MIPS/LD: Fix a segfault from ELF `e_flags' access with non-ELF output BFD

Fix a commit 861fb55ab5 ("Defer allocation of R_MIPS_REL32 GOT
slots"), <https://sourceware.org/ml/binutils/2008-08/msg00096.html>,
regression and a more recent:

FAIL: ld-unique/pr21529

new LD test case failure, observed with all the relevant MIPS targets
whenever the linker is invoked with one or more ELF inputs and the
output format set to `binary'.

The culprit is a segmentation fault caused in `mips_before_allocation'
by a null pointer dereference, where an attempt is made to access the
ELF file header's `e_flags' member, for the purpose of determining
whether to produce a PLT and copy relocations, without first checking
that the output BFD is ELF.  The `e_flags' member is stored in BFD's
private data pointed to by `tdep', which in the case of the `binary' BFD
is null, causing the segmentation fault.  With other non-ELF BFDs such
as SREC `tdep' is not null and consequently no crash may happen and in
that case random data will be interpreted as it was `e_flags'.

Disable the access to `e_flags' then and all the associated checks and
consequently never produce a PLT and copy relocations if output is not a
MIPS ELF BFD, matching `_bfd_mips_elf_merge_private_bfd_data' that does
not process `e_flags' in that case either and therefore does not let us
decide here anyway if all the input objects included in the link are
suitable for use with a PLT and copy relocations.

	ld/
	* emultempl/mipself.em (mips_before_allocation): Avoid ELF
	processing if not MIPS ELF.
	* testsuite/ld-mips-elf/binary.d: New test.
	* testsuite/ld-mips-elf/binary.ld: New test linker script.
	* testsuite/ld-mips-elf/binary.s: New test source.
	* testsuite/ld-mips-elf/mips-elf.exp: Run the new test.

ld/ChangeLog

@@ -1,3 +1,12 @@
+2017-07-07  Maciej W. Rozycki  <macro@imgtec.com>
+
+	* emultempl/mipself.em (mips_before_allocation): Avoid ELF
+	processing if not MIPS ELF.
+	* testsuite/ld-mips-elf/binary.d: New test.
+	* testsuite/ld-mips-elf/binary.ld: New test linker script.
+	* testsuite/ld-mips-elf/binary.s: New test source.
+	* testsuite/ld-mips-elf/mips-elf.exp: Run the new test.
+
 2017-07-07  Alan Modra  <amodra@gmail.com>
 
 	* testsuite/ld-powerpc/ppc476-shared.lnk: Align .bss.

ld/emultempl/mipself.em

@@ -214,6 +214,8 @@ mips_create_output_section_statements (void)
 static void
 mips_before_allocation (void)
 {
+  if (is_mips_elf (link_info.output_bfd))
+    {
       flagword flags;
 
       flags = elf_elfheader (link_info.output_bfd)->e_flags;
@@ -221,6 +223,7 @@ mips_before_allocation (void)
 	  && !link_info.nocopyreloc
 	  && (flags & (EF_MIPS_PIC | EF_MIPS_CPIC)) == EF_MIPS_CPIC)
 	_bfd_mips_elf_use_plts_and_copy_relocs (&link_info);
+    }
 
   gld${EMULATION_NAME}_before_allocation ();
 }

ld/testsuite/ld-mips-elf/binary.d

@@ -0,0 +1,8 @@
+#objdump: -b binary -s
+#name: MIPS link ELF into binary output format
+#ld: -r --oformat=binary -T binary.ld
+
+.*: +file format binary
+
+Contents of section \.data:
+ 0000 61626364 65666768 696a6b6c 6d6e6f70  abcdefghijklmnop

ld/testsuite/ld-mips-elf/binary.ld

@@ -0,0 +1,5 @@
+SECTIONS
+{
+  .data : { *(.data) }
+  /DISCARD/ : { *(*) }
+}

ld/testsuite/ld-mips-elf/binary.s

@@ -0,0 +1,2 @@
+	.data
+	.ascii	"abcdefghijklmnop"

ld/testsuite/ld-mips-elf/mips-elf.exp

@@ -1213,3 +1213,6 @@ run_ld_link_tests [list \
 	"relax-offset-umips"]]
 rename prune_warnings ""
 rename mips_old_prune_warnings prune_warnings
+
+# Verify that we can link ELF input into the `binary' output format.
+run_dump_test "binary"