This routine runs though the provided symbol table and uses the offsets generated by the previous pass and the pointers generated when the symbol table was read in to create the structured hierarchy required by coff. It changes each pointer to a symbol into the index into the symbol table of the asymbol.

• coff_write_symbols

This routine runs through the symbol table and patches up the symbols from their internal form into the coff way, calls the bit twiddlers, and writes out the table to the file.

3.3.2.6 coff_symbol_type

The hidden information for an asymbol is described in a combined_entry_type:

typedef struct coff_ptr_struct
{
  /* Remembers the offset from the first symbol in the file for
     this symbol.  Generated by coff_renumber_symbols.  */
  unsigned int offset;

  /* Selects between the elements of the union below.  */
  unsigned int is_sym : 1;

  /* Selects between the elements of the x_sym.x_tagndx union.  If set,
     p is valid and the field will be renumbered.  */
  unsigned int fix_tag : 1;

  /* Selects between the elements of the x_sym.x_fcnary.x_fcn.x_endndx
     union.  If set, p is valid and the field will be renumbered.  */
  unsigned int fix_end : 1;

  /* Selects between the elements of the x_csect.x_scnlen union.  If set,
     p is valid and the field will be renumbered.  */
  unsigned int fix_scnlen : 1;

  /* If set, u.syment.n_value contains a pointer to a symbol.  The final
     value will be the offset field.  Used for XCOFF C_BSTAT symbols.  */
  unsigned int fix_value : 1;

  /* If set, u.syment.n_value is an index into the line number entries.
     Used for XCOFF C_BINCL/C_EINCL symbols.  */
  unsigned int fix_line : 1;

  /* The container for the symbol structure as read and translated
     from the file.  */
  union
  {
    union internal_auxent auxent;
    struct internal_syment syment;
  } u;

 /* An extra pointer which can used by format based on COFF (like XCOFF)
    to provide extra information to their backend.  */
 void *extrap;
} combined_entry_type;

/* Each canonical asymbol really looks like this: */

typedef struct coff_symbol_struct
{
  /* The actual symbol which the rest of BFD works with */
  asymbol symbol;

  /* A pointer to the hidden information for this symbol */
  combined_entry_type *native;

  /* A pointer to the linenumber information for this symbol */
  struct lineno_cache_entry *lineno;

  /* Have the line numbers been relocated yet ? */
  bool done_lineno;
} coff_symbol_type;

3.3.2.7 bfd_coff_backend_data

typedef struct
{
  void (*_bfd_coff_swap_aux_in)
    (bfd *, void *, int, int, int, int, void *);

  void (*_bfd_coff_swap_sym_in)
    (bfd *, void *, void *);

  void (*_bfd_coff_swap_lineno_in)
    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_aux_out)
    (bfd *, void *, int, int, int, int, void *);

  unsigned int (*_bfd_coff_swap_sym_out)
    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_lineno_out)
    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_reloc_out)
    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_filehdr_out)
    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_aouthdr_out)
    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_scnhdr_out)
    (bfd *, void *, void *);

  unsigned int _bfd_filhsz;
  unsigned int _bfd_aoutsz;
  unsigned int _bfd_scnhsz;
  unsigned int _bfd_symesz;
  unsigned int _bfd_auxesz;
  unsigned int _bfd_relsz;
  unsigned int _bfd_linesz;
  unsigned int _bfd_filnmlen;
  bool _bfd_coff_long_filenames;

  bool _bfd_coff_long_section_names;
  bool (*_bfd_coff_set_long_section_names)
    (bfd *, int);

  unsigned int _bfd_coff_default_section_alignment_power;
  bool _bfd_coff_force_symnames_in_strings;
  unsigned int _bfd_coff_debug_string_prefix_length;
  unsigned int _bfd_coff_max_nscns;

  void (*_bfd_coff_swap_filehdr_in)
    (bfd *, void *, void *);

  void (*_bfd_coff_swap_aouthdr_in)
    (bfd *, void *, void *);

  void (*_bfd_coff_swap_scnhdr_in)
    (bfd *, void *, void *);

  void (*_bfd_coff_swap_reloc_in)
    (bfd *abfd, void *, void *);

  bool (*_bfd_coff_bad_format_hook)
    (bfd *, void *);

  bool (*_bfd_coff_set_arch_mach_hook)
    (bfd *, void *);

