sysprof: replace backtrace with libunwind

`backtrace` fails to unwind the host stack in LuaJIT since there
are no frame pointers during the VM calls, even with the
`-fno-omit-frame-pointer` option set. Sometimes, those failures
cause crashes. Moreover, `backtrace` is not signal-safe, which is
vital for the sysprof. This commit replaces it with the
libunwind-based unwinder, which makes use of additional runtime
information to provide robust unwinding even if there are no frame
pointers and is signal-safe. Because Tarantool uses its own fiber
unwinder, the new default unwinder is not built into bundled
builds.

Also, this commit enables C API tests, which used to crash with
`backtrace`, and disables an assertion regarding the successful
backtracer function setting, as it is unnecessary.

Part of tarantool/tarantool#781

CMakeLists.txt

@@ -97,7 +97,10 @@ endif()
 # tracebacks and unwinding are not affected -- the assembler part
 # has frame unwind information and GCC emits it where needed (x64)
 # or with -g.
+# The '-fno-omit-frame-pointer` is set later, depending on sysprof
+# and libunwind support.
 AppendFlags(CMAKE_C_FLAGS -fomit-frame-pointer -fno-stack-protector)
+AppendFlags(CMAKE_C_FLAGS -fno-stack-protector)
 
 # Redefined to benefit from expanding macros in gdb.
 set(CMAKE_C_FLAGS_DEBUG "-g -ggdb3")
@@ -212,6 +215,21 @@ endif()
 option(LUAJIT_DISABLE_SYSPROF "LuaJIT platform and Lua profiler support" OFF)
 if(LUAJIT_DISABLE_SYSPROF)
   AppendFlags(TARGET_C_FLAGS -DLUAJIT_DISABLE_SYSPROF)
+else()
+  if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND
+    (
+      CMAKE_SYSTEM_PROCESSOR STREQUAL "i386" OR
+      CMAKE_SYSTEM_PROCESSOR STREQUAL "i686" OR
+      CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64"
+    )
+  )
+    AppendFlags(CMAKE_C_FLAGS -fno-omit-frame-pointer -fasynchronous-unwind-tables)
+    if(ENABLE_BUNDLED_LUAJIT)
+        AppendFlags(TARGET_C_FLAGS -DLUAJIT_EXTERNAL_SYSPROF_BACKTRACER)
+    else()
+        find_package(LibUnwind MODULE REQUIRED)
+    endif()
+  endif()
 endif()
 
 # Switch to harder (and slower) hash function when a collision

cmake/FindLibUnwind.cmake

@@ -0,0 +1,43 @@
+#[========================================================================[.rst:
+FindLibUnwind
+--------
+Finds the libunwind library.
+
+Result Variables
+^^^^^^^^^^^^^^^^
+``LIBUNWIND_FOUND``
+  True if the system has the libunwind library.
+``LIBUNWIND_INCLUDE_DIR``
+  Include directory needed to use libunwind.
+``LIBUNWIND_LIBRARIES``
+  Libraries needed to link to libunwind.
+
+Cache Variables
+^^^^^^^^^^^^^^^
+``LIBUNWIND_INCLUDE_DIR``
+  The directory containing ``libunwind.h``.
+``LIBUNWIND_LIBRARIES``
+  The paths to the libunwind libraries.
+#]========================================================================]
+
+include(FindPackageHandleStandardArgs)
+
+find_package(PkgConfig QUIET)
+pkg_check_modules(PC_LIBUNWIND QUIET libunwind)
+
+find_path(LIBUNWIND_INCLUDE_DIR libunwind.h ${PC_LIBUNWIND_INCLUDE_DIRS})
+if(LIBUNWIND_INCLUDE_DIR)
+    include_directories(${LIBUNWIND_INCLUDE_DIR})
+endif()
+
+find_library(LIBUNWIND_LIBRARY NAMES unwind PATHS ${PC_LIBUNWIND_LIBRARY_DIRS})
+
+set(LIBUNWIND_PLATFORM_LIBRARY_NAME "unwind-${CMAKE_SYSTEM_PROCESSOR}")
+find_library(LIBUNWIND_PLATFORM_LIBRARY ${LIBUNWIND_PLATFORM_LIBRARY_NAME}
+             ${PC_LIBUNWIND_LIBRARY_DIRS})
+set(LIBUNWIND_LIBRARIES ${LIBUNWIND_LIBRARY} ${LIBUNWIND_PLATFORM_LIBRARY})
+
+find_package_handle_standard_args(LibUnwind
+      REQUIRED_VARS LIBUNWIND_INCLUDE_DIR LIBUNWIND_LIBRARIES)
+
+mark_as_advanced(LIBUNWIND_INCLUDE_DIR LIBUNWIND_LIBRARIES)

src/CMakeLists.txt

@@ -284,6 +284,10 @@ add_dependencies(core_shared buildvm_output)
 
 list(APPEND TARGET_LIBS m)
 
