Use fallthrough macro

As of the Linux 5.9 kernel a fallthrough macro has been added which
should be used to anotate all intentional fallthrough paths.  Once
all of the kernel code paths have been updated to use fallthrough
the -Wimplicit-fallthrough option will because the default.  To
avoid warnings in the OpenZFS code base when this happens apply
the fallthrough macro.

Additional reading: https://lwn.net/Articles/794944/

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12441

cmd/mount_zfs/mount_zfs.c

@@ -369,7 +369,7 @@ main(int argc, char **argv)
 				return (MOUNT_SYSERR);
 			}
 #endif
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			(void) fprintf(stderr, gettext("filesystem "
 			    "'%s' can not be mounted: %s\n"), dataset,

cmd/zdb/zdb.c

@@ -4625,7 +4625,7 @@ dump_path_impl(objset_t *os, uint64_t obj, char *name, uint64_t *retobj)
 	case DMU_OT_DIRECTORY_CONTENTS:
 		if (s != NULL && *(s + 1) != '\0')
 			return (dump_path_impl(os, child_obj, s + 1, retobj));
-		/* FALLTHROUGH */
+		fallthrough;
 	case DMU_OT_PLAIN_FILE_CONTENTS:
 		if (retobj != NULL) {
 			*retobj = child_obj;

cmd/zfs/zfs_main.c

@@ -7466,6 +7466,7 @@ unshare_unmount(int op, int argc, char **argv)
 				if (zfs_prop_get_int(zhp, ZFS_PROP_CANMOUNT) ==
 				    ZFS_CANMOUNT_NOAUTO)
 					continue;
+				break;
 			default:
 				break;
 			}

cmd/zpool_influxdb/zpool_influxdb.c

@@ -118,7 +118,7 @@ escape_string(const char *s)
 		case '=':
 		case '\\':
 			*d++ = '\\';
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			*d = *c;
 		}

config/Rules.am

@@ -26,6 +26,7 @@ AM_LIBTOOLFLAGS = --silent
 AM_CFLAGS  = -std=gnu99 -Wall -Wstrict-prototypes -Wmissing-prototypes
 AM_CFLAGS += -fno-strict-aliasing
 AM_CFLAGS += $(NO_OMIT_FRAME_POINTER)
+AM_CFLAGS += $(IMPLICIT_FALLTHROUGH)
 AM_CFLAGS += $(DEBUG_CFLAGS)
 AM_CFLAGS += $(ASAN_CFLAGS)
 AM_CFLAGS += $(CODE_COVERAGE_CFLAGS) $(NO_FORMAT_ZERO_LENGTH)

config/always-compiler-options.m4

@@ -161,6 +161,29 @@ AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_CC_NO_UNUSED_BUT_SET_VARIABLE], [
 	AC_SUBST([NO_UNUSED_BUT_SET_VARIABLE])
 ])
 
+dnl #
+dnl # Check if gcc supports -Wimplicit-fallthrough option.
+dnl #
+AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_CC_IMPLICIT_FALLTHROUGH], [
+	AC_MSG_CHECKING([whether $CC supports -Wimplicit-fallthrough])
+
+	saved_flags="$CFLAGS"
+	CFLAGS="$CFLAGS -Werror -Wimplicit-fallthrough"
+
+	AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [])], [
+		IMPLICIT_FALLTHROUGH=-Wimplicit-fallthrough
+		AC_DEFINE([HAVE_IMPLICIT_FALLTHROUGH], 1,
+			[Define if compiler supports -Wimplicit-fallthrough])
+		AC_MSG_RESULT([yes])
+	], [
+		IMPLICIT_FALLTHROUGH=
+		AC_MSG_RESULT([no])
+	])
+
+	CFLAGS="$saved_flags"
+	AC_SUBST([IMPLICIT_FALLTHROUGH])
+])
+
 dnl #
 dnl # Check if gcc supports -fno-omit-frame-pointer option.
 dnl #

