Re: RFA: Skip ARM ELF Mapping symbols when showing disassembly

[Andrew Cagney <ac131313 at redhat dot com>]

Nick,

>   I have recently committed a patch to the arm-elf port of GAS which
>   causes it to generate special mapping symbols as required by the ARM
>   ELF spec.  Unfortunately this now means that when GDB shows a
>   disassembly it can select one of the mapping symbols instead of the
>   proper function name symbol.

(GDB has minimal (=== asymbol), partial and full symbols.  IN the below 
"symbol" refers to the minimal symbol.)

Hmm, what information do those mapping symbols provide?  Dig dig (from 
tc-arm.c) ...

      4.4.7 Mapping and tagging symbols

      A section of an ARM ELF file can contain a mixture of ARM code,
      Thumb code, and data.  There are inline transitions between code
      and data at literal pool boundaries. There can also be inline
      transitions between ARM code and Thumb code, for example in
      ARM-Thumb inter-working veneers.  Linkers, machine-level
      debuggers, profiling tools, and disassembly tools need to map
      images accurately. For example, setting an ARM breakpoint on a
      Thumb location, or in a literal pool, can crash the program
      being debugged, ruining the debugging session.

      ARM ELF entities are mapped (see section 4.4.7.1 below) and
      tagged (see section 4.4.7.2 below) using local symbols (with
      binding STB_LOCAL).  To assist consumers, mapping and tagging
      symbols should be collated first in the symbol table, before
      other symbols with binding STB_LOCAL.

      To allow properly collated mapping and tagging symbols to be
      skipped by consumers that have no interest in them, the first
      such symbol should have the name $m and its st_value field equal
      to the total number of mapping and tagging symbols (including
      the $m) in the symbol table.

      4.4.7.1 Mapping symbols

      $a    Labels the first byte of a sequence of ARM instructions.
            Its type is STT_FUNC.

      $d    Labels the first byte of a sequence of data items.
            Its type is STT_OBJECT.

      $t    Labels the first byte of a sequence of Thumb instructions.
            Its type is STT_FUNC.

      This list of mapping symbols may be extended in the future.

      Section-relative mapping symbols

      Mapping symbols defined in a section define a sequence of
      half-open address intervals that cover the address range of the
      section. Each interval starts at the address defined by a
      mapping symbol, and continues up to, but not including, the
      address defined by the next (in address order) mapping symbol or
      the end of the section. A corollary is that there must be a
      mapping symbol defined at the beginning of each section.
      Consumers can ignore the size of a section-relative mapping
      symbol. Producers can set it to 0.

      Absolute mapping symbols

      Because of the need to crystallize a Thumb address with the
      Thumb-bit set, absolute symbol of type STT_FUNC (symbols of type
      STT_FUNC defined in section SHN_ABS) need to be mapped with $a
      or $t.

      The extent of a mapping symbol defined in SHN_ABS is [st_value,
      st_value + st_size), or [st_value, st_value + 1) if st_size = 0,
      where [x, y) denotes the half-open address range from x,
      inclusive, to y, exclusive.

      In the absence of a mapping symbol, a consumer can interpret a
      function symbol with an odd value as the Thumb code address
      obtained by clearing the least significant bit of the
      value. This interpretation is deprecated, and it may not work in
      the future.

   Note - the Tagging symbols ($b, $f, $p $m) have been dropped from
   the EABI (which is still under development), so they are not
   implemented here.  */

So GDB and objdump both need this information?  How does objdump handle 
all this?

- should there be a BFD method that lets GDB better identify a user 
visibile [minimal] symbol (gdb's elf reader currently contains what can 
only be described as heuristics).

- should there be a BFD method that, given an address and symbol table, 
indicates the relevant ISA/ABI?

- or even just have BFD export name->addr and addr->name methods?

Andrew