Re: RFC: Should AArch64 *_NC relocs complain on overflow ?

[Nick Clifton <nickc at redhat dot com>]

Hi Marcus, Hi Jiong,

> NC relocations do complain if the relocation requires some minimum alignment 
> and the final relocated value does not meet that requirement.  Consider, for 
> example, instructions with an implicit scaling of an address offset, 
> relocations used for those instruction, including the NC form will gripe if 
> the offset being inserted into the instruction is not a multiple of the 
> implicit scaling implied.

OK that makes sense, 

It would be nice if we could generate a more helpful warning message for the user.
What do you think of a patch like the one below ?  With this applied we get output 
like the following:

1.c:(.text+0x4): relocation truncated to fit: R_AARCH64_LDST32_ABS_LO12_NC against symbol `var_2' defined in COMMON section in tmpdir/dump1.o
1.c:(.text+0x4): warning: Possibly the symbol is being referenced via a differently aligned type to its declaration>, no expected output

Cheers
  Nick

diff --git a/bfd/elfnn-aarch64.c b/bfd/elfnn-aarch64.c
index 292470df..be0ddb5 100644
--- a/bfd/elfnn-aarch64.c
+++ b/bfd/elfnn-aarch64.c
@@ -6405,10 +6405,6 @@ elfNN_aarch64_relocate_section (bfd *output_bfd,
 	  break;
 	}
 
-      if (!save_addend)
-	addend = 0;
-
-
       /* Dynamic relocs are not propagated for SEC_DEBUGGING sections
          because such sections are not SEC_ALLOC and thus ld.so will
          not process them.  */
@@ -6448,6 +6444,31 @@ elfNN_aarch64_relocate_section (bfd *output_bfd,
 		     name, input_bfd, input_section, rel->r_offset);
 		  return FALSE;
 		}
+	      /* Overflow can occur when a variable is referenced with a type
+		 that has a larger alignment than the type with which it was
+		 declared. eg:
+		   file1.c: extern int foo; int a (void) { return foo; }
+		   file2.c: char bar, foo, baz;
+		 If the variable is placed into a data section at an offset
+		 that is incompatible with the larger alignment requirement
+		 overflow will occur.  (Strictly speaking this is not overflow
+		 but rather an alignment problem, but the bfd_reloc_ error
+		 enum does not have a value to cover that situation).
+
+		 Try to catch this situation here and provide a more helpful
+		 error message to the user.  */
+	      if (addend & ((1 << howto->rightshift) - 1)
+		  /* FIXME: Are we testing all of the appropriate reloc
+		     types here ?  */
+		  && (real_r_type == BFD_RELOC_AARCH64_LDST16_LO12
+		      || real_r_type == BFD_RELOC_AARCH64_LDST32_LO12
+		      || real_r_type == BFD_RELOC_AARCH64_LDST64_LO12
+		      || real_r_type == BFD_RELOC_AARCH64_LDST128_LO12))
+		{
+		  info->callbacks->warning
+		    (info, _("Possibly the symbol is being referenced via a differently aligned type to its declaration"),
+		     name, input_bfd, input_section, rel->r_offset);
+		}
 	      break;
 
 	    case bfd_reloc_undefined:
@@ -6482,6 +6503,9 @@ elfNN_aarch64_relocate_section (bfd *output_bfd,
 	      break;
 	    }
 	}
+
+      if (!save_addend)
+	addend = 0;
     }
 
   return TRUE;
diff --git a/ld/testsuite/ld-aarch64/aarch64-elf.exp b/ld/testsuite/ld-aarch64/aarch64-elf.exp
index 939539e..d0b33cf 100644
--- a/ld/testsuite/ld-aarch64/aarch64-elf.exp
+++ b/ld/testsuite/ld-aarch64/aarch64-elf.exp
@@ -159,6 +159,8 @@ run_dump_test "emit-relocs-537"
 run_dump_test "emit-relocs-537-overflow"
 run_dump_test "emit-relocs-538"
 
+run_dump_test "reloc-overflow-bad"
+
 # test addend correctness when --emit-relocs specified for non-relocatable obj.
 run_dump_test "emit-relocs-local-addend"
 # test addend correctness when -r specified.
--- /dev/null	2016-02-08 07:59:08.480962732 +0000
+++ ld/testsuite/ld-aarch64/reloc-overflow-bad.d	2016-02-08 12:34:55.696124085 +0000
@@ -0,0 +1,7 @@
+#source: reloc-overflow-1.s
+#source: reloc-overflow-2.s
+#ld: -e0
+#error: .*Possibly the symbol.*
+
+
+
--- /dev/null	2016-02-08 07:59:08.480962732 +0000
+++ ld/testsuite/ld-aarch64/reloc-overflow-1.s	2016-02-08 12:30:59.584834873 +0000
@@ -0,0 +1,14 @@
+        .file   "1.c"
+        .text
+        .align  2
+        .p2align 3,,7
+        .global dec
+        .arch armv8-a+fp+simd
+        //.tune generic
+        .type   dec, %function
+dec:
+        adrp    x0, var_2
+        ldr     w0, [x0, #:lo12:var_2]
+        ret
+        .size   dec, .-dec
+        .ident  "GCC: (GNU) 6.0.0 20160208 (experimental) [trunk revision 233206]"
--- /dev/null	2016-02-08 07:59:08.480962732 +0000
+++ ld/testsuite/ld-aarch64/reloc-overflow-2.s	2016-02-08 12:31:36.679037435 +0000
@@ -0,0 +1,5 @@
+        .file   "2.c"
+        .comm   var_3,1,1
+        .comm   var_2,1,1
+        .comm   var_1,1,1
+        .ident  "GCC: (GNU) 6.0.0 20160208 (experimental) [trunk revision 233206]"