config/zfs-build.m4

@@ -211,6 +211,7 @@ AC_DEFUN([ZFS_AC_CONFIG_ALWAYS], [
 
 	ZFS_AC_CONFIG_ALWAYS_CC_NO_UNUSED_BUT_SET_VARIABLE
 	ZFS_AC_CONFIG_ALWAYS_CC_NO_BOOL_COMPARE
+	ZFS_AC_CONFIG_ALWAYS_CC_IMPLICIT_FALLTHROUGH
 	ZFS_AC_CONFIG_ALWAYS_CC_FRAME_LARGER_THAN
 	ZFS_AC_CONFIG_ALWAYS_CC_NO_FORMAT_TRUNCATION
 	ZFS_AC_CONFIG_ALWAYS_CC_NO_FORMAT_ZERO_LENGTH

include/os/freebsd/linux/compiler.h

@@ -67,6 +67,7 @@
 #define	__always_inline			inline
 #define	noinline			__noinline
 #define	____cacheline_aligned		__aligned(CACHE_LINE_SIZE)
+#define	fallthrough			__attribute__((__fallthrough__))
 
 #if !defined(_KERNEL) && !defined(_STANDALONE)
 #define	likely(x)			__builtin_expect(!!(x), 1)

include/os/linux/kernel/linux/compiler_compat.h

@@ -28,6 +28,14 @@
 
 #include <linux/compiler.h>
 
+#if !defined(fallthrough)
+#if defined(HAVE_IMPLICIT_FALLTHROUGH)
+#define	fallthrough		__attribute__((__fallthrough__))
+#else
+#define	fallthrough		((void)0)
+#endif
+#endif
+
 #if !defined(READ_ONCE)
 #define	READ_ONCE(x)		ACCESS_ONCE(x)
 #endif

include/os/linux/zfs/sys/zfs_context_os.h

@@ -25,6 +25,7 @@
 
 #include <linux/dcache_compat.h>
 #include <linux/utsname_compat.h>
+#include <linux/compiler_compat.h>
 #include <linux/module.h>
 
 #if THREAD_SIZE >= 16384

include/sys/zfs_context.h

@@ -766,7 +766,6 @@ extern void spl_fstrans_unmark(fstrans_cookie_t);
 extern int __spl_pf_fstrans_check(void);
 extern int kmem_cache_reap_active(void);
 
-#define	____cacheline_aligned
 
 /*
  * Kernel modules

lib/libspl/include/sys/feature_tests.h

@@ -27,6 +27,15 @@
 #ifndef _SYS_FEATURE_TESTS_H
 #define	_SYS_FEATURE_TESTS_H
 
-#define	__NORETURN	__attribute__((__noreturn__))
+#define	____cacheline_aligned
+#define	__NORETURN		__attribute__((__noreturn__))
+
+#if !defined(fallthrough)
+#if defined(HAVE_IMPLICIT_FALLTHROUGH)
+#define	fallthrough		__attribute__((__fallthrough__))
+#else
+#define	fallthrough		((void)0)
+#endif
+#endif
 
 #endif

lib/libzfs/libzfs_dataset.c

@@ -1363,9 +1363,8 @@ zfs_valid_proplist(libzfs_handle_t *hdl, zfs_type_t type, nvlist_t *nvl,
 				(void) zfs_error(hdl, EZFS_BADPROP, errbuf);
 				goto error;
 			}
+			fallthrough;
 		}
-			/* FALLTHROUGH */
-
 
 		case ZFS_PROP_SHARESMB:
 		case ZFS_PROP_SHARENFS:
@@ -3775,8 +3774,8 @@ zfs_create(libzfs_handle_t *hdl, const char *path, zfs_type_t type,
 			if (type == ZFS_TYPE_VOLUME)
 				return (zfs_error(hdl, EZFS_VOLTOOBIG,
 				    errbuf));
+			fallthrough;
 #endif
-			/* FALLTHROUGH */
 		default:
 			return (zfs_standard_error(hdl, errno, errbuf));
 		}

lib/libzfs/libzfs_pool.c

@@ -314,7 +314,7 @@ zpool_get_prop(zpool_handle_t *zhp, zpool_prop_t prop, char *buf,
 				    len);
 				break;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			(void) strlcpy(buf, "-", len);
 			break;
@@ -405,7 +405,7 @@ zpool_get_prop(zpool_handle_t *zhp, zpool_prop_t prop, char *buf,
 				(void) snprintf(buf, len, "-");
 				break;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			(void) snprintf(buf, len, "%llu", (u_longlong_t)intval);
 		}

