Re: [PATCH] Fix type and alignment of ARM/Thumb EABI mapping symbols

[Julian Brown <julian@codesourcery.com>]

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Richard Earnshaw wrote:
> On Sun, 2005-03-27 at 21:25, Julian Brown wrote:
> 
>>Hi,
>>
>>This patch fixes two problems: the type of mapping symbols is made 
>>STT_NOTYPE (from STT_FUNC or STT_OBJECT from ARM/Thumb or data 
>>respectively), and the alignment is corrected to not have the low bit 
>>set for Thumb mapping symbols, in accordance with the latest version of 
>>the ARM AAELF spec.
>>
>>Unfortunately altering the symbol type to STT_NOTYPE for mapping symbols 
>>affects the output of disassembly from objdump. $a and $t were 
>>previously treated as functions. When objdump tried to find the closest 
>>symbol for possibly-symbol-relative offsets previously, it found those 
>>mapping symbols since compare_symbols places functions first in a sorted 
>>version of the symbol table. They were then rejected by the 
>>symbol_is_valid hook (arm_symbol_is_valid) from the disassemble_info 
>>struct, and the section name was printed instead.
>>
>>When the type of mapping symbols is changed to STT_NOTYPE, they are no 
>>longer placed first in the sorted symbol list, so another symbol is used 
>>instead. This causes test regressions, e.g. a symbol-offset address 
>>which was previously output as:
>>
>>   bl      8224 <.text-0xc>
>>
>>was output instead as:
>>
>>   bl      8224 <_start-0xc>
>>
>>In the interest of preserving existing behaviour, I have fixed this by 
>>adding a new disassemble_info hook which allows the symbol table used 
>>for disassembly to have a target-specific hook for altering each symbol 
>>(or remove it) in the remove_useless_symbols function. This sets the 
>>BSF_FUNCTION flag for mapping symbols, which makes things behave the way 
>>they used to, and possibly provides useful functionality for other 
>>platforms as well.
>>
>>It may be better to just change the expected test output instead. Thoughts?
>>
> 
> 
> I think the tests should be fixed.  Other targets (those that don't have
> mapping symbols) would almost certainly output _start in the example
> above, so the existing behaviour should be considered undesirable at
> best and probably just buggy.
> 
> That should allow you to simplify your patch so that you don't need to
> touch MI parts.

OK, this is a simpler version of the patch which alters the expected 
test output instead (only ARM-specific code is now touched).

Tested with cross to arm-none-eabi and arm-none-symbianelf, and with all 
targets on i686-pc-linux-gnu. OK to apply?

ChangeLog:

   * bfd/elf32-arm.c (arm_elf_find_function): Include STT_NOTYPE in test
   for mapping symbols.
   * gas/config/tc-arm.c (mapping_state): Update documentation in
   function comment. Change type of mapping symbols to BSF_NO_TYPE.
   (is_arm_mapping_symbol_name): New function.
   (arm_adjust_symtab): Don't change type of mapping symbols here.
   * gas/testsuite/gas/arm/mapping.d: Update expected output.
   * ld/testsuite/ld-arm/arm-app-abs32.d: Likewise.
   * ld/testsuite/ld-arm/arm-app.d: Likewise.
   * ld/testsuite/ld-arm/mixed-app.d: Likewise.

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? bfd/doc/bfd.info
? bfd/doc/bfd.info-1
? gas/doc/as.info
? gas/doc/as.info-1
Index: bfd/elf32-arm.c
===================================================================
RCS file: /cvs/src/src/bfd/elf32-arm.c,v
retrieving revision 1.28
diff -c -p -r1.28 elf32-arm.c
*** bfd/elf32-arm.c	29 Mar 2005 16:54:19 -0000	1.28
--- bfd/elf32-arm.c	29 Mar 2005 21:13:42 -0000
*************** arm_elf_find_function (bfd *         abf
*** 4754,4765 ****
  	  break;
  	case STT_FUNC:
  	case STT_ARM_TFUNC:
  	  /* Skip $a and $t symbols.  */
  	  if ((q->symbol.flags & BSF_LOCAL)
  	      && is_arm_mapping_symbol_name (q->symbol.name))
  	    continue;
  	  /* Fall through.  */
- 	case STT_NOTYPE:
  	  if (bfd_get_section (&q->symbol) == section
  	      && q->symbol.value >= low_func
  	      && q->symbol.value <= offset)
--- 4754,4765 ----
  	  break;
  	case STT_FUNC:
  	case STT_ARM_TFUNC:
+ 	case STT_NOTYPE:
  	  /* Skip $a and $t symbols.  */
  	  if ((q->symbol.flags & BSF_LOCAL)
  	      && is_arm_mapping_symbol_name (q->symbol.name))
  	    continue;
  	  /* Fall through.  */
  	  if (bfd_get_section (&q->symbol) == section
  	      && q->symbol.value >= low_func
  	      && q->symbol.value <= offset)
Index: gas/config/tc-arm.c
===================================================================
RCS file: /cvs/src/src/gas/config/tc-arm.c,v
retrieving revision 1.199
diff -c -p -r1.199 tc-arm.c
*** gas/config/tc-arm.c	29 Mar 2005 16:54:21 -0000	1.199
--- gas/config/tc-arm.c	29 Mar 2005 21:13:47 -0000
*************** validate_offset_imm (unsigned int val, i
*** 1243,1316 ****
  \f
  #ifdef OBJ_ELF
  /* This code is to handle mapping symbols as defined in the ARM ELF spec.
!    (This text is taken from version B-02 of the spec):
  
!       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
--- 1243,1308 ----
  \f
  #ifdef OBJ_ELF
  /* This code is to handle mapping symbols as defined in the ARM ELF spec.
!    (This text is taken from version 0.62 RC 2 of the spec):
  
!       4.5.7 Mapping symbols
  
!       A section of an 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, and potentially other tools, need to map images correctly (for
!       example, to support byte swapping to produce a BE-8 image from a BE-32
!       object file).  To support this, a number of symbols, termed mapping
!       symbols appear in the symbol table to denote the start of a sequence of
!       bytes of the appropriate type.  All mapping symbols have type STT_NOTYPE
!       and binding STB_LOCAL.  The st_size field is unused and must be zero.
! 
!       The mapping symbols are defined in Table 4-6, Mapping symbols.  It is an
!       error for a relocation to reference a mapping symbol.  Two forms of
!       mapping symbol are supported:
! 
!         * a short form, that uses a dollar character and a single letter
!         denoting the class.  This form can be used when an object producer
!         creates mapping symbols automatically, and minimizes symbol table
!         space
! 
!         * a longer form, where the short form is extended with a period and
!         then any sequence of characters that are legal for a symbol.  This
!         form can be used when assembler files have to be annotated manually
!         and the assembler does not support multiple definitions of symbols.
! 
!       Table 4-6, Mapping symbols
! 
!       Name         Meaning
! 
!       $a           Start of a sequence of ARM instructions
!       $a.<any...>
! 
!       $d           Start of a sequence of data items (for example, a literal
!       $d.<any...>  pool)
! 
!       $t           Start of a sequence of Thumb instructions
!       $t.<any...>
! 
!       4.5.7.1 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 the 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 section must
!       have a mapping symbol defined at the beginning of the section; however,
!       if the section contains only data then the mapping symbol may be omitted.
! 
!       4.5.7.2 Absolute mapping symbols
! 
!       Mapping symbols are no-longer required for the absolute section.  The
!       equivalent information is now conveyed by the type of the absolute
!       symbol.
  
     Note - the Tagging symbols ($b, $f, $p $m) have been dropped from
     the EABI (which is still under development), so they are not
*************** mapping_state (enum mstate state)
*** 1336,1350 ****
      {
      case MAP_DATA:
        symname = "$d";
!       type = BSF_OBJECT;
        break;
      case MAP_ARM:
        symname = "$a";
!       type = BSF_FUNCTION;
        break;
      case MAP_THUMB:
        symname = "$t";
!       type = BSF_FUNCTION;
        break;
      case MAP_UNDEFINED:
        return;
--- 1328,1342 ----
      {
      case MAP_DATA:
        symname = "$d";
!       type = BSF_NO_FLAGS;
        break;
      case MAP_ARM:
        symname = "$a";
!       type = BSF_NO_FLAGS;
        break;
      case MAP_THUMB:
        symname = "$t";
!       type = BSF_NO_FLAGS;
        break;
      case MAP_UNDEFINED:
        return;
*************** arm_frob_label (symbolS * sym)
*** 13678,13683 ****
--- 13670,13685 ----
      }
  }
  
+ /* Recognize short-form or long-form mapping symbol names.  */
+ 
+ static bfd_boolean
+ is_arm_mapping_symbol_name (const char* name)
+ {
+   return name && name[0] == '$'
+          && (name[1] == 'a' || name[1] == 'd' || name[1] == 't')
+          && (name[2] == '\0' || name[2] == '.');
+ }
+ 
  /* Adjust the symbol table.  This marks Thumb symbols as distinct from
     ARM ones.  */
  
*************** arm_adjust_symtab (void)
*** 13738,13751 ****
  	  elf_sym = elf_symbol (symbol_get_bfdsym (sym));
  	  bind = ELF_ST_BIND (elf_sym->internal_elf_sym.st_info);
  
! 	  /* If it's a .thumb_func, declare it as so,
! 	     otherwise tag label as .code 16.  */
! 	  if (THUMB_IS_FUNC (sym))
! 	    elf_sym->internal_elf_sym.st_info =
! 	      ELF_ST_INFO (bind, STT_ARM_TFUNC);
! 	  else
! 	    elf_sym->internal_elf_sym.st_info =
! 	      ELF_ST_INFO (bind, STT_ARM_16BIT);
  	}
      }
  #endif
--- 13740,13756 ----
  	  elf_sym = elf_symbol (symbol_get_bfdsym (sym));
  	  bind = ELF_ST_BIND (elf_sym->internal_elf_sym.st_info);
  
! 	  if (!is_arm_mapping_symbol_name (elf_sym->symbol.name))
! 	    { 
! 	      /* If it's a .thumb_func, declare it as so,
! 		 otherwise tag label as .code 16.  */
! 	      if (THUMB_IS_FUNC (sym))
! 		elf_sym->internal_elf_sym.st_info =
! 		  ELF_ST_INFO (bind, STT_ARM_TFUNC);
! 	      else
! 		elf_sym->internal_elf_sym.st_info =
! 		  ELF_ST_INFO (bind, STT_ARM_16BIT);
! 	    }
  	}
      }
  #endif
Index: gas/testsuite/gas/arm/mapping.d
===================================================================
RCS file: /cvs/src/src/gas/testsuite/gas/arm/mapping.d,v
retrieving revision 1.4
diff -c -p -r1.4 mapping.d
*** gas/testsuite/gas/arm/mapping.d	11 Dec 2004 04:32:36 -0000	1.4
--- gas/testsuite/gas/arm/mapping.d	29 Mar 2005 21:13:47 -0000
*************** SYMBOL TABLE:
*** 9,18 ****
  0+00 l    d  .text	0+0 (|.text)
  0+00 l    d  .data	0+0 (|.data)
  0+00 l    d  .bss	0+0 (|.bss)
! 0+00 l     F .text	0+0 \$a
! 0+08 l     F .text	0+0 \$t
! 0+00 l     O .data	0+0 \$d
  0+00 l    d  foo	0+0 (|foo)
! 0+00 l     F foo	0+0 \$t
  0+00 g       .text	0+0 mapping
  0+08 g     F .text	0+0 thumb_mapping
--- 9,18 ----
  0+00 l    d  .text	0+0 (|.text)
  0+00 l    d  .data	0+0 (|.data)
  0+00 l    d  .bss	0+0 (|.bss)
! 0+00 l       .text	0+0 \$a
! 0+08 l       .text	0+0 \$t
! 0+00 l       .data	0+0 \$d
  0+00 l    d  foo	0+0 (|foo)
! 0+00 l       foo	0+0 \$t
  0+00 g       .text	0+0 mapping
  0+08 g     F .text	0+0 thumb_mapping
Index: ld/testsuite/ld-arm/arm-app-abs32.d
===================================================================
RCS file: /cvs/src/src/ld/testsuite/ld-arm/arm-app-abs32.d,v
retrieving revision 1.2
diff -c -p -r1.2 arm-app-abs32.d
*** ld/testsuite/ld-arm/arm-app-abs32.d	17 Nov 2004 17:50:27 -0000	1.2
--- ld/testsuite/ld-arm/arm-app-abs32.d	29 Mar 2005 21:14:01 -0000
*************** Disassembly of section .plt:
*** 8,14 ****
  
  .* <.plt>:
      .*:	e52de004 	str	lr, \[sp, #-4\]!
!     .*:	e59fe004 	ldr	lr, \[pc, #4\]	; .* <.plt\+0x10>
      .*:	e08fe00e 	add	lr, pc, lr
      .*:	e5bef008 	ldr	pc, \[lr, #8\]!
      .*:	.* 	.*
--- 8,14 ----
  
  .* <.plt>:
      .*:	e52de004 	str	lr, \[sp, #-4\]!
!     .*:	e59fe004 	ldr	lr, \[pc, #4\]	; .* <_start-0x10>
      .*:	e08fe00e 	add	lr, pc, lr
      .*:	e5bef008 	ldr	pc, \[lr, #8\]!
      .*:	.* 	.*
Index: ld/testsuite/ld-arm/arm-app.d
===================================================================
RCS file: /cvs/src/src/ld/testsuite/ld-arm/arm-app.d,v
retrieving revision 1.2
diff -c -p -r1.2 arm-app.d
*** ld/testsuite/ld-arm/arm-app.d	17 Nov 2004 17:50:27 -0000	1.2
--- ld/testsuite/ld-arm/arm-app.d	29 Mar 2005 21:14:01 -0000
*************** Disassembly of section .plt:
*** 8,14 ****
  
  .* <.plt>:
   .*:	e52de004 	str	lr, \[sp, #-4\]!
!  .*:	e59fe004 	ldr	lr, \[pc, #4\]	; .* <.plt\+0x10>
   .*:	e08fe00e 	add	lr, pc, lr
   .*:	e5bef008 	ldr	pc, \[lr, #8\]!
   .*:	.*
--- 8,14 ----
  
  .* <.plt>:
   .*:	e52de004 	str	lr, \[sp, #-4\]!
!  .*:	e59fe004 	ldr	lr, \[pc, #4\]	; .* <_start-0x10>
   .*:	e08fe00e 	add	lr, pc, lr
   .*:	e5bef008 	ldr	pc, \[lr, #8\]!
   .*:	.*
*************** Disassembly of section .text:
*** 27,33 ****
  .* <app_func>:
   .*:	e1a0c00d 	mov	ip, sp
   .*:	e92dd800 	stmdb	sp!, {fp, ip, lr, pc}
!  .*:	ebfffff4 	bl	.* <.text-0xc>
   .*:	e89d6800 	ldmia	sp, {fp, sp, lr}
   .*:	e12fff1e 	bx	lr
  
--- 27,33 ----
  .* <app_func>:
   .*:	e1a0c00d 	mov	ip, sp
   .*:	e92dd800 	stmdb	sp!, {fp, ip, lr, pc}
!  .*:	ebfffff4 	bl	.* <_start-0xc>
   .*:	e89d6800 	ldmia	sp, {fp, sp, lr}
   .*:	e12fff1e 	bx	lr
  
Index: ld/testsuite/ld-arm/mixed-app.d
===================================================================
RCS file: /cvs/src/src/ld/testsuite/ld-arm/mixed-app.d,v
retrieving revision 1.1
diff -c -p -r1.1 mixed-app.d
*** ld/testsuite/ld-arm/mixed-app.d	17 Nov 2004 17:50:27 -0000	1.1
--- ld/testsuite/ld-arm/mixed-app.d	29 Mar 2005 21:14:01 -0000
*************** Disassembly of section .plt:
*** 8,14 ****
  
  .* <.plt>:
   .*:	e52de004 	str	lr, \[sp, #-4\]!
!  .*:	e59fe004 	ldr	lr, \[pc, #4\]	; .* <.plt\+0x10>
   .*:	e08fe00e 	add	lr, pc, lr
   .*:	e5bef008 	ldr	pc, \[lr, #8\]!
   .*:	.*
--- 8,14 ----
  
  .* <.plt>:
   .*:	e52de004 	str	lr, \[sp, #-4\]!
!  .*:	e59fe004 	ldr	lr, \[pc, #4\]	; .* <_start-0x2c>
   .*:	e08fe00e 	add	lr, pc, lr
   .*:	e5bef008 	ldr	pc, \[lr, #8\]!
   .*:	.*
*************** Disassembly of section .text:
*** 34,40 ****
  .* <app_func>:
   .*:	e1a0c00d 	mov	ip, sp
   .*:	e92dd800 	stmdb	sp!, {fp, ip, lr, pc}
!  .*:	ebffffe. 	bl	.* <.text-0x..>
   .*:	e89d6800 	ldmia	sp, {fp, sp, lr}
   .*:	e12fff1e 	bx	lr
   .*:	e1a00000 	nop			\(mov r0,r0\)
--- 34,40 ----
  .* <app_func>:
   .*:	e1a0c00d 	mov	ip, sp
   .*:	e92dd800 	stmdb	sp!, {fp, ip, lr, pc}
!  .*:	ebffffe. 	bl	.* <_start-0x..>
   .*:	e89d6800 	ldmia	sp, {fp, sp, lr}
   .*:	e12fff1e 	bx	lr
   .*:	e1a00000 	nop			\(mov r0,r0\)
*************** Disassembly of section .text:
*** 49,55 ****
  
  .* <app_tfunc>:
   .*:	b500      	push	{lr}
!  .*:	(ffc.f7ff|f7ffffc.) 	bl	.* <.text-0x..>
   .*:	bd00      	pop	{pc}
   .*:	4770      	bx	lr
   .*:	46c0      	nop			\(mov r8, r8\)
--- 49,55 ----
  
  .* <app_tfunc>:
   .*:	b500      	push	{lr}
!  .*:	(ffc.f7ff|f7ffffc.) 	bl	.* <_start-0x..>
   .*:	bd00      	pop	{pc}
   .*:	4770      	bx	lr
   .*:	46c0      	nop			\(mov r8, r8\)