2 files changed, 12 insertions, 5 deletions

diff --git a/DESIGN b/DESIGN
index b6e2050..b25b37c 100644
--- a/DESIGN
+++ b/DESIGN
@@ -116,13 +116,20 @@ Build hardening the shipped binary
 ----------------------------------
 
 The production and debug binaries carry -fstack-protector-strong, PIE and
-_FORTIFY_SOURCE=2; a GNU-style linker also gets full RELRO. Measured cost on
+_FORTIFY_SOURCE=2; every linker but Apple's also gets full RELRO (-z relro,-z now). Measured cost on
 the announce hot path is 300 to 305 nanoseconds, under two per cent, and the
 individual flags sit inside the run-to-run noise: add_peer has few stack
 buffers, most buffer sizes are known at compile time so FORTIFY emits no
 runtime check, and PIE on arm64 is almost free. On x86-64 PIE costs a little
 more. End to end it is invisible, since a UDP announce is mostly kernel time.
 
+RELRO is gated on the operating system because Apple's linker rejects -z. The
+branch runs in the shell through a != assignment, the one form both bmake and
+GNU make 4 accept; $(shell) is GNU only and GNU make 3.81, which stock macOS
+ships, has neither. On that old make the != line is inert and leaves RELRO
+empty, which is the right answer for macOS anyway, so every platform resolves
+correctly without a make conditional.
+
 The flags are not in OT_CFLAGS, so the sanitizer test builds do not inherit
 them. AddressSanitizer checks harder than FORTIFY does, and the two clash on
 FreeBSD, where a FORTIFY plus ASan build aborted a multi hash scrape that the
diff --git a/Makefile b/Makefile
index eed77a0..7ed552c 100644
--- a/Makefile
+++ b/Makefile
@@ -46,9 +46,9 @@ OT_CFLAGS+=-Wall -Wextra -Wwrite-strings -pipe -pthread
 
 HARDENING=-fstack-protector-strong -fPIE -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2
 HARDENING_LD=-pie
-# Full RELRO is free at runtime and makes the GOT read-only, but Apple's linker
-# does not accept -z, so enable it for the GNU-style linkers (Linux, the BSDs):
-#HARDENING_LD+=-Wl,-z,relro,-z,now
+# Full RELRO on every linker that takes it, which is all but Apple's. See DESIGN.
+RELRO_LD!=      case `uname -s` in Darwin) ;; *) printf %s -Wl,-z,relro,-z,now ;; esac
+HARDENING_LD+=$(RELRO_LD)
 OT_LDFLAGS+=-pthread
 
 OPTS_debug=-D_DEBUG -g -ggdb
@@ -78,7 +78,7 @@ $(BINARY).debug: $(SOURCES) $(HEADERS) $(BUILDDEPS)
         $(CC) -o $@ $(OPTS_debug) $(COMPILE) $(HARDENING) $(SOURCES) $(LINK) $(HARDENING_LD)
 
 $(BUILDSTAMP): FORCE
-        @echo "$(CFLAGS) $(COMPILE) $(LINK)" | cmp -s - $@ 2>/dev/null || echo "$(CFLAGS) $(COMPILE) $(LINK)" > $@
+        @echo "$(CFLAGS) $(COMPILE) $(HARDENING) $(LINK) $(HARDENING_LD)" | cmp -s - $@ 2>/dev/null || echo "$(CFLAGS) $(COMPILE) $(HARDENING) $(LINK) $(HARDENING_LD)" > $@
 
 clean:
         rm -rf $(BINARIES) $(BINARIES:=.dSYM) $(BUILDSTAMP) *.o *~ src/*.o src/*~