tar,unzip: postpone creation of symlinks with "suspicious" targets

This mostly reverts commit bc9bbeb2b81001e8731cd2ae501c8fccc8d87cc7
"libarchive: do not extract unsafe symlinks unless $EXTRACT_UNSAFE_SYMLINKS=1"

Users report that it is somewhat too restrictive. See
https://bugs.busybox.net/show_bug.cgi?id=8411

In particular, this interferes with unpacking of busybox-based
filesystems with links like "sbin/applet" -> "../bin/busybox".

The change is made smaller by deleting ARCHIVE_EXTRACT_QUIET flag -
it is unused since 2010, and removing conditionals on it
allows commonalizing some error message codes.

function                                             old     new   delta
create_or_remember_symlink                             -      94     +94
create_symlinks_from_list                              -      64     +64
tar_main                                            1002    1006      +4
unzip_main                                          2732    2724      -8
data_extract_all                                     984     891     -93
unsafe_symlink_target                                147       -    -147
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 1/2 up/down: 162/-248)          Total: -86 bytes

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

4 files changed, 49 insertions, 65 deletions

diff --git a/archival/libarchive/data_extract_all.c b/archival/libarchive/data_extract_all.c
index d3a6df5..8fa69ff 100644
--- a/archival/libarchive/data_extract_all.c
+++ b/archival/libarchive/data_extract_all.c
@@ -107,9 +107,7 @@ void FAST_FUNC data_extract_all(archive_handle_t *archive_handle)
 			}
 		}
 		else if (existing_sb.st_mtime >= file_header->mtime) {
-			if (!(archive_handle->ah_flags & ARCHIVE_EXTRACT_QUIET)
-			 && !S_ISDIR(file_header->mode)
-			) {
+			if (!S_ISDIR(file_header->mode)) {
 				bb_error_msg("%s not created: newer or "
 					"same age file exists", dst_name);
 			}
@@ -125,7 +123,7 @@ void FAST_FUNC data_extract_all(archive_handle_t *archive_handle)
 	/* Handle hard links separately */
 	if (hard_link) {
 		res = link(hard_link, dst_name);
-		if (res != 0 && !(archive_handle->ah_flags & ARCHIVE_EXTRACT_QUIET)) {
+		if (res != 0) {
 			/* shared message */
 			bb_perror_msg("can't create %slink '%s' to '%s'",
 				"hard",	dst_name, hard_link
@@ -165,10 +163,9 @@ void FAST_FUNC data_extract_all(archive_handle_t *archive_handle)
 	}
 	case S_IFDIR:
 		res = mkdir(dst_name, file_header->mode);
-		if ((res == -1)
+		if ((res != 0)
 		 && (errno != EISDIR) /* btw, Linux doesn't return this */
 		 && (errno != EEXIST)
-		 && !(archive_handle->ah_flags & ARCHIVE_EXTRACT_QUIET)
 		) {
 			bb_perror_msg("can't make dir %s", dst_name);
 		}
@@ -198,27 +195,16 @@ void FAST_FUNC data_extract_all(archive_handle_t *archive_handle)
 		 *
 		 * Untarring bug.tar would otherwise place evil.py in '/tmp'.
 		 */
-		if (!unsafe_symlink_target(file_header->link_target)) {
-			res = symlink(file_header->link_target, dst_name);
-			if (res != 0
-			 && !(archive_handle->ah_flags & ARCHIVE_EXTRACT_QUIET)
-			) {
-				/* shared message */
-				bb_perror_msg("can't create %slink '%s' to '%s'",
-					"sym",
-					dst_name, file_header->link_target
-				);
-			}
-		}
+		create_or_remember_symlink(&archive_handle->symlink_placeholders,
+				file_header->link_target,
+				dst_name);
 		break;
 	case S_IFSOCK:
 	case S_IFBLK:
 	case S_IFCHR:
 	case S_IFIFO:
 		res = mknod(dst_name, file_header->mode, file_header->device);
-		if ((res == -1)
-		 && !(archive_handle->ah_flags & ARCHIVE_EXTRACT_QUIET)
-		) {
+		if (res != 0) {
 			bb_perror_msg("can't create node %s", dst_name);
 		}
 		break;
diff --git a/archival/libarchive/unsafe_symlink_target.c b/archival/libarchive/unsafe_symlink_target.c
index 441ba8b..8dcafea 100644
--- a/archival/libarchive/unsafe_symlink_target.c
+++ b/archival/libarchive/unsafe_symlink_target.c
@@ -5,44 +5,37 @@
 #include "libbb.h"
 #include "bb_archive.h"
 
-int FAST_FUNC unsafe_symlink_target(const char *target)
+void FAST_FUNC create_or_remember_symlink(llist_t **symlink_placeholders,
+		const char *target,
+		const char *linkname)
 {
-	const char *dot;
-
-	if (target[0] == '/') {
-		const char *var;
- unsafe:
-		var = getenv("EXTRACT_UNSAFE_SYMLINKS");
-		if (var) {
-			if (LONE_CHAR(var, '1'))
-				return 0; /* pretend it's safe */
-			return 1; /* "UNSAFE!" */
-		}
-		bb_error_msg("skipping unsafe symlink to '%s' in archive,"
-			" set %s=1 to extract",
-			target,
-			"EXTRACT_UNSAFE_SYMLINKS"
+	if (target[0] == '/' || strstr(target, "..")) {
+		llist_add_to(symlink_placeholders,
+			xasprintf("%s%c%s", linkname, '\0', target)
+		);
+		return;
+	}
+	if (symlink(target, linkname) != 0) {
+		/* shared message */
+		bb_perror_msg_and_die("can't create %slink '%s' to '%s'",
+			"sym", linkname, target
 		);
-		/* Prevent further messages */
-		setenv("EXTRACT_UNSAFE_SYