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


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.
? 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 ****
  
  #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 ----
  
  #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			\(mov r8, r8\)

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