Fix cross-endian interoperability of zstd

It turns out that layouts of union bitfields are a pain, and the
current code results in an inconsistent layout between BE and LE
systems, leading to zstd-active datasets on one erroring out on
the other.

Switch everyone over to the LE layout, and add compatibility code
to read both.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12008
Closes #12022

cmd/zdb/zdb.c

@@ -2259,7 +2259,8 @@ snprintf_zstd_header(spa_t *spa, char *blkbuf, size_t buflen,
 		(void) snprintf(blkbuf + strlen(blkbuf),
 		    buflen - strlen(blkbuf),
 		    " ZSTD:size=%u:version=%u:level=%u:EMBEDDED",
-		    zstd_hdr.c_len, zstd_hdr.version, zstd_hdr.level);
+		    zstd_hdr.c_len, zfs_get_hdrversion(&zstd_hdr),
+		    zfs_get_hdrlevel(&zstd_hdr));
 		return;
 	}
 
@@ -2283,7 +2284,8 @@ snprintf_zstd_header(spa_t *spa, char *blkbuf, size_t buflen,
 	(void) snprintf(blkbuf + strlen(blkbuf),
 	    buflen - strlen(blkbuf),
 	    " ZSTD:size=%u:version=%u:level=%u:NORMAL",
-	    zstd_hdr.c_len, zstd_hdr.version, zstd_hdr.level);
+	    zstd_hdr.c_len, zfs_get_hdrversion(&zstd_hdr),
+	    zfs_get_hdrlevel(&zstd_hdr));
 
 	abd_return_buf_copy(pabd, buf, BP_GET_LSIZE(bp));
 }

include/sys/zstd/zstd.h

@@ -56,21 +56,24 @@ typedef struct zfs_zstd_header {
 
 	/*
 	 * Version and compression level
-	 * We use a union to be able to big endian encode a single 32 bit
-	 * unsigned integer, but still access the individual bitmasked
-	 * components easily.
+	 * We used to use a union to reference compression level
+	 * and version easily, but as it turns out, relying on the
+	 * ordering of bitfields is not remotely portable.
+	 * So now we have get/set functions in zfs_zstd.c for
+	 * manipulating this in just the right way forever.
 	 */
-	union {
-		uint32_t raw_version_level;
-		struct {
-			uint32_t version : 24;
-			uint8_t level;
-		};
-	};
-
+	uint32_t raw_version_level;
 	char data[];
 } zfs_zstdhdr_t;
 
+/*
+ * Simple struct to pass the data from raw_version_level around.
+ */
+typedef struct zfs_zstd_meta {
+	uint8_t level;
+	uint32_t version;
+} zfs_zstdmeta_t;
+
 /*
  * kstat helper macros
  */
@@ -94,6 +97,129 @@ int zfs_zstd_decompress(void *s_start, void *d_start, size_t s_len,
     size_t d_len, int n);
 void zfs_zstd_cache_reap_now(void);
 
