x86, relocs: ignore R_386_NONE in kernel relocation entries

For relocatable 32bit kernels, boot/compressed/relocs.c processes
relocation entries in the kernel image and appends it to the kernel
image such that boot/compressed/head_32.S can relocate the kernel.
The kernel image is one statically linked object and only uses two
relocation types - R_386_PC32 and R_386_32, of the two only the latter
needs massaging during kernel relocation and thus handled by relocs.
R_386_PC32 is ignored and all other relocation types are considered
error.

When the target of a relocation resides in a discarded section,
binutils doesn't throw away the relocation record but nullifies it by
changing it to R_386_NONE, which unfortunately makes relocs fail.

The problem was triggered by yet out-of-tree x86 stack unwind patches
but given the binutils behavior, ignoring R_386_NONE is the right
thing to do.

The problem has been tracked down to binutils behavior by Jan Beulich.

[ Impact: fix build with certain binutils by ignoring R_386_NONE ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jan Beulich <JBeulich@novell.com>
Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <4A1B8150.40702@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/boot/compressed/relocs.c

@@ -504,8 +504,11 @@ static void walk_relocs(void (*visit)(Elf32_Rel *rel, Elf32_Sym *sym))
 			if (sym->st_shndx == SHN_ABS) {
 				continue;
 			}
-			if (r_type == R_386_PC32) {
-				/* PC relative relocations don't need to be adjusted */
+			if (r_type == R_386_NONE || r_type == R_386_PC32) {
+				/*
+				 * NONE can be ignored and and PC relative
+				 * relocations don't need to be adjusted.
+				 */
 			}
 			else if (r_type == R_386_32) {
 				/* Visit relocations that need to be adjusted */