tar: handle the case when opened created tarball happens to have fd#0

Reproducer:
    exec 0>&-
    exec 1>&-
    tar czf z.tar.gz FILE

function                                             old     new   delta
vfork_compressor                                     229     257     +28

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

1 files changed, 18 insertions, 6 deletions

diff --git a/archival/tar.c b/archival/tar.c
index 9239d8ee4..120c77f3b 100644
--- a/archival/tar.c
+++ b/archival/tar.c
@@ -611,6 +611,7 @@ static void NOINLINE vfork_compressor(int tar_fd, const char *gzip)
 
         if (xvfork() == 0) {
                 /* child */
+                int tfd;
                 /* NB: close _first_, then move fds! */
                 close(data.wr);
 #  if WAIT_FOR_CHILD
@@ -619,8 +620,23 @@ static void NOINLINE vfork_compressor(int tar_fd, const char *gzip)
                  * parent waits for this close to happen */
                 fcntl(status.wr, F_SETFD, FD_CLOEXEC);
 #  endif
+                /* copy it: parent's tar_fd variable must not change */
+                tfd = tar_fd;
+                if (tfd == 0) {
+                        /* Output tar fd may be zero.
+                         * xmove_fd(data.rd, 0) would destroy it.
+                         * Reproducer:
+                         *  exec 0>&-
+                         *  exec 1>&-
+                         *  tar czf Z.tar.gz FILE
+                         * Swapping move_fd's order wouldn't work:
+                         * data.rd is 1 and _it_ would be destroyed.
+                         */
+                        xmove_fd(tfd, 3);
+                        tfd = 3;
+                }
                 xmove_fd(data.rd, 0);
-                xmove_fd(tar_fd, 1);
+                xmove_fd(tfd, 1);
                 /* exec gzip/bzip2 program/applet */
                 BB_EXECLP(gzip, gzip, "-f", (char *)0);
                 vfork_exec_errno = errno;
@@ -715,11 +731,7 @@ static NOINLINE int writeTarFile(
         return errorFlag;
 }
 
-#else /* !FEATURE_TAR_CREATE */
-
-# define writeTarFile(...) 0
-
-#endif
+#endif /* FEATURE_TAR_CREATE */
 
 #if ENABLE_FEATURE_TAR_FROM
 static llist_t *append_file_list_to_list(llist_t *list)