Make ./snabb binary more portable

This will enable us to release Snabb Switch as a single executable
file for any modern Linux/x86_64.

./snabb will link with any GLIBC version >= 2.7 (released 2007).  This
makes the binary linker-compatible with older Linux distributions such
as Ubuntu 10.04 and CentOS 6.

Tested when built on Ubuntu 14.04. Building on other distributions may
yield different results, but a selftest script is included to catch that.

(There may be other compatibility issues with older kernels and
CPUs. This only takes care of the dynamic linking.)

Makefile

@@ -5,7 +5,7 @@ COBJ   = $(CSRC:.c=.o)
 
 LUAJIT_O := deps/luajit/src/libluajit.a
 
-LUAJIT_CFLAGS := -DLUAJIT_USE_PERFTOOLS -DLUAJIT_USE_GDBJIT -DLUAJIT_NUMMODE=3
+LUAJIT_CFLAGS := -DLUAJIT_USE_PERFTOOLS -DLUAJIT_USE_GDBJIT -DLUAJIT_NUMMODE=3 -include $(CURDIR)/gcc-preinclude.h
 
 all: $(LUAJIT_O)
 	cd src && $(MAKE)

gcc-preinclude.h

@@ -0,0 +1,6 @@
+// Force an old symbol version on memcpy.
+// See: http://www.win.tue.nl/~aeb/linux/misc/gcc-semibug.html
+//      https://rjpower9000.wordpress.com/2012/04/09/fun-with-shared-libraries-version-glibc_2-14-not-found/
+__asm__(".symver memcpy,memcpy@GLIBC_2.2.5");
+
+

src/Makefile

@@ -45,7 +45,7 @@ snabb: $(LUAOBJ) $(HOBJ) $(COBJ) $(ASMOBJ)
 	$(E) "LINK      $@"
 	$(Q) gcc -Wl,--no-as-needed -Wl,-E -Werror -Wall -o $@ $^ \
 	    ../deps/luajit/src/libluajit.a \
-	    -lc -ldl -lm -lrt -lpthread
+	    -lrt -lc -ldl -lm -lpthread
 	@echo -n "Firmware: "
 	@ln -fs snabb snabbswitch
 	@ls -sh snabb
@@ -99,7 +99,7 @@ $(LUAOBJ): obj/%_lua.o: %.lua Makefile | $(OBJDIR)
 
 $(COBJ): obj/%_c.o: %.c $(CHDR) Makefile | $(OBJDIR)
 	$(E) "C         $@"
-	$(Q) gcc -Wl,-E -I ../deps/luajit/src -I . -c -Wall -Werror -o $@ $<
+	$(Q) gcc -Wl,-E -I ../deps/luajit/src -I . -include $(CURDIR)/../gcc-preinclude.h -c -Wall -Werror -o $@ $<
 
 $(HOBJ): obj/%_h.o: %.h Makefile | $(OBJDIR)
 	$(E) "H         $@"

src/selftest.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+echo "selftest: ./snabb binary portability"
+glibc=$(objdump -T snabb | grep GLIBC | sed -e 's/^.*GLIBC_//' -e 's/ .*//' | sort -nr | head -1)
+if [ "$glibc" == "2.7" ]; then
+    echo "ok: links with glibc >= 2.7"
+else
+    echo "error: requires glibc $glibc (> 2.7)"
+    exit 1
+fi