debug/elf: change R_ARM_REL32 to R_ARM_ABS32 in applyRelocationsARM

The original version of applyRelocationsARM was added in
http://golang.org/cl/7266.  It was added to fix the ARM build, which
had been broken by http://golang.org/cl/6780.

Before CL 6780, there was no relocation processing for ARM.  CL 6780
changed the code to require relocation processing for every supported
target.  CL 7266 fixed the ARM build by adding a relocation processing
function, but in fact no actual processing was done.  The code only
looked for REL32 relocations, but ARM debug info has no such
relocations.  The test case added in CL 7266 doesn't have any either.

This didn't matter because no relocation processing was required on
ARM, at least not for GCC-generated debug info.  GCC generates ABS32
relocations, but only against section symbols which have the value 0.
Therefore, the addition done by correct handling of ABS32 doesn't
change anything.

Clang, however, generates ABS32 relocations against local symbols,
some of which have non-zero values.  For those, we need to handle
ABS32 relocations.

This patch corrects the CL 7266 to look for ABS32 relocations instead
of REL32 relocations.  The code was already written to correctly
handle ABS32 relocations, it just mistakenly said REL32.

This is the ARM equivalent of https://golang.org/cl/96680045, which
fixed the same problem in the same way for clang on 386.

With this patch, clang-3.5 can be used to build Go on ARM GNU/Linux.

Fixes #8980.

Change-Id: I0c2d72eadfe6373bde99cd03eee40de6a582dda1
Reviewed-on: https://go-review.googlesource.com/11222Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/debug/elf/file.go

@@ -653,7 +653,7 @@ func (f *File) applyRelocationsARM(dst []byte, rels []byte) error {
 		sym := &symbols[symNo-1]
 
 		switch t {
-		case R_ARM_REL32:
+		case R_ARM_ABS32:
 			if rel.Off+4 >= uint32(len(dst)) {
 				continue
 			}

src/debug/elf/file_test.go

@@ -272,6 +272,12 @@ var relocationTests = []relocationTest{
 			{0, &dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "GNU C 4.9.2 20141224 (prerelease) -march=armv7-a -mfloat-abi=hard -mfpu=vfpv3-d16 -mtls-dialect=gnu -g", Class: dwarf.ClassString}, {Attr: dwarf.AttrLanguage, Val: int64(1), Class: dwarf.ClassConstant}, {Attr: dwarf.AttrName, Val: "go-relocation-test-gcc492.c", Class: dwarf.ClassString}, {Attr: dwarf.AttrCompDir, Val: "/root/go/src/debug/elf/testdata", Class: dwarf.ClassString}, {Attr: dwarf.AttrLowpc, Val: uint64(0x0), Class: dwarf.ClassAddress}, {Attr: dwarf.AttrHighpc, Val: int64(0x28), Class: dwarf.ClassConstant}, {Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr}}}},
 		},
 	},
+	{
+		"testdata/go-relocation-test-clang-arm.obj",
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{dwarf.Field{Attr: dwarf.AttrProducer, Val: "Debian clang version 3.5.0-10 (tags/RELEASE_350/final) (based on LLVM 3.5.0)", Class: dwarf.ClassString}, dwarf.Field{Attr: dwarf.AttrLanguage, Val: int64(12), Class: dwarf.ClassConstant}, dwarf.Field{Attr: dwarf.AttrName, Val: "hello.c", Class: dwarf.ClassString}, dwarf.Field{Attr: dwarf.AttrStmtList, Val: int64(0x0), Class: dwarf.ClassLinePtr}, dwarf.Field{Attr: dwarf.AttrCompDir, Val: "/tmp", Class: dwarf.ClassString}, dwarf.Field{Attr: dwarf.AttrLowpc, Val: uint64(0x0), Class: dwarf.ClassAddress}, dwarf.Field{Attr: dwarf.AttrHighpc, Val: int64(48), Class: dwarf.ClassConstant}}}},
+		},
+	},
 	{
 		"testdata/go-relocation-test-gcc5-ppc.obj",
 		[]relocationTestEntry{