lib/libzfs/libzfs_sendrecv.c

@@ -4866,7 +4866,7 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 				(void) zfs_error(hdl, EZFS_BUSY, errbuf);
 				break;
 			}
-			/* fallthru */
+			fallthrough;
 		default:
 			(void) zfs_standard_error(hdl, ioctl_errno, errbuf);
 		}

lib/libzfs/libzfs_util.c

@@ -598,7 +598,7 @@ zfs_setprop_error(libzfs_handle_t *hdl, zfs_prop_t prop, int err,
 			break;
 		}
 #endif
-		/* FALLTHROUGH */
+		fallthrough;
 	default:
 		(void) zfs_standard_error(hdl, err, errbuf);
 	}

lib/libzfsbootenv/lzbe_device.c

@@ -63,7 +63,7 @@ lzbe_set_boot_device(const char *pool, lzbe_flags_t flag, const char *device)
 			/* Drop this nvlist */
 			fnvlist_free(nv);
 		}
-		/* FALLTHROUGH */
+		fallthrough;
 	case lzbe_replace:
 		nv = fnvlist_alloc();
 		break;

lib/libzutil/zutil_import.c

@@ -1032,10 +1032,11 @@ zpool_read_label(int fd, nvlist_t **config, int *num_labels)
 					// This shouldn't be possible to
 					// encounter, die if we do.
 					ASSERT(B_FALSE);
+					fallthrough;
 				case EOPNOTSUPP:
 				case ENOSYS:
 					do_slow = B_TRUE;
-					/* FALLTHROUGH */
+					fallthrough;
 				case 0:
 				default:
 					(void) aio_return(&aiocbs[l]);

module/icp/core/kcf_prov_tabs.c

@@ -377,7 +377,7 @@ kcf_provider_zero_refcnt(kcf_provider_desc_t *desc)
 			mutex_exit(&desc->pd_lock);
 			break;
 		}
-		/* FALLTHROUGH */
+		fallthrough;
 
 	case CRYPTO_HW_PROVIDER:
 	case CRYPTO_LOGICAL_PROVIDER:

module/icp/io/aes.c

@@ -976,7 +976,7 @@ aes_encrypt_atomic(crypto_provider_handle_t provider,
 	case AES_GMAC_MECH_INFO_TYPE:
 		if (plaintext->cd_length != 0)
 			return (CRYPTO_ARGUMENTS_BAD);
-		/* FALLTHROUGH */
+		fallthrough;
 	case AES_GCM_MECH_INFO_TYPE:
 		length_needed = plaintext->cd_length + aes_ctx.ac_tag_len;
 		break;

module/lua/lcode.c

@@ -8,6 +8,10 @@
 #define lcode_c
 #define LUA_CORE
 
+#if defined(HAVE_IMPLICIT_FALLTHROUGH)
+#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+#endif
+
 #include <sys/lua/lua.h>
 
 #include "lcode.h"

module/lua/lgc.c

@@ -676,7 +676,7 @@ static void freeobj (lua_State *L, GCObject *o) {
     case LUA_TUSERDATA: luaM_freemem(L, o, sizeudata(gco2u(o))); break;
     case LUA_TSHRSTR:
       G(L)->strt.nuse--;
-      /* FALLTHROUGH */
+      fallthrough;
     case LUA_TLNGSTR: {
       luaM_freemem(L, o, sizestring(gco2ts(o)));
       break;

module/lua/llex.c

@@ -477,7 +477,7 @@ static int llex (LexState *ls, SemInfo *seminfo) {
         else if (!lisdigit(ls->current)) return '.';
         /* else go through */
       }
-        /* FALLTHROUGH */
+        fallthrough;
       case '0': case '1': case '2': case '3': case '4':
       case '5': case '6': case '7': case '8': case '9': {
         read_numeral(ls, seminfo);

module/lua/lstrlib.c

@@ -501,7 +501,7 @@ static const char *match (MatchState *ms, const char *s, const char *p) {
             }
             case '+':  /* 1 or more repetitions */
               s++;  /* 1 match already done */
-              /* FALLTHROUGH */
+              fallthrough;
             case '*':  /* 0 or more repetitions */
               s = max_expand(ms, s, p, ep);
               break;

