diff --git a/lib/libc/x86/sys/Makefile.inc b/lib/libc/x86/sys/Makefile.inc
--- a/lib/libc/x86/sys/Makefile.inc
+++ b/lib/libc/x86/sys/Makefile.inc
@@ -12,3 +12,11 @@
 .if ${MACHINE_CPUARCH} == "amd64" && ${MK_HYPERV} != "no"
 CFLAGS+=	-DWANT_HYPERV
 .endif
+# We can't use sanitizer instrumentation on ifuncs called during sanitizer
+# runtime startup.
+.if ${MK_ASAN} != "no"
+CFLAGS.__vdso_gettc.c+=-fno-sanitize=address
+.endif
+.if ${MK_UBSAN} != "no"
+CFLAGS.__vdso_gettc.c+=-fno-sanitize=undefined
+.endif