Use memset to zero stack allocations containing unions

C99 6.7.8.17 says that when an undesignated initialiser is used, only
the first element of a union is initialised. If the first element is not
the largest within the union, how the remaining space is initialised is
up to the compiler.

GCC extends the initialiser to the entire union, while Clang treats the
remainder as padding, and so initialises according to whatever
automatic/implicit initialisation rules are currently active.

When Linux is compiled with CONFIG_INIT_STACK_ALL_PATTERN,
-ftrivial-auto-var-init=pattern is added to the kernel CFLAGS. This flag
sets the policy for automatic/implicit initialisation of variables on
the stack.

Taken together, this means that when compiling under
CONFIG_INIT_STACK_ALL_PATTERN on Clang, the "zero" initialiser will only
zero the first element in a union, and the rest will be filled with a
pattern. This is significant for aes_ctx_t, which in
aes_encrypt_atomic() and aes_decrypt_atomic() is initialised to zero,
but then used as a gcm_ctx_t, which is the fifth element in the union,
and thus gets pattern initialisation. Later, it's assumed to be zero,
resulting in a hang.

As confusing and undiscoverable as it is, by the spec, we are at fault
when we initialise a structure containing a union with the zero
initializer. As such, this commit replaces these uses with an explicit
memset(0).

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes openzfs#16135
Closes openzfs#16206

cmd/zstream/zstream_redup.c

@@ -186,14 +186,16 @@ static void
 zfs_redup_stream(int infd, int outfd, boolean_t verbose)
 {
 	int bufsz = SPA_MAXBLOCKSIZE;
-	dmu_replay_record_t thedrr = { 0 };
+	dmu_replay_record_t thedrr;
 	dmu_replay_record_t *drr = &thedrr;
 	redup_table_t rdt;
 	zio_cksum_t stream_cksum;
 	uint64_t numbuckets;
 	uint64_t num_records = 0;
 	uint64_t num_write_byref_records = 0;
 
+	memset(&thedrr, 0, sizeof (dmu_replay_record_t));
+
 #ifdef _ILP32
 	uint64_t max_rde_size = SMALLEST_POSSIBLE_MAX_RDT_MB << 20;
 #else

lib/libzfs/libzfs_sendrecv.c

@@ -2170,7 +2170,8 @@ zfs_send_saved(zfs_handle_t *zhp, sendflags_t *flags, int outfd,
 static int
 send_conclusion_record(int fd, zio_cksum_t *zc)
 {
-	dmu_replay_record_t drr = { 0 };
+	dmu_replay_record_t drr;
+	memset(&drr, 0, sizeof (dmu_replay_record_t));
 	drr.drr_type = DRR_END;
 	if (zc != NULL)
 		drr.drr_u.drr_end.drr_checksum = *zc;
@@ -2272,7 +2273,8 @@ send_prelim_records(zfs_handle_t *zhp, const char *from, int fd,
 	}
 
 	if (!dryrun) {
-		dmu_replay_record_t drr = { 0 };
+		dmu_replay_record_t drr;
+		memset(&drr, 0, sizeof (dmu_replay_record_t));
 		/* write first begin record */
 		drr.drr_type = DRR_BEGIN;
 		drr.drr_u.drr_begin.drr_magic = DMU_BACKUP_MAGIC;

module/icp/io/aes.c

@@ -832,12 +832,14 @@ aes_encrypt_atomic(crypto_mechanism_t *mechanism,
     crypto_key_t *key, crypto_data_t *plaintext, crypto_data_t *ciphertext,
     crypto_spi_ctx_template_t template)
 {
-	aes_ctx_t aes_ctx = {{{{0}}}};
+	aes_ctx_t aes_ctx;
 	off_t saved_offset;
 	size_t saved_length;
 	size_t length_needed;
 	int ret;
 
+	memset(&aes_ctx, 0, sizeof (aes_ctx_t));
+
 	ASSERT(ciphertext != NULL);
 
 	/*
@@ -956,12 +958,14 @@ aes_decrypt_atomic(crypto_mechanism_t *mechanism,
     crypto_key_t *key, crypto_data_t *ciphertext, crypto_data_t *plaintext,
     crypto_spi_ctx_template_t template)
 {
-	aes_ctx_t aes_ctx = {{{{0}}}};
+	aes_ctx_t aes_ctx;
 	off_t saved_offset;
 	size_t saved_length;
 	size_t length_needed;
 	int ret;
 
+	memset(&aes_ctx, 0, sizeof (aes_ctx_t));
+
 	ASSERT(plaintext != NULL);
 
 	/*

tests/zfs-tests/cmd/libzfs_input_check.c

@@ -521,13 +521,15 @@ test_send_new(const char *snapshot, int fd)
 static void
 test_recv_new(const char *dataset, int fd)
 {
-	dmu_replay_record_t drr = { 0 };
+	dmu_replay_record_t drr;
 	nvlist_t *required = fnvlist_alloc();
 	nvlist_t *optional = fnvlist_alloc();
 	nvlist_t *props = fnvlist_alloc();
 	char snapshot[MAXNAMELEN + 32];
 	ssize_t count;
 
+	memset(&drr, 0, sizeof (dmu_replay_record_t));
+
 	int cleanup_fd = open(ZFS_DEV, O_RDWR);
 	if (cleanup_fd == -1) {
 		(void) fprintf(stderr, "open(%s) failed: %s\n", ZFS_DEV,