+if(NOT LUAJIT_DISABLE_SYSPROF)
+  list(APPEND TARGET_LIBS ${LIBUNWIND_LIBRARIES})
+endif()
+
 set(LIB_OBJECTS_STATIC
   $<TARGET_OBJECTS:vm_static>
   $<TARGET_OBJECTS:core_static>

src/Makefile.original

@@ -292,6 +292,18 @@ ifneq (,$(findstring LJ_TARGET_PS3 1,$(TARGET_TESTARCH)))
   TARGET_XLIBS+= -lpthread
 endif
 
+ifneq (,$(findstring LJ_HASSYSPROF 1,$(TARGET_TESTARCH)))
+  # Try to link against libunwind to determine whether it
+  # is present in the system.
+  HAS_LIBUNWIND=$(shell $(TARGET_LD) -lunwind -lunwind-x86_64 -c 2>/dev/null; echo $$?)
+ifneq (,$(HAS_LIBUNWIND))
+  $(error libunwind is required for sysprof)
+else
+  TARGET_XLIBS+= -lunwind -lunwind-x86_64
+  TARGET_XCFLAGS+= -fno-omit-frame-pointer -fasynchronous-unwind-tables
+endif
+endif
+
 TARGET_XCFLAGS+= $(CCOPT_$(TARGET_LJARCH))
 TARGET_ARCH+= $(patsubst %,-DLUAJIT_TARGET=LUAJIT_ARCH_%,$(TARGET_LJARCH))
 

src/lj_sysprof.c

@@ -26,7 +26,17 @@
 
 #include <pthread.h>
 #include <errno.h>
-#include <execinfo.h>
+
+#ifndef LUAJIT_EXTERNAL_SYSPROF_BACKTRACER
+/*
+** We only need local unwinding, then a special implementation
+** can be selected which may run much faster than the generic
+** implementation which supports both kinds of unwinding, local
+** and remote.
+*/
+#define UNW_LOCAL_ONLY
+#include <libunwind.h>
+#endif
 
 /*
 ** Number of profiler frames we need to omit during stack
@@ -85,6 +95,56 @@ static struct sysprof sysprof = {0};
 
 /* --- Stream ------------------------------------------------------------- */
 
+#ifndef LUAJIT_EXTERNAL_SYSPROF_BACKTRACER
+
+static ssize_t collect_stack(void **buffer, int size)
+{
+  int frame_no = 0;
+  unw_context_t unw_ctx;
+  unw_cursor_t unw_cur;
+
+  int rc = unw_getcontext(&unw_ctx);
+  if (rc != 0)
+    return -1;
+
+  rc = unw_init_local(&unw_cur, &unw_ctx);
+  if (rc != 0)
+    return -1;
+
+  for (; frame_no < size; ++frame_no) {
+    unw_word_t ip;
+    rc = unw_get_reg(&unw_cur, UNW_REG_IP, &ip);
+    if (rc != 0)
+      return -1;
+
+    buffer[frame_no] = (void *)ip;
+    rc = unw_step(&unw_cur);
+    if (rc <= 0)
+      break;
+  }
+  return frame_no;
+}
+
+static void default_backtrace_host(void *(writer)(int frame_no, void *addr))
+{
+  static void *backtrace_buf[SYSPROF_BACKTRACE_FRAME_MAX] = {};
+
+  struct sysprof *sp = &sysprof;
+  int max_depth = sp->opt.mode == LUAM_SYSPROF_LEAF
+                  ? SYSPROF_HANDLER_STACK_DEPTH + 1
+                  : SYSPROF_BACKTRACE_FRAME_MAX;
+  const int depth = collect_stack(backtrace_buf, max_depth);
+  int level;
+
+  lj_assertX(depth <= max_depth, "backtrace is too deep");
+  lj_assertX(depth != -1, "failed to collect backtrace");
+  for (level = SYSPROF_HANDLER_STACK_DEPTH; level < depth; ++level) {
+    if (!writer(level - SYSPROF_HANDLER_STACK_DEPTH + 1, backtrace_buf[level]))
+      return;
+  }
+}
+#endif
+
 static const uint8_t ljp_header[] = {'l', 'j', 'p', LJP_FORMAT_VERSION,
                                       0x0, 0x0, 0x0};
 