module/lua/ltable.c

@@ -492,7 +492,7 @@ const TValue *luaH_get (Table *t, const TValue *key) {
         return luaH_getint(t, k);  /* use specialized version */
       /* else go through */
     }
-      /* FALLTHROUGH */
+      fallthrough;
     default: {
       Node *n = mainposition(t, key);
       do {  /* check whether `key' is somewhere in the chain */

module/os/freebsd/zfs/zfs_acl.c

@@ -269,7 +269,7 @@ zfs_ace_fuid_size(void *acep)
 		    entry_type == OWNING_GROUP ||
 		    entry_type == ACE_EVERYONE)
 			return (sizeof (zfs_ace_hdr_t));
-		/* FALLTHROUGH */
+		fallthrough;
 	default:
 		return (sizeof (zfs_ace_t));
 	}
@@ -2153,7 +2153,7 @@ zfs_zaccess_aces_check(znode_t *zp, uint32_t *working_mode,
 			break;
 		case OWNING_GROUP:
 			who = gowner;
-			/* FALLTHROUGH */
+			fallthrough;
 		case ACE_IDENTIFIER_GROUP:
 			checkit = zfs_groupmember(zfsvfs, who, cr);
 			break;

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -960,7 +960,7 @@ zfs_lookup(vnode_t *dvp, const char *nm, vnode_t **vpp,
 				cnp->cn_flags |= SAVENAME;
 				break;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		case DELETE:
 			if (error == 0)
 				cnp->cn_flags |= SAVENAME;

module/os/linux/zfs/zfs_acl.c

@@ -269,7 +269,7 @@ zfs_ace_fuid_size(void *acep)
 		    entry_type == OWNING_GROUP ||
 		    entry_type == ACE_EVERYONE)
 			return (sizeof (zfs_ace_hdr_t));
-		/* FALLTHROUGH */
+		fallthrough;
 	default:
 		return (sizeof (zfs_ace_t));
 	}
@@ -2317,7 +2317,7 @@ zfs_zaccess_aces_check(znode_t *zp, uint32_t *working_mode,
 			break;
 		case OWNING_GROUP:
 			who = gowner;
-			/* FALLTHROUGH */
+			fallthrough;
 		case ACE_IDENTIFIER_GROUP:
 			checkit = zfs_groupmember(zfsvfs, who, cr);
 			break;

module/os/linux/zfs/zfs_znode.c

@@ -430,7 +430,7 @@ zfs_inode_set_ops(zfsvfs_t *zfsvfs, struct inode *ip)
 	case S_IFBLK:
 		(void) sa_lookup(ITOZ(ip)->z_sa_hdl, SA_ZPL_RDEV(zfsvfs), &rdev,
 		    sizeof (rdev));
-		/* FALLTHROUGH */
+		fallthrough;
 	case S_IFIFO:
 	case S_IFSOCK:
 		init_special_inode(ip, ip->i_mode, rdev);

module/zfs/abd.c

@@ -1066,10 +1066,10 @@ abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
 		switch (parity) {
 			case 3:
 				len = MIN(caiters[2].iter_mapsize, len);
-				/* falls through */
+				fallthrough;
 			case 2:
 				len = MIN(caiters[1].iter_mapsize, len);
-				/* falls through */
+				fallthrough;
 			case 1:
 				len = MIN(caiters[0].iter_mapsize, len);
 		}
@@ -1179,11 +1179,11 @@ abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
 			case 3:
 				len = MIN(xiters[2].iter_mapsize, len);
 				len = MIN(citers[2].iter_mapsize, len);
-				/* falls through */
+				fallthrough;
 			case 2:
 				len = MIN(xiters[1].iter_mapsize, len);
 				len = MIN(citers[1].iter_mapsize, len);
-				/* falls through */
+				fallthrough;
 			case 1:
 				len = MIN(xiters[0].iter_mapsize, len);
 				len = MIN(citers[0].iter_mapsize, len);

module/zfs/dsl_prop.c

@@ -749,7 +749,7 @@ dsl_prop_set_sync_impl(dsl_dataset_t *ds, const char *propname,
 		ASSERT(err == 0 || err == ENOENT);
 		err = zap_remove(mos, zapobj, inheritstr, tx);
 		ASSERT(err == 0 || err == ENOENT);
-		/* FALLTHROUGH */
+		fallthrough;
 	case (ZPROP_SRC_NONE | ZPROP_SRC_RECEIVED):
 		/*
 		 * remove propname$recvd