+/*
+ * So, the reason we have all these complicated set/get functions is that
+ * originally, in the zstd "header" we wrote out to disk, we used a 32-bit
+ * bitfield to store the "level" (8 bits) and "version" (24 bits).
+ *
+ * Unfortunately, bitfields make few promises about how they're arranged in
+ * memory...
+ *
+ * By way of example, if we were using version 1.4.5 and level 3, it'd be
+ * level = 0x03, version = 10405/0x0028A5, which gets broken into Vhigh = 0x00,
+ * Vmid = 0x28, Vlow = 0xA5. We include these positions below to help follow
+ * which data winds up where.
+ *
+ * As a consequence, we wound up with little endian platforms with a layout
+ * like this in memory:
+ *
+ *      0       8      16      24      32
+ *      +-------+-------+-------+-------+
+ *      | Vlow  | Vmid  | Vhigh | level |
+ *      +-------+-------+-------+-------+
+ *        =A5     =28     =00     =03
+ *
+ * ...and then, after being run through BE_32(), serializing this out to
+ * disk:
+ *
+ *      0       8      16      24      32
+ *      +-------+-------+-------+-------+
+ *      | level | Vhigh | Vmid  | Vlow  |
+ *      +-------+-------+-------+-------+
+ *        =03     =00     =28     =A5
+ *
+ * while on big-endian systems, since BE_32() is a noop there, both in
+ * memory and on disk, we wind up with:
+ *
+ *      0       8      16      24      32
+ *      +-------+-------+-------+-------+
+ *      | Vhigh | Vmid  | Vlow  | level |
+ *      +-------+-------+-------+-------+
+ *        =00     =28     =A5     =03
+ *
+ * (Vhigh is always 0 until version exceeds 6.55.35. Vmid and Vlow are the
+ * other two bytes of the "version" data.)
+ *
+ * So now we use the BF32_SET macros to get consistent behavior (the
+ * ondisk LE encoding, since x86 currently rules the world) across
+ * platforms, but the "get" behavior requires that we check each of the
+ * bytes in the aforementioned former-bitfield for 0x00, and from there,
+ * we can know which possible layout we're dealing with. (Only the two
+ * that have been observed in the wild are illustrated above, but handlers
+ * for all 4 positions of 0x00 are implemented.
+ */
+
+static inline void
+zfs_get_hdrmeta(const zfs_zstdhdr_t *blob, zfs_zstdmeta_t *res)
+{
+	uint32_t raw = blob->raw_version_level;
+	uint8_t findme = 0xff;
+	int shift;
+	for (shift = 0; shift < 4; shift++) {
+		findme = BF32_GET(raw, 8*shift, 8);
+		if (findme == 0)
+			break;
+	}
+	switch (shift) {
+	case 0:
+		res->level = BF32_GET(raw, 24, 8);
+		res->version = BSWAP_32(raw);
+		res->version = BF32_GET(res->version, 8, 24);
+		break;
+	case 1:
+		res->level = BF32_GET(raw, 0, 8);
+		res->version = BSWAP_32(raw);
+		res->version = BF32_GET(res->version, 0, 24);
+		break;
+	case 2:
+		res->level = BF32_GET(raw, 24, 8);
+		res->version = BF32_GET(raw, 0, 24);
+		break;
+	case 3:
+		res->level = BF32_GET(raw, 0, 8);
+		res->version = BF32_GET(raw, 8, 24);
+		break;
+	default:
+		res->level = 0;
+		res->version = 0;
+		break;
+	}
+}
+
+static inline uint8_t
+zfs_get_hdrlevel(const zfs_zstdhdr_t *blob)
+{
+	uint8_t level = 0;
+	zfs_zstdmeta_t res;
+	zfs_get_hdrmeta(blob, &res);
+	level = res.level;
+	return (level);
+}
+
+static inline uint32_t
+zfs_get_hdrversion(const zfs_zstdhdr_t *blob)
+{
+	uint32_t version = 0;
+	zfs_zstdmeta_t res;
+	zfs_get_hdrmeta(blob, &res);
+	version = res.version;
+	return (version);
+
+}
+
+static inline void
+zfs_set_hdrversion(zfs_zstdhdr_t *blob, uint32_t version)
+{
+	BF32_SET(blob->raw_version_level, 0, 24, version);
+}
+
+static inline void
+zfs_set_hdrlevel(zfs_zstdhdr_t *blob, uint8_t level)
+{
+	BF32_SET(blob->raw_version_level, 24, 8, level);
+}
+
+
 #ifdef	__cplusplus
 }
 #endif

module/zstd/Makefile.in

@@ -33,6 +33,7 @@ $(obj)/zfs_zstd.o: c_flags += -include $(zstd_include)/zstd_compat_wrapper.h
 
 $(MODULE)-objs += zfs_zstd.o
 $(MODULE)-objs += lib/zstd.o
+$(MODULE)-objs += zstd_sparc.o
 
 all:
 	mkdir -p lib

module/zstd/include/sparc_compat.h

@@ -0,0 +1,4 @@
+#if defined(__sparc)
+uint64_t __bswapdi2(uint64_t in);
+uint32_t __bswapsi2(uint32_t in);
+#endif

module/zstd/zfs_zstd.c

@@ -380,6 +380,7 @@ zstd_enum_to_level(enum zio_zstd_levels level, int16_t *zstd_level)
 	return (1);
 }
 
+
 /* Compress block using zstd */
 size_t
 zfs_zstd_compress(void *s_start, void *d_start, size_t s_len, size_t d_len,
@@ -477,8 +478,8 @@ zfs_zstd_compress(void *s_start, void *d_start, size_t s_len, size_t d_len,
 	 * As soon as such incompatibility occurs, handling code needs to be
 	 * added, differentiating between the versions.
 	 */
-	hdr->version = ZSTD_VERSION_NUMBER;
-	hdr->level = level;
+	zfs_set_hdrversion(hdr, ZSTD_VERSION_NUMBER);
+	zfs_set_hdrlevel(hdr, level);
 	hdr->raw_version_level = BE_32(hdr->raw_version_level);
 
 	return (c_len + sizeof (*hdr));
@@ -504,6 +505,7 @@ zfs_zstd_decompress_level(void *s_start, void *d_start, size_t s_len,
 	 * not modify the original data that may be used again later.
 	 */
 	hdr_copy.raw_version_level = BE_32(hdr->raw_version_level);
+	uint8_t curlevel = zfs_get_hdrlevel(&hdr_copy);
 
 	/*
 	 * NOTE: We ignore the ZSTD version for now. As soon as any
@@ -516,13 +518,13 @@ zfs_zstd_decompress_level(void *s_start, void *d_start, size_t s_len,
 	 * An invalid level is a strong indicator for data corruption! In such
 	 * case return an error so the upper layers can try to fix it.
 	 */
-	if (zstd_enum_to_level(hdr_copy.level, &zstd_level)) {
+	if (zstd_enum_to_level(curlevel, &zstd_level)) {
 		ZSTDSTAT_BUMP(zstd_stat_dec_inval);
 		return (1);
 	}
 
 	ASSERT3U(d_len, >=, s_len);
-	ASSERT3U(hdr_copy.level, !=, ZIO_COMPLEVEL_INHERIT);
+	ASSERT3U(curlevel, !=, ZIO_COMPLEVEL_INHERIT);
 
 	/* Invalid compressed buffer size encoded at start */
 	if (c_len + sizeof (*hdr) > s_len) {
@@ -553,7 +555,7 @@ zfs_zstd_decompress_level(void *s_start, void *d_start, size_t s_len,
 	}
 
 	if (level) {
-		*level = hdr_copy.level;
+		*level = curlevel;
 	}
 
 	return (0);
@@ -783,7 +785,7 @@ module_exit(zstd_fini);
 
 ZFS_MODULE_DESCRIPTION("ZSTD Compression for ZFS");
 ZFS_MODULE_LICENSE("Dual BSD/GPL");
-ZFS_MODULE_VERSION(ZSTD_VERSION_STRING);
+ZFS_MODULE_VERSION(ZSTD_VERSION_STRING "a");
 
 EXPORT_SYMBOL(zfs_zstd_compress);
 EXPORT_SYMBOL(zfs_zstd_decompress_level);

module/zstd/zstd_sparc.c

@@ -0,0 +1,11 @@
+#ifdef __sparc__
+#include <stdint.h>
+#include <sys/byteorder.h>
+#include "include/sparc_compat.h"
+uint64_t __bswapdi2(uint64_t in) {
+	return (BSWAP_64(in));
+}
+uint32_t __bswapsi2(uint32_t in) {
+	return (BSWAP_32(in));
+}
+#endif