objtool: Allow !PC relative relocations

Objtool doesn't currently much like per-cpu usage in alternatives:

arch/x86/entry/entry_64.o: warning: objtool: .altinstr_replacement+0xf: unsupported relocation in alternatives section
  f:   65 c7 04 25 00 00 00 00 00 00 00 80     movl   $0x80000000,%gs:0x0      13: R_X86_64_32S        __x86_call_depth

Since the R_X86_64_32S relocation is location invariant (it's
computation doesn't include P - the address of the location itself),
it can be trivially allowed.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111145.806607235@infradead.org

tools/objtool/arch/x86/decode.c

@@ -73,6 +73,30 @@ unsigned long arch_jump_destination(struct instruction *insn)
 	return insn->offset + insn->len + insn->immediate;
 }
 
+bool arch_pc_relative_reloc(struct reloc *reloc)
+{
+	/*
+	 * All relocation types where P (the address of the target)
+	 * is included in the computation.
+	 */
+	switch (reloc->type) {
+	case R_X86_64_PC8:
+	case R_X86_64_PC16:
+	case R_X86_64_PC32:
+	case R_X86_64_PC64:
+
+	case R_X86_64_PLT32:
+	case R_X86_64_GOTPC32:
+	case R_X86_64_GOTPCREL:
+		return true;
+
+	default:
+		break;
+	}
+
+	return false;
+}
+
 #define ADD_OP(op) \
 	if (!(op = calloc(1, sizeof(*op)))) \
 		return -1; \

tools/objtool/check.c

@@ -1645,7 +1645,7 @@ static int handle_group_alt(struct objtool_file *file,
 		 * accordingly.
 		 */
 		alt_reloc = insn_reloc(file, insn);
-		if (alt_reloc &&
+		if (alt_reloc && arch_pc_relative_reloc(alt_reloc) &&
 		    !arch_support_alt_relocation(special_alt, insn, alt_reloc)) {
 
 			WARN_FUNC("unsupported relocation in alternatives section",

tools/objtool/include/objtool/arch.h

@@ -93,4 +93,6 @@ bool arch_is_rethunk(struct symbol *sym);
 
 int arch_rewrite_retpolines(struct objtool_file *file);
 
+bool arch_pc_relative_reloc(struct reloc *reloc);
+
 #endif /* _ARCH_H */