[ARM] Relocations against STN_UNDEF

[Thomas Schwinge]

Hello!

The ELF standard says that STN_UNDEF (symbol index zero, the undefined
symbol index) is to be marked as SHN_UNDEF, undefined symbol.  There is
one exception however: during relocations processing, a relocation
against STN_UNDEF shall be treated as a symbol value of zero.

For example on ARM, the bl instruction does a PC-relative jump, so to
jump to an absolute value of 0x10000, ``bl 0x10000'' has to keep its
relocation until the final linking is done, to be converted to the proper
PC-relative offset.

    $ cat < ~/sgxx/issue8612/bl_ABS.s
            bl 0x10000
    $ "$PWD"_install/bin/*-as -o bl_ABS.o ~/sgxx/issue8612/bl_ABS.s
    $ "$PWD"_install/bin/*-readelf -r bl_ABS.o 
    
    Relocation section '.rel.text' at offset 0x25c contains 1 entries:
     Offset     Info    Type            Sym.Value  Sym. Name
    00000000  0000001c R_ARM_CALL       

This info value translates to relocation type 28 (R_ARM_CALL) against
symbol index zero (STN_UNDEF).

On 2010-09-24 00:12, Alan Modra wrote:
> On Thu, Sep 23, 2010 at 05:37:01PM +0200, Thomas Schwinge wrote:
>>     $ "$PWD"_install/bin/*-ld -o bl_ABS bl_ABS.o
>>     /scratch/thomas/binutils/HEAD_build_arm-none-eabi_install/bin/arm-none-eabi-ld: warning: cannot find entry symbol _start; defaulting to 0000000000008000
>>     bl_ABS.o:(.text+0x0): undefined reference to `no symbol'
>
> ARM fails because the arm backend specifically checks for undefined
> local symbols.  I'd say the arm backend check needs fixing (or
> removing) rather than changing the common elflink code.

Here is a patch to fix this issue in the ARM backend.  There are no
regressions with a --target=arm-none-eabi config.

bfd/
2010-09-24  Thomas Schwinge  <thomas@codesourcery.com>

	* elf32-arm.c (elf32_arm_final_link_relocate)
	(elf32_arm_relocate_section): Handle relocations against STN_UNDEF.

Index: bfd/elf32-arm.c
===================================================================
RCS file: /cvs/src/src/bfd/elf32-arm.c,v
retrieving revision 1.241
diff -u -p -r1.241 elf32-arm.c
--- bfd/elf32-arm.c	26 Aug 2010 10:32:26 -0000	1.241
+++ bfd/elf32-arm.c	24 Sep 2010 11:16:45 -0000
@@ -7224,12 +7224,12 @@ elf32_arm_final_link_relocate (reloc_how
 
 	  /* A branch to an undefined weak symbol is turned into a jump to
 	     the next instruction unless a PLT entry will be created.
-	     Do the same for local undefined symbols.
+	     Do the same for local undefined symbols (but not for STN_UNDEF).
 	     The jump to the next instruction is optimized as a NOP depending
 	     on the architecture.  */
 	  if (h ? (h->root.type == bfd_link_hash_undefweak
 		   && !(splt != NULL && h->plt.offset != (bfd_vma) -1))
-	      : bfd_is_und_section (sym_sec))
+	      : r_symndx != STN_UNDEF && bfd_is_und_section (sym_sec))
 	    {
 	      value = (bfd_get_32 (input_bfd, hit_data) & 0xf0000000);
 
@@ -8903,9 +8903,11 @@ elf32_arm_relocate_section (bfd *       
 	     undefined symbol.  This is a daft object file, but we
 	     should at least do something about it.  V4BX & NONE
 	     relocations do not use the symbol and are explicitly
-	     allowed to use the undefined symbol, so allow those.  */
+	     allowed to use the undefined symbol, so allow those.
+	     Likewise for relocations against STN_UNDEF.  */
 	  if (r_type != R_ARM_V4BX
 	      && r_type != R_ARM_NONE
+	      && r_symndx != STN_UNDEF
 	      && bfd_is_und_section (sec)
 	      && ELF_ST_BIND (sym->st_info) != STB_WEAK)
 	    {

Success with patched BFD:

    $ "$PWD"_install/bin/*-ld -o bl_ABS bl_ABS.o
    /scratch/thomas/binutils/HEAD_build_arm-none-eabi_install/bin/arm-none-eabi-ld: warning: cannot find entry symbol _start; defaulting to 0000000000008000
    $ "$PWD"_install/bin/*-readelf -r bl_ABS
    
    There are no relocations in this file.
    $ "$PWD"_install/bin/*-objdump -dr bl_ABS
    
    bl_ABS:     file format elf32-littlearm
    
    
    Disassembly of section .text:
    
    00008000 <__data_start-0x8004>:
        8000:       eb001ffe        bl      10000 <__bss_end__-0x4>


Regards,
 Thomas
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