xz compression detection: avoid the need to seek

function                                             old     new   delta
unpack_unxz                                           12      67     +55
unpack_xz_stream                                    2357    2373     +16
xmalloc_read                                         197     199      +2
setup_unzip_on_fd                                    118      99     -19
rpm2cpio_main                                        222     203     -19
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/2 up/down: 73/-38)             Total: 35 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

5 files changed, 30 insertions, 14 deletions

diff --git a/archival/bbunzip.c b/archival/bbunzip.c
index b243afb..832a7bb 100644
--- a/archival/bbunzip.c
+++ b/archival/bbunzip.c
@@ -373,6 +373,15 @@ int unlzma_main(int argc UNUSED_PARAM, char **argv)
 static
 IF_DESKTOP(long long) int FAST_FUNC unpack_unxz(unpack_info_t *info UNUSED_PARAM)
 {
+	struct {
+		uint32_t v1;
+		uint16_t v2;
+	} magic;
+	xread(STDIN_FILENO, &magic, 6);
+	if (magic.v1 != XZ_MAGIC1a || magic.v2 != XZ_MAGIC2a) {
+		bb_error_msg("invalid magic");
+		return -1;
+	}
 	return unpack_xz_stream(STDIN_FILENO, STDOUT_FILENO);
 }
 int unxz_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
diff --git a/archival/libunarchive/decompress_unxz.c b/archival/libunarchive/decompress_unxz.c
index 5d7513a..721acd9 100644
--- a/archival/libunarchive/decompress_unxz.c
+++ b/archival/libunarchive/decompress_unxz.c
@@ -56,9 +56,11 @@ unpack_xz_stream(int src_fd, int dst_fd)
 	if (!crc32_table)
 		crc32_table = crc32_filltable(NULL, /*endian:*/ 0);
 
-	membuf = xmalloc(2 * BUFSIZ);
 	memset(&iobuf, 0, sizeof(iobuf));
+	/* Preload XZ file signature */
+	membuf = (void*) strcpy(xmalloc(2 * BUFSIZ), HEADER_MAGIC);
 	iobuf.in = membuf;
+	iobuf.in_size = HEADER_MAGIC_SIZE;
 	iobuf.out = membuf + BUFSIZ;
 	iobuf.out_size = BUFSIZ;
 
diff --git a/archival/rpm2cpio.c b/archival/rpm2cpio.c
index 1f67fa8..b23d5af 100644
--- a/archival/rpm2cpio.c
+++ b/archival/rpm2cpio.c
@@ -92,8 +92,8 @@ int rpm2cpio_main(int argc UNUSED_PARAM, char **argv)
 			xread(rpm_fd, magic.b32, sizeof(magic.b32[0]));
 			if (magic.b32[0] != XZ_MAGIC2)
 				goto no_magic;
-			/* unpack_xz_stream wants fd at position 0 */
-			xlseek(rpm_fd, -6, SEEK_CUR);
+			/* unpack_xz_stream wants fd at position 6, no need to seek */
+			//xlseek(rpm_fd, -6, SEEK_CUR);
 			unpack = unpack_xz_stream;
 		} else {
  no_magic:
diff --git a/include/unarchive.h b/include/unarchive.h
index b4cf160..3b8e157 100644
--- a/include/unarchive.h
+++ b/include/unarchive.h
@@ -7,16 +7,21 @@ PUSH_AND_SET_FUNCTION_VISIBILITY_TO_HIDDEN
 enum {
 #if BB_BIG_ENDIAN
 	COMPRESS_MAGIC = 0x1f9d,
-	GZIP_MAGIC = 0x1f8b,
-	BZIP2_MAGIC = ('B'<<8) + 'Z',
-	XZ_MAGIC1 = (0xfd<<8) + '7',
-	XZ_MAGIC2 = ((((('z'<<8) + 'X')<<8) + 'Z')<<8) + 0,
+	GZIP_MAGIC  = 0x1f8b,
+	BZIP2_MAGIC = 'B' * 256 + 'Z',
+	XZ_MAGIC1   = 0xfd * 256 + '7',
+	XZ_MAGIC2   = (('z' * 256 + 'X') * 256 + 'Z') * 256 + 0,
+	/* Different form: 32 bits, then 16 bits: */
+	XZ_MAGIC1a  = ((0xfd * 256 + '7') * 256 + 'z') * 256 + 'X',
+	XZ_MAGIC2a  = 'Z' * 256 + 0,
 #else
 	COMPRESS_MAGIC = 0x9d1f,
-	GZIP_MAGIC = 0x8b1f,
-	BZIP2_MAGIC = ('Z'<<8) + 'B',
-	XZ_MAGIC1 = ('7'<<8) + 0xfd,
-	XZ_MAGIC2 = (((((0<<8) + 'Z')<<8) + 'X')<<8) + 'z',
+	GZIP_MAGIC  = 0x8b1f,
+	BZIP2_MAGIC = 'Z' * 256 + 'B',
+	XZ_MAGIC1   = '7' * 256 + 0xfd,
+	XZ_MAGIC2   = ((0 * 256 + 'Z') * 256 + 'X') * 256 + 'z',
+	XZ_MAGIC1a  = (('X' * 256 + 'z') * 256 + '7') * 256 + 0xfd,
+	XZ_MAGIC2a  = 0 * 256 + 'Z',
 #endif
 };
 
@@ -196,7 +201,7 @@ typedef struct inflate_unzip_result {
 } inflate_unzip_result;
 
 IF_DESKTOP(long long) int inflate_unzip(inflate_unzip_result *res, off_t compr_size, int src_fd, int dst_fd) FAST_FUNC;
-/* xz unpacker takes .xz stream from offset 0 */
+/* xz unpacker takes .xz stream from offset 6 */
 IF_DESKTOP(long long) int unpack_xz_stream(int src_fd, int dst_fd) FAST_FUNC;
 /* lzma unpacker takes .lzma stream from offset 0 */
 IF_DESKTOP(long long) int unpack_lzma_stream(int src_fd, int dst_fd) FAST_FUNC;
diff --git a/libbb/read_printf.c b/libbb/read_printf.c
index 3aee075..6780b94 100644
--- a/libbb/read_printf.c
+++ b/libbb/read_printf.c
@@ -296,8 +296,8 @@ void FAST_FUNC setup_unzip_on_fd(int fd /*, int fail_if_not_detected*/)
 		if (magic.b32[0] == XZ_MAGIC2) {
 # if BB_MMU
 			xformer = unpack_xz_stream;
-			/* unpack_xz_stream wants fd at position 0 */
-			xlseek(fd, offset, SEEK_CUR);
+			/* unpack_xz_stream wants fd at position 6, no need to seek */
+			//xlseek(fd, offset, SEEK_CUR);
 # else
 			xformer_prog = "unxz";
 # endif