autotools: add -Wimplicit-fallthrough

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

CODING_STYLE.md

@@ -633,3 +633,32 @@ string.
 Unless you have a valid reason to accept truncation you must check whether
 truncation has occurred, treat it as an error, and handle the error
 appropriately.
+
+## 25) Use `__fallthrough__` in switch statements
+
+If LXC detects that the compiler is new enough it will tell it to check
+`switch` statements for non-documented fallthroughs. Please always place
+a `__fallthrough__` after a `case` which falls through the next one.
+
+```c
+int lxc_attach_run_command(void *payload)
+{
+	int ret = -1;
+	lxc_attach_command_t *cmd = payload;
+
+	ret = execvp(cmd->program, cmd->argv);
+	if (ret < 0) {
+		switch (errno) {
+		case ENOEXEC:
+			ret = 126;
+			break;
+		case ENOENT:
+			ret = 127;
+			break;
+		}
+	}
+
+	SYSERROR("Failed to exec \"%s\"", cmd->program);
+	return ret;
+}
+```

config/ax_check_compile_flag.m4

@@ -0,0 +1,53 @@
+# ===========================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])
+#
+# DESCRIPTION
+#
+#   Check whether the given FLAG works with the current language's compiler
+#   or gives an error.  (Warnings, however, are ignored)
+#
+#   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
+#   success/failure.
+#
+#   If EXTRA-FLAGS is defined, it is added to the current language's default
+#   flags (e.g. CFLAGS) when the check is done.  The check is thus made with
+#   the flags: "CFLAGS EXTRA-FLAGS FLAG".  This can for example be used to
+#   force the compiler to issue an error when a bad flag is given.
+#
+#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
+#
+#   NOTE: Implementation based on AX_CFLAGS_GCC_OPTION. Please keep this
+#   macro in sync with AX_CHECK_{PREPROC,LINK}_FLAG.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Guido U. Draheim <guidod@gmx.de>
+#   Copyright (c) 2011 Maarten Bosmans <mkbosmans@gmail.com>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 6
+
+AC_DEFUN([AX_CHECK_COMPILE_FLAG],
+[AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
+AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
+AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
+  ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
+  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
+  AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
+    [AS_VAR_SET(CACHEVAR,[yes])],
+    [AS_VAR_SET(CACHEVAR,[no])])
+  _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
+AS_VAR_IF(CACHEVAR,yes,
+  [m4_default([$2], :)],
+  [m4_default([$3], :)])
+AS_VAR_POPDEF([CACHEVAR])dnl
+])dnl AX_CHECK_COMPILE_FLAGS

configure.ac

@@ -683,6 +683,9 @@ AC_PROG_SED
 # See if we support thread-local storage.
 LXC_CHECK_TLS
 
+AX_CHECK_COMPILE_FLAG([-fdiagnostics-color], [CFLAGS="$CFLAGS -fdiagnostics-color"],,[-Werror])
+AX_CHECK_COMPILE_FLAG([-fdiagnostics-color], [CFLAGS="$CFLAGS -Wimplicit-fallthrough"],,[-Werror])
+
 CFLAGS="$CFLAGS -Wvla -std=gnu11"
 if test "x$enable_werror" = "xyes"; then
 	CFLAGS="$CFLAGS -Werror"

src/lxc/attach.c

@@ -1520,8 +1520,10 @@ int lxc_attach_run_command(void *payload)
 		switch (errno) {
 		case ENOEXEC:
 			ret = 126;
+			break;
 		case ENOENT:
 			ret = 127;
+			break;
 		}
 	}
 

src/lxc/cmd/lxc_init.c

@@ -39,6 +39,7 @@
 #include <lxc/lxccontainer.h>
 #include <lxc/version.h>
 
+#include "compiler.h"
 #include "error.h"
 #include "initutils.h"
 #include "log.h"
@@ -466,7 +467,7 @@ out:
 	exit(exit_with);
 }
 
-static void print_usage(const struct option longopts[])
+__noreturn__ static void print_usage_exit(const struct option longopts[])
 
 {
 	fprintf(stderr, "Usage: lxc-init [-n|--name=NAME] [-h|--help] [--usage] [--version]\n\
@@ -474,7 +475,7 @@ static void print_usage(const struct option longopts[])
 	exit(0);
 }
 
-static void print_version(void)
+__noreturn__ static void print_version_exit(void)
 {
 	printf("%s\n", LXC_VERSION);
 	exit(0);
@@ -531,9 +532,9 @@ static int arguments_parse(struct arguments *args, int argc,
 			args->lxcpath = optarg;
 			break;
 		case OPT_USAGE:
-			print_usage(args->options);
+			print_usage_exit(args->options);
 		case OPT_VERSION:
-			print_version();
+			print_version_exit();
 		case '?':
 			print_help();
 			exit(EXIT_FAILURE);

src/lxc/compiler.h

@@ -32,4 +32,18 @@
 #endif
 #endif
 
+#if defined(__GNUC__) && __GNUC__ >= 7 || defined(__clang__)
+#define __fallthrough__ __attribute__((fallthrough))
+#else
+#define __fallthrough__
+#endif
+
+#ifndef _noreturn_
+#if __STDC_VERSION__ >= 201112L
+#define __noreturn__ _Noreturn
+#else
+#define __noreturn__ __attribute__((noreturn))
+#endif
+#endif
+
 #endif /* __LXC_COMPILER_H */

src/lxc/tools/arguments.c

@@ -37,6 +37,7 @@
 #include <lxc/version.h>
 
 #include "arguments.h"
+#include "compiler.h"
 #include "namespace.h"
 #include "initutils.h"
 
@@ -87,7 +88,7 @@ is2big:
 	return -1;
 }
 
-static void print_usage_exit(const struct option longopts[],
+__noreturn__ static void print_usage_exit(const struct option longopts[],
 					  const struct lxc_arguments *a_args)
 
 {
@@ -134,13 +135,14 @@ static void print_usage_exit(const struct option longopts[],
 	exit(0);
 }
 
-static void print_version_exit()
+__noreturn__ static void print_version_exit()
 {
 	printf("%s\n", lxc_get_version());
 	exit(0);
 }
 
-static void print_help_exit(const struct lxc_arguments *args, int code)
+__noreturn__ static void print_help_exit(const struct lxc_arguments *args,
+					 int code)
 {
 	fprintf(stderr, "\
 Usage: %s %s\