MIPS/BFD: Fix section symbol name fetching in relocation

Symbol table entries for section symbols are different between IRIX and
traditional MIPS ELF targets in that IRIX entries have their `st_name'
member pointing at the section's name in the string table section, while
traditional entries have 0 there and the section header string table has
to be referred via the relevant section header's `shn_name' member
instead.

This is chosen with the `elf_backend_name_local_section_symbols' backend
and can be observed with `readelf -s' output for an IRIX object:

Symbol table '.symtab' contains 12 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 SECTION LOCAL  DEFAULT    1 .text
     2: 00000000     0 SECTION LOCAL  DEFAULT    3 .data
     3: 00000000     0 SECTION LOCAL  DEFAULT    4 .bss
     4: 00000000     0 SECTION LOCAL  DEFAULT    5 .reginfo
     5: 00000000     0 SECTION LOCAL  DEFAULT    6 .MIPS.abiflags
     6: 00000000     0 SECTION LOCAL  DEFAULT    7 .pdr
     7: 00000000     0 SECTION LOCAL  DEFAULT    9 .gnu.attributes
     8: 00002000    16 FUNC    GLOBAL DEFAULT    1 foo
     9: 00004008     0 FUNC    LOCAL  DEFAULT    1 abar
    10: 00002008     0 FUNC    LOCAL  DEFAULT    1 afoo
    11: 00004000    16 FUNC    GLOBAL DEFAULT    1 bar

and a corresponding traditional object:

Symbol table '.symtab' contains 12 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 SECTION LOCAL  DEFAULT    1
     2: 00000000     0 SECTION LOCAL  DEFAULT    3
     3: 00000000     0 SECTION LOCAL  DEFAULT    4
     4: 00004008     0 FUNC    LOCAL  DEFAULT    1 abar
     5: 00002008     0 FUNC    LOCAL  DEFAULT    1 afoo
     6: 00000000     0 SECTION LOCAL  DEFAULT    5
     7: 00000000     0 SECTION LOCAL  DEFAULT    6
     8: 00000000     0 SECTION LOCAL  DEFAULT    7
     9: 00000000     0 SECTION LOCAL  DEFAULT    9
    10: 00002000    16 FUNC    GLOBAL DEFAULT    1 foo
    11: 00004000    16 FUNC    GLOBAL DEFAULT    1 bar

respectively.  Consequently the right way to retrieve a section symbol's
name has to be chosen in `mips_elf_calculate_relocation' for the purpose
of error reporting.

Originally we produced symbol tables in the traditional object format
only and we handled it correctly until it was lost in a rewrite with:

commit 7403cb6305f5660fccc8869d3333a731102ae978
Author: Mark Mitchell <mark@codesourcery.com>
Date:   Wed Jun 30 20:13:43 1999 +0000

probably because of the extra pointer indirection added which made the
same expression have a different meaning.

With the addition of IRIX symbol table format with:

commit 174fd7f9556183397625dbfa99ef68ecd325c74b
Author: Richard Sandiford <rdsandiford@googlemail.com>
Date:   Mon Feb 9 08:04:00 2004 +0000

the bug has been partially covered and now when a relocation error is
triggered with an IRIX object the offending section symbol is correctly
reported:

tmpdir/dump0.o: In function `foo':
(.text+0x2000): relocation truncated to fit: R_MIPS_26 against `.text'
tmpdir/dump0.o: In function `bar':
(.text+0x4000): relocation truncated to fit: R_MIPS_26 against `.text'

because `bfd_elf_string_from_elf_section' retrieves the name from the
string table section.  With a traditional object however the function
returns an empty string and consequently `no symbol' is printed instead:

tmpdir/dump0.o: In function `foo':
(.text+0x2000): relocation truncated to fit: R_MIPS_26 against `no symbol'
tmpdir/dump0.o: In function `bar':
(.text+0x4000): relocation truncated to fit: R_MIPS_26 against `no symbol'

Restore the original semantics so that the section name is always
correctly retrieved.

	bfd/
	* elfxx-mips.c (mips_elf_calculate_relocation): Also use the
	section name if `bfd_elf_string_from_elf_section' returns an
	empty string.

	ld/
	* testsuite/ld-mips-elf/reloc-local-overflow.d: New test.
	* testsuite/ld-mips-elf/reloc-local-overflow.s: Source for the
	new test.
	* testsuite/ld-mips-elf/mips-elf.exp: Run the new test.

2 files changed, 7 insertions, 1 deletions

diff --git a/bfd/ChangeLog b/bfd/ChangeLog
index bb9cb3c..5bace1e 100644
--- a/bfd/ChangeLog
+++ b/bfd/ChangeLog
@@ -1,3 +1,9 @@
+2016-05-27  Maciej W. Rozycki  <macro@imgtec.com>
+
+	* elfxx-mips.c (mips_elf_calculate_relocation): Also use the
+	section name if `bfd_elf_string_from_elf_section' returns an
+	empty string.
+
 2016-05-26  Maciej W. Rozycki  <macro@imgtec.com>
 
 	* elfxx-mips.c (_bfd_mips_elf_relocate_section)
diff --git a/bfd/elfxx-mips.c b/bfd/elfxx-mips.c
index 3e7b488..2fff306 100644
--- a/bfd/elfxx-mips.c
+++ b/bfd/elfxx-mips.c
@@ -5322,7 +5322,7 @@ mips_elf_calculate_relocation (bfd *abfd, bfd *input_bfd,
       *namep = bfd_elf_string_from_elf_section (input_bfd,
 						symtab_hdr->sh_link,
 						sym->st_name);
-      if (*namep == '\0')
+      if (*namep == NULL || **namep == '\0')
 	*namep = bfd_section_name (input_bfd, sec);
 
       target_is_16_bit_code_p = ELF_ST_IS_MIPS16 (sym->st_other);