module/zfs/spa.c

@@ -9710,7 +9710,7 @@ spa_activity_in_progress(spa_t *spa, zpool_wait_activity_t activity,
 	case ZPOOL_WAIT_RESILVER:
 		if ((*in_progress = vdev_rebuild_active(spa->spa_root_vdev)))
 			break;
-		/* fall through */
+		fallthrough;
 	case ZPOOL_WAIT_SCRUB:
 	{
 		boolean_t scanning, paused, is_scrub;

module/zfs/vdev_label.c

@@ -1305,7 +1305,7 @@ vdev_label_read_bootenv(vdev_t *rvd, nvlist_t *bootenv)
 				nvlist_free(config);
 				break;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			/* Check for FreeBSD zfs bootonce command string */
 			buf = abd_to_buf(abd);

module/zfs/vdev_raidz_math_scalar.c

@@ -142,7 +142,7 @@ static const struct {
 		a.b[6] = mul_lt[a.b[6]];				\
 		a.b[5] = mul_lt[a.b[5]];				\
 		a.b[4] = mul_lt[a.b[4]];				\
-		/* falls through */					\
+		fallthrough;						\
 	case 4:								\
 		a.b[3] = mul_lt[a.b[3]];				\
 		a.b[2] = mul_lt[a.b[2]];				\

module/zfs/zfs_replay.c

@@ -362,7 +362,7 @@ zfs_replay_create_acl(void *arg1, void *arg2, boolean_t byteswap)
 		zfsvfs->z_fuid_replay = zfs_replay_fuids(fuidstart,
 		    (void *)&name, lracl->lr_fuidcnt, lracl->lr_domcnt,
 		    lr->lr_uid, lr->lr_gid);
-		/* FALLTHROUGH */
+		fallthrough;
 	case TX_CREATE_ACL_ATTR:
 		if (name == NULL) {
 			lrattr = (lr_attr_t *)(caddr_t)(lracl + 1);
@@ -394,7 +394,7 @@ zfs_replay_create_acl(void *arg1, void *arg2, boolean_t byteswap)
 		zfsvfs->z_fuid_replay = zfs_replay_fuids(fuidstart,
 		    (void *)&name, lracl->lr_fuidcnt, lracl->lr_domcnt,
 		    lr->lr_uid, lr->lr_gid);
-		/* FALLTHROUGH */
+		fallthrough;
 	case TX_MKDIR_ACL_ATTR:
 		if (name == NULL) {
 			lrattr = (lr_attr_t *)(caddr_t)(lracl + 1);
@@ -519,7 +519,7 @@ zfs_replay_create(void *arg1, void *arg2, boolean_t byteswap)
 		    zfs_replay_fuid_domain(start, &start,
 		    lr->lr_uid, lr->lr_gid);
 		name = (char *)start;
-		/* FALLTHROUGH */
+		fallthrough;
 
 	case TX_CREATE:
 		if (name == NULL)
@@ -537,7 +537,7 @@ zfs_replay_create(void *arg1, void *arg2, boolean_t byteswap)
 		    zfs_replay_fuid_domain(start, &start,
 		    lr->lr_uid, lr->lr_gid);
 		name = (char *)start;
-		/* FALLTHROUGH */
+		fallthrough;
 
 	case TX_MKDIR:
 		if (name == NULL)

module/zfs/zio_compress.c

@@ -214,7 +214,7 @@ zio_compress_to_feature(enum zio_compress comp)
 	case ZIO_COMPRESS_ZSTD:
 		return (SPA_FEATURE_ZSTD_COMPRESS);
 	default:
-		/* fallthru */;
+		break;
 	}
 	return (SPA_FEATURE_NONE);
 }

tests/zfs-tests/cmd/mkbusy/mkbusy.c

@@ -122,7 +122,7 @@ main(int argc, char *argv[])
 		switch (sbuf.st_mode & S_IFMT) {
 			case S_IFDIR:
 				isdir = B_TRUE;
-				/* FALLTHROUGH */
+				fallthrough;
 			case S_IFLNK:
 			case S_IFCHR:
 			case S_IFBLK: