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>

1 files changed, 9 insertions, 0 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;