[PATCH v3] aarch64: Emit jump slot for conditional branch to undefined symbols

The linker silently writes out a conditional branch to 0 if the
target symbol in R_AARCH64_CONDBR19 or R_AARCH64_TSTBR14 relocations is
undefined.  Emit a PLT instead so that behaviour is the same for these
relocations as the llvm linker.

The special behaviour for undefined weak symbols, where conditional
branches to such symbols result in a branch unto themselves, has been
retained.  This is because the weak-undefined.s test explicitly checks
for that, leading me to conclude that it's expected behaviour.

bfd	* elfnn-aarch64.c (elfNN_aarch64_final_link_relocate): Club
	BFD_RELOC_AARCH64_BRANCH19 and BFD_RELOC_AARCH64_TSTBR14
	cases with BFD_RELOC_AARCH64_JUMP26.
	(elfNN_aarch64_check_relocs): Likewise.

ld	* testsuite/ld-aarch64/aarch64-elf.exp: New test
	emit-relocs-560.
	* testsuite/ld-aarch64/emit-relocs-560.d: New file.
	* testsuite/ld-aarch64/emit-relocs-560.s: New file.

bfd/ChangeLog

@@ -1,3 +1,10 @@
+2020-05-19  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>
+
+	* elfnn-aarch64.c (elfNN_aarch64_final_link_relocate): Club
+	BFD_RELOC_AARCH64_BRANCH19 and BFD_RELOC_AARCH64_TSTBR14
+	cases with BFD_RELOC_AARCH64_JUMP26.
+	(elfNN_aarch64_check_relocs): Likewise.
+
 2020-05-19  Alan Modra  <amodra@gmail.com>
 
 	* aix5ppc-core.c (xcoff64_core_file_matches_executable_p): Use

bfd/elfnn-aarch64.c

@@ -5494,6 +5494,7 @@ elfNN_aarch64_final_link_relocate (reloc_howto_type *howto,
   bfd_vma orig_value = value;
   bfd_boolean resolved_to_zero;
   bfd_boolean abs_symbol_p;
+  bfd_boolean via_plt_p;
 
   globals = elf_aarch64_hash_table (info);
 
@@ -5515,6 +5516,8 @@ elfNN_aarch64_final_link_relocate (reloc_howto_type *howto,
 		  : bfd_is_und_section (sym_sec));
   abs_symbol_p = h != NULL && bfd_is_abs_symbol (&h->root);
 
+  via_plt_p = (globals->root.splt != NULL && h != NULL
+	       && h->plt.offset != (bfd_vma) - 1);
 
   /* Since STT_GNU_IFUNC symbol must go through PLT, we handle
      it here if it is defined in a non-shared object.  */
@@ -5850,12 +5853,23 @@ elfNN_aarch64_final_link_relocate (reloc_howto_type *howto,
 	value += signed_addend;
       break;
 
+    case BFD_RELOC_AARCH64_BRANCH19:
+    case BFD_RELOC_AARCH64_TSTBR14:
+      /* A conditional branch to an undefined weak symbol is converted to a
+	 branch to itself.  */
+      if (weak_undef_p && !via_plt_p)
+	{
+	  value = _bfd_aarch64_elf_resolve_relocation (input_bfd, bfd_r_type,
+						       place, value,
+						       signed_addend,
+						       weak_undef_p);
+	  break;
+	}
+      /* Fall through.  */
     case BFD_RELOC_AARCH64_CALL26:
     case BFD_RELOC_AARCH64_JUMP26:
       {
 	asection *splt = globals->root.splt;
-	bfd_boolean via_plt_p =
-	  splt != NULL && h != NULL && h->plt.offset != (bfd_vma) - 1;
 
 	/* A call to an undefined weak symbol is converted to a jump to
 	   the next instruction unless a PLT entry will be created.
@@ -5943,7 +5957,6 @@ elfNN_aarch64_final_link_relocate (reloc_howto_type *howto,
     case BFD_RELOC_AARCH64_32:
 #endif
     case BFD_RELOC_AARCH64_ADD_LO12:
-    case BFD_RELOC_AARCH64_BRANCH19:
     case BFD_RELOC_AARCH64_LDST128_LO12:
     case BFD_RELOC_AARCH64_LDST16_LO12:
     case BFD_RELOC_AARCH64_LDST32_LO12:
@@ -5959,7 +5972,6 @@ elfNN_aarch64_final_link_relocate (reloc_howto_type *howto,
     case BFD_RELOC_AARCH64_MOVW_G2_NC:
     case BFD_RELOC_AARCH64_MOVW_G2_S:
     case BFD_RELOC_AARCH64_MOVW_G3:
-    case BFD_RELOC_AARCH64_TSTBR14:
       value = _bfd_aarch64_elf_resolve_relocation (input_bfd, bfd_r_type,
 						   place, value,
 						   signed_addend, weak_undef_p);
@@ -8022,6 +8034,8 @@ elfNN_aarch64_check_relocs (bfd *abfd, struct bfd_link_info *info,
 	    break;
 	  }
 
+	case BFD_RELOC_AARCH64_BRANCH19:
+	case BFD_RELOC_AARCH64_TSTBR14:
 	case BFD_RELOC_AARCH64_CALL26:
 	case BFD_RELOC_AARCH64_JUMP26:
 	  /* If this is a local symbol then we resolve it

ld/ChangeLog

@@ -1,3 +1,10 @@
+2020-05-19  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>
+
+	* testsuite/ld-aarch64/aarch64-elf.exp: New test
+	emit-relocs-560.
+	* testsuite/ld-aarch64/emit-relocs-560.d: New file.
+	* testsuite/ld-aarch64/emit-relocs-560.s: New file.
+
 2020-05-19  Alan Modra  <amodra@gmail.com>
 
 	* emultempl/beos.em (sort_by_file_name): Use bfd_get_filename

ld/testsuite/ld-aarch64/aarch64-elf.exp

@@ -236,6 +236,7 @@ run_dump_test_lp64 "emit-relocs-557"
 run_dump_test_lp64 "emit-relocs-558"
 run_dump_test_lp64 "emit-relocs-558-overflow"
 run_dump_test_lp64 "emit-relocs-559"
+run_dump_test_lp64 "emit-relocs-560"
 
 run_dump_test "reloc-overflow-bad"
 

ld/testsuite/ld-aarch64/emit-relocs-560.d

@@ -0,0 +1,8 @@
+#source: emit-relocs-560.s
+#ld: -shared
+#readelf: -r
+
+Relocation section '.rela.plt' at offset 0x[0-9a-f]+ contains 2 entries:
+  Offset          Info           Type           Sym. Value    Sym. Name \+ Addend
+[0-9a-f]+  000100000402 R_AARCH64_JUMP_SL 0000000000000000 baz \+ 0
+[0-9a-f]+  000200000402 R_AARCH64_JUMP_SL 0000000000000000 bar \+ 0

ld/testsuite/ld-aarch64/emit-relocs-560.s

@@ -0,0 +1,3 @@
+foo:
+	tbz	x0, 1, bar
+	cbnz	x1, baz