[ubsan] Support varlen arrays at end of struct as [1] or []

The --enable-ubsan configure feature complains when we index variable
length arrays at the end of some structs because the arrays size is
defined to be 1. Newer C compilers support declaring these as
variable-sized using [] in the struct member type definition, which also
quiets the UBSAN complaint. Test for this support in configure and use
it if it is supported by the compiler.

Signed-off-by: Coleman Kane <ckane@colemankane.org>

config/zfs-build.m4

@@ -231,6 +231,7 @@ AC_DEFUN([ZFS_AC_CONFIG_ALWAYS], [
 	ZFS_AC_CONFIG_ALWAYS_CPPCHECK
 	ZFS_AC_CONFIG_ALWAYS_SHELLCHECK
 	ZFS_AC_CONFIG_ALWAYS_PARALLEL
+	ZFS_AC_CONFIG_ALWAYS_CC_VARSIZE_ARRAY_IN_STRUCT
 ])
 
 AC_DEFUN([ZFS_AC_CONFIG], [
@@ -630,3 +631,29 @@ AC_DEFUN([ZFS_AC_PACKAGE], [
 		ZFS_AC_ALIEN
 	])
 ])
+
+dnl #
+dnl # Test whether C compiler supports variable-length array at the
+dnl # end of a struct definition, and if it needs a constant or not
+dnl # declaring a size
+dnl #
+AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_CC_VARSIZE_ARRAY_IN_STRUCT], [
+	AC_MSG_CHECKING(
+	    [if C compiler handles empty-index var-length array members])
+	AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+		struct s {
+			int a;
+			int v[];
+		};
+
+		struct s test_array __attribute__((unused));
+	])], [
+		AC_MSG_RESULT([yes])
+		AC_DEFINE([VARLEN_ARRAY_IDX], [],
+		    [Index to use for variable-length array struct members])
+	], [
+		AC_MSG_RESULT([no])
+		AC_DEFINE([VARLEN_ARRAY_IDX], 1,
+		    [Index to use for variable-length array struct members])
+	])
+])

include/os/linux/spl/sys/kmem_cache.h

@@ -108,7 +108,8 @@ typedef struct spl_kmem_magazine {
 	uint32_t		skm_refill;	/* Batch refill size */
 	struct spl_kmem_cache	*skm_cache;	/* Owned by cache */
 	unsigned int		skm_cpu;	/* Owned by cpu */
-	void			*skm_objs[0];	/* Object pointers */
+	/* Object pointers */
+	void			*skm_objs[VARLEN_ARRAY_IDX];
 } spl_kmem_magazine_t;
 
 typedef struct spl_kmem_obj {

include/sys/sa_impl.h

@@ -177,8 +177,11 @@ typedef struct sa_hdr_phys {
 	 *
 	 */
 	uint16_t sa_layout_info;
-	uint16_t sa_lengths[1];	/* optional sizes for variable length attrs */
-	/* ... Data follows the lengths.  */
+	uint16_t sa_lengths[VARLEN_ARRAY_IDX];
+	/*
+	 * optional sizes for variable length attrs
+	 * ... Data follows the lengths.
+	 */
 } sa_hdr_phys_t;
 
 #define	SA_HDR_LAYOUT_NUM(hdr) BF32_GET(hdr->sa_layout_info, 0, 10)

include/sys/vdev_raidz_impl.h

@@ -130,7 +130,8 @@ typedef struct raidz_row {
 	uint64_t rr_offset;		/* Logical offset for *_io_verify() */
 	uint64_t rr_size;		/* Physical size for *_io_verify() */
 #endif
-	raidz_col_t rr_col[0];		/* Flexible array of I/O columns */
+	/* Flexible array of I/O columns */
+	raidz_col_t rr_col[VARLEN_ARRAY_IDX];
 } raidz_row_t;
 
 typedef struct raidz_map {
@@ -139,7 +140,7 @@ typedef struct raidz_map {
 	int rm_nskip;			/* RAIDZ sectors skipped for padding */
 	int rm_skipstart;		/* Column index of padding start */
 	const raidz_impl_ops_t *rm_ops;	/* RAIDZ math operations */
-	raidz_row_t *rm_row[0];		/* flexible array of rows */
+	raidz_row_t *rm_row[VARLEN_ARRAY_IDX];	/* flexible array of rows */
 } raidz_map_t;
 
 

include/sys/zap_impl.h

@@ -61,7 +61,7 @@ typedef struct mzap_phys {
 	uint64_t mz_salt;
 	uint64_t mz_normflags;
 	uint64_t mz_pad[5];
-	mzap_ent_phys_t mz_chunk[1];
+	mzap_ent_phys_t mz_chunk[VARLEN_ARRAY_IDX];
 	/* actually variable size depending on block size */
 } mzap_phys_t;
 

include/sys/zap_leaf.h

@@ -132,7 +132,7 @@ typedef struct zap_leaf_phys {
 	 * with the ZAP_LEAF_CHUNK() macro.
 	 */
 
-	uint16_t l_hash[1];
+	uint16_t l_hash[VARLEN_ARRAY_IDX];
 } zap_leaf_phys_t;
 
 typedef union zap_leaf_chunk {