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>

3 files changed, 14 insertions, 3 deletions

diff --git a/archival/bbunzip.c b/archival/bbunzip.c
index b243afb2e..832a7bbf3 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 5d7513a2e..721acd907 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 1f67fa887..b23d5afa9 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: