1 files changed, 27 insertions, 6 deletions

diff --git a/src/lib/libcrypto/bn/s2n_bignum_internal.h b/src/lib/libcrypto/bn/s2n_bignum_internal.h
index b82db7d019..4217ca95af 100644
--- a/src/lib/libcrypto/bn/s2n_bignum_internal.h
+++ b/src/lib/libcrypto/bn/s2n_bignum_internal.h
@@ -14,14 +14,14 @@
 
 #ifdef __APPLE__
 #   define S2N_BN_SYMBOL(NAME) _##NAME
+#   if defined(__AARCH64EL__) || defined(__ARMEL__)
+#     define __LF %%
+#   else
+#     define __LF ;
+#   endif
 #else
 #   define S2N_BN_SYMBOL(name) name
-#endif
-
-#ifdef __CET__
-#   include <cet.h>
-#else
-#   define _CET_ENDBR
+#   define __LF ;
 #endif
 
 #define S2N_BN_SYM_VISIBILITY_DIRECTIVE(name) .globl S2N_BN_SYMBOL(name)
@@ -34,3 +34,24 @@
 #else
 #   define S2N_BN_SYM_PRIVACY_DIRECTIVE(name)  /* NO-OP: S2N_BN_SYM_PRIVACY_DIRECTIVE */
 #endif
+
+// Enable indirect branch tracking support unless explicitly disabled
+// with -DNO_IBT. If the platform supports CET, simply inherit this from
+// the usual header. Otherwise manually define _CET_ENDBR, used at each
+// x86 entry point, to be the ENDBR64 instruction, with an explicit byte
+// sequence for compilers/assemblers that don't know about it. Note that
+// it is safe to use ENDBR64 on all platforms, since the encoding is by
+// design interpreted as a NOP on all pre-CET x86_64 processors. The only
+// downside is a small increase in code size and potentially a modest
+// slowdown from executing one more instruction.
+
+#if NO_IBT
+#   if defined(_CET_ENDBR)
+#     error "The s2n-bignum build option NO_IBT was configured, but _CET_ENDBR is defined in this compilation unit. That is weird, so failing the build."
+#   endif
+#   define _CET_ENDBR
+#elif defined(__CET__)
+#   include <cet.h>
+#elif !defined(_CET_ENDBR)
+#   define _CET_ENDBR .byte 0xf3,0x0f,0x1e,0xfa
+#endif