diff --git a/lib/libc/x86/sys/Makefile.inc b/lib/libc/x86/sys/Makefile.inc
index 41e0d1a8ad66..eff3a1085ec6 100644
--- a/lib/libc/x86/sys/Makefile.inc
+++ b/lib/libc/x86/sys/Makefile.inc
@@ -1,14 +1,22 @@
 # $FreeBSD$
 
 .PATH:	${LIBC_SRCTOP}/x86/sys
 
 SRCS+= \
 	__vdso_gettc.c \
 	pkru.c
 
 MAN+=	\
 	pkru.3
 
 .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