objtool/x86: allow syscall instruction

The syscall instruction is used in Xen PV mode for doing hypercalls.
Allow syscall to be used in the kernel in case it is tagged with an
unwind hint for objtool.

This is part of XSA-466 / CVE-2024-53241.

Reported-by: Andrew Cooper <[e-mail]>
Signed-off-by: Juergen Gross <[e-mail]>
Co-developed-by: Peter Zijlstra <[e-mail]>

tools/objtool/check.c

···
 			break;
 
 		case INSN_CONTEXT_SWITCH:
-			if (func && (!next_insn || !next_insn->hint)) {
-				WARN_INSN(insn, "unsupported instruction in callable function");
-				return 1;
+			if (func) {
+				if (!next_insn || !next_insn->hint) {
+					WARN_INSN(insn, "unsupported instruction in callable function");
+					return 1;
+				}
+				break;
 			}
 			return 0;