tar: prevent empty file to be treated as valid tarball

function                                             old     new   delta
tar_main                                             879     895     +16

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

2 files changed, 35 insertions, 3 deletions

diff --git a/archival/tar.c b/archival/tar.c
index 3877ea4..bd61abd 100644
--- a/archival/tar.c
+++ b/archival/tar.c
@@ -1196,8 +1196,14 @@ int tar_main(int argc UNUSED_PARAM, char **argv)
 		/*tar_handle->offset = 0; - already is */
 	}
 
+	/* Zero processed headers (== empty file) is not a valid tarball.
+	 * We (ab)use bb_got_signal as exitcode here,
+	 * because check_errors_in_children() uses _it_ as error indicator.
+	 */
+	bb_got_signal = EXIT_FAILURE;
+
 	while (get_header_tar(tar_handle) == EXIT_SUCCESS)
-		continue;
+		bb_got_signal = EXIT_SUCCESS; /* saw at least one header, good */
 
 	/* Check that every file that should have been extracted was */
 	while (tar_handle->accept) {
@@ -1213,8 +1219,9 @@ int tar_main(int argc UNUSED_PARAM, char **argv)
 		close(tar_handle->src_fd);
 
 	if (SEAMLESS_COMPRESSION || OPT_COMPRESS) {
+		/* Set bb_got_signal to 1 if a child died with !0 exitcode */
 		check_errors_in_children(0);
-		return bb_got_signal;
 	}
-	return EXIT_SUCCESS;
+
+	return bb_got_signal;
 }
diff --git a/testsuite/tar.tests b/testsuite/tar.tests
index c947d33..93ba5f0 100755
--- a/testsuite/tar.tests
+++ b/testsuite/tar.tests
@@ -15,6 +15,15 @@ mkdir tar.tempdir && cd tar.tempdir || exit 1
 
 # testing "test name" "script" "expected result" "file input" "stdin"
 
+testing "Empty file is not a tarball" '\
+tar xvf - 2>&1; echo $?
+' "\
+tar: short read
+1
+" \
+"" ""
+SKIP=
+
 optional FEATURE_SEAMLESS_GZ
 testing "Empty file is not a tarball.tar.gz" '\
 tar xvzf - 2>&1; echo $?
@@ -26,6 +35,22 @@ tar: short read
 "" ""
 SKIP=
 
+testing "Two zeroed blocks is a ('truncated') empty tarball" '\
+dd if=/dev/zero bs=512 count=2 2>/dev/null | strace -o /tmp/zzz -tt -s99 tar xvf - 2>&1; echo $?
+' "\
+0
+" \
+"" ""
+SKIP=
+
+testing "Twenty zeroed blocks is an empty tarball" '\
+dd if=/dev/zero bs=512 count=20 2>/dev/null | tar xvf - 2>&1; echo $?
+' "\
+0
+" \
+"" ""
+SKIP=
+
 optional FEATURE_TAR_CREATE FEATURE_LS_SORTFILES
 testing "tar hardlinks and repeated files" '\
 rm -rf input_* test.tar 2>/dev/null