Add an ELF loader that can load and run an executable in the sandbox

makefile

 CFLAGS = -g -O0 -Wall -Werror -Wextra -Wno-missing-field-initializers         \
          -Wno-unused-parameter -I.
 LDFLAGS = -g
 CPPFLAGS =
 DEPFLAGS = -MMD -MF $@.d
 MODS := allocator preload library debug maps x86_decode securemem sandbox     \
         syscall_entrypoint system_call_table                                  \
         trusted_thread trusted_thread_asm trusted_process                     \
         access exit fault_handler_asm clone                                   \
         getpid gettid ioctl ipc madvise mmap mprotect                         \
         munmap open prctl reference_trusted_thread sigaction sigprocmask      \
         socketcall stat tls_setup tls_setup_helper
 OBJS64 := $(shell echo ${MODS} | xargs -n 1 | sed -e 's/$$/.o64/')
 OBJS32 := $(shell echo ${MODS} | xargs -n 1 | sed -e 's/$$/.o32/')
 ALL_OBJS = $(OBJS32) $(OBJS64) tests/test_syscalls.o64 tests/test_syscalls.o32 \
            tests/clone_test_helper.o64 tests/clone_test_helper.o32 \
            timestats.o playground.o
 DEP_FILES = $(wildcard $(foreach f,$(ALL_OBJS),$(f).d))
 
 include $(DEP_FILES)
 
 .SUFFIXES: .o64 .o32
 
-all: testbin timestats demo
+all: testbin timestats demo elf_loader_32 elf_loader_64
 
 clean:
         -rm -f playground playground.o
         -rm -f $(ALL_OBJS)
         -rm -f $(DEP_FILES)
         -rm -f preload64.so
         -rm -f preload32.so
         -rm -f testbin64 testbin.o64
         -rm -f testbin32 testbin.o32
         -rm -f timestats timestats.o
@@ skipping 10 matching lines @@
 tests/test_syscalls.o64 tests/test_syscalls.o32: tests/test-list.h
 
 tests/test-list.h: tests/list_tests.py tests/test_syscalls.cc
         python tests/list_tests.py tests/test_syscalls.cc > $@
 
 run_tests_64: $(OBJS64) tests/test_syscalls.o64 tests/clone_test_helper.o64
         g++ -m64 $^ -lpthread -lutil -o $@
 run_tests_32: $(OBJS32) tests/test_syscalls.o32 tests/clone_test_helper.o32
         g++ -m32 $^ -lpthread -lutil -o $@
 
+# Link these as PIEs so that they stay out of the way of any
+# fixed-position executable that gets loaded later.
+elf_loader_64: elf_loader.o64 $(OBJS64)
+        g++ -pie -m64 $^ -o $@
+elf_loader_32: elf_loader.o32 $(OBJS32)
+        g++ -pie -m32 $^ -o $@
+
 demo: playground preload32.so preload64.so
         ./playground /bin/ls $(HOME)
 
 testbin: testbin32 testbin64
 
 gdb: testbin64
         gdb $<
 
 valgrind: testbin64
         valgrind --db-attach=yes ./$<
@@ skipping 39 matching lines @@
 
 .cc.o32:
         ${CXX} ${CFLAGS} ${CPPFLAGS} ${DEPFLAGS} -m32 -fPIC -c -o $@ $<
 
 .c.o32:
         ${CC} ${CFLAGS} ${CPPFLAGS} ${DEPFLAGS} -m32 --std=gnu99 -fPIC \
                 -c -o $@ $<
 
 .S.o32:
         ${CC} ${CFLAGS} ${CPPFLAGS} ${DEPFLAGS} -m32 -c -o $@ $<