@@ -205,24 +265,6 @@ static void *stream_frame_host(int frame_no, void *addr)
   return addr;
 }
 
-static void default_backtrace_host(void *(writer)(int frame_no, void *addr))
-{
-  static void *backtrace_buf[SYSPROF_BACKTRACE_FRAME_MAX] = {};
-
-  struct sysprof *sp = &sysprof;
-  int max_depth = sp->opt.mode == LUAM_SYSPROF_LEAF
-                  ? SYSPROF_HANDLER_STACK_DEPTH + 1
-                  : SYSPROF_BACKTRACE_FRAME_MAX;
-  const int depth = backtrace(backtrace_buf, max_depth);
-  int level;
-
-  lj_assertX(depth <= max_depth, "depth of C stack is too big");
-  for (level = SYSPROF_HANDLER_STACK_DEPTH; level < depth; ++level) {
-    if (!writer(level - SYSPROF_HANDLER_STACK_DEPTH + 1, backtrace_buf[level]))
-      return;
-  }
-}
-
 static void stream_backtrace_host(struct sysprof *sp)
 {
   lj_assertX(sp->backtracer != NULL, "uninitialized sysprof backtracer");
@@ -427,20 +469,16 @@ int lj_sysprof_set_backtracer(luam_Sysprof_backtracer backtracer) {
 
   if (sp->state != SPS_IDLE)
     return PROFILE_ERRUSE;
-  if (backtracer == NULL) {
+
+  if (backtracer == NULL)
+#ifndef LUAJIT_EXTERNAL_SYSPROF_BACKTRACER
     sp->backtracer = default_backtrace_host;
-    /*
-    ** XXX: `backtrace` is not signal-safe, according to man,
-    ** because it is lazy loaded on the first call, which triggers
-    ** allocations. We need to call `backtrace` before starting profiling
-    ** to avoid lazy loading.
-    */
-    void *dummy = NULL;
-    backtrace(&dummy, 1);
-  }
-  else {
+#else
+    return PROFILE_ERRUSE;
+#endif
+  else
     sp->backtracer = backtracer;
-  }
+
   if (!is_unconfigured(sp)) {
     sp->state = SPS_IDLE;
   }

test/tarantool-c-tests/misclib-sysprof-capi.test.c

@@ -121,13 +121,6 @@ static int validation(void *test_state)
 	return TEST_EXIT_SUCCESS;
 }
 
-/*
- * FIXME: The following two tests are disabled because sometimes
- * `backtrace` dynamically loads a platform-specific unwinder,
- * which is not signal-safe.
- */
-
-#if 0
 /*
  * Structure given as ctx to sysprof writer and on_stop callback.
  */
@@ -239,7 +232,6 @@ static int check_profile_func(lua_State *L)
 		    == PROFILE_SUCCESS);
 	assert_true(luaM_sysprof_set_on_stop(on_stop_cb_default)
 		    == PROFILE_SUCCESS);
-	assert_true(luaM_sysprof_set_backtracer(NULL) == PROFILE_SUCCESS);
 
 	status = luaM_sysprof_start(L, &opt);
 	assert_true(status == PROFILE_SUCCESS);
@@ -270,33 +262,24 @@ static int check_profile_func(lua_State *L)
 
 	return TEST_EXIT_SUCCESS;
 }
-#endif
 
 static int profile_func_jitoff(void *test_state)
 {
-	UNUSED(test_state);
-	return todo("Need to replace backtrace with libunwind first");
-#if 0
 	lua_State *L = test_state;
 	utils_get_aux_lfunc(L);
 	(void)luaJIT_setmode(L, 0, LUAJIT_MODE_ENGINE | LUAJIT_MODE_OFF);
 	(void)luaJIT_setmode(L, 0, LUAJIT_MODE_ENGINE | LUAJIT_MODE_FLUSH);
 	check_profile_func(L);
 	(void)luaJIT_setmode(L, 0, LUAJIT_MODE_ENGINE | LUAJIT_MODE_ON);
 	return TEST_EXIT_SUCCESS;
-#endif
 }
 
 static int profile_func_jiton(void *test_state)
 {
-	UNUSED(test_state);
-	return todo("Need to replace backtrace with libunwind first");
-#if 0
 	lua_State *L = test_state;
 	utils_get_aux_lfunc(L);
 	check_profile_func(L);
 	return TEST_EXIT_SUCCESS;
-#endif
 }
 
 int main(void)