  void * (*_bfd_coff_mkobject_hook)
    (bfd *, void *, void *);

  bool (*_bfd_styp_to_sec_flags_hook)
    (bfd *, void *, const char *, asection *, flagword *);

  void (*_bfd_set_alignment_hook)
    (bfd *, asection *, void *);

  bool (*_bfd_coff_slurp_symbol_table)
    (bfd *);

  bool (*_bfd_coff_symname_in_debug)
    (bfd *, struct internal_syment *);

  bool (*_bfd_coff_pointerize_aux_hook)
    (bfd *, combined_entry_type *, combined_entry_type *,
     unsigned int, combined_entry_type *);

  bool (*_bfd_coff_print_aux)
    (bfd *, FILE *, combined_entry_type *, combined_entry_type *,
     combined_entry_type *, unsigned int);

  bool (*_bfd_coff_reloc16_extra_cases)
    (bfd *, struct bfd_link_info *, struct bfd_link_order *, arelent *,
     bfd_byte *, size_t *, size_t *);

  int (*_bfd_coff_reloc16_estimate)
    (bfd *, asection *, arelent *, unsigned int,
     struct bfd_link_info *);

  enum coff_symbol_classification (*_bfd_coff_classify_symbol)
    (bfd *, struct internal_syment *);

  bool (*_bfd_coff_compute_section_file_positions)
    (bfd *);

  bool (*_bfd_coff_start_final_link)
    (bfd *, struct bfd_link_info *);

  bool (*_bfd_coff_relocate_section)
    (bfd *, struct bfd_link_info *, bfd *, asection *, bfd_byte *,
     struct internal_reloc *, struct internal_syment *, asection **);

  reloc_howto_type *(*_bfd_coff_rtype_to_howto)
    (bfd *, asection *, struct internal_reloc *,
     struct coff_link_hash_entry *, struct internal_syment *, bfd_vma *);

  bool (*_bfd_coff_adjust_symndx)
    (bfd *, struct bfd_link_info *, bfd *, asection *,
     struct internal_reloc *, bool *);

  bool (*_bfd_coff_link_add_one_symbol)
    (struct bfd_link_info *, bfd *, const char *, flagword,
     asection *, bfd_vma, const char *, bool, bool,
     struct bfd_link_hash_entry **);

  bool (*_bfd_coff_link_output_has_begun)
    (bfd *, struct coff_final_link_info *);

  bool (*_bfd_coff_final_link_postscript)
    (bfd *, struct coff_final_link_info *);

  bool (*_bfd_coff_print_pdata)
    (bfd *, void *);

} bfd_coff_backend_data;

3.3.2.8 Writing relocations

To write relocations, the back end steps though the canonical relocation table and create an internal_reloc. The symbol index to use is removed from the offset field in the symbol table supplied. The address comes directly from the sum of the section base address and the relocation offset; the type is dug directly from the howto field. Then the internal_reloc is swapped into the shape of an external_reloc and written out to disk.

3.3.2.9 Reading linenumbers

Creating the linenumber table is done by reading in the entire coff linenumber table, and creating another table for internal use.

A coff linenumber table is structured so that each function is marked as having a line number of 0. Each line within the function is an offset from the first line in the function. The base of the line number information for the table is stored in the symbol associated with the function.

Note: The PE format uses line number 0 for a flag indicating a new source file.

The information is copied from the external to the internal table, and each symbol which marks a function is marked by pointing its...

How does this work ?

3.3.2.10 Reading relocations

Coff relocations are easily transformed into the internal BFD form (arelent).

Reading a coff relocation table is done in the following stages:

• Read the entire coff relocation table into memory.
• Process each relocation in turn; first swap it from the external to the internal form.
• Turn the symbol referenced in the relocation’s symbol index into a pointer into the canonical symbol table. This table is the same as the one returned by a call to bfd_canonicalize_symtab. The back end will call that routine and save the result if a canonicalization hasn’t been done.
• The reloc index is turned into a pointer to a howto structure, in a back end specific way. For instance, the 386 uses the r_type to directly produce an index into a howto table vector.
• Note that arelent.addend for COFF is often not what most people understand as a relocation addend, but rather an adjustment to the relocation addend stored in section contents of relocatable object files. The value found in section contents may also be confusing, depending on both symbol value and addend somewhat similar to the field value for a final-linked object. See CALC_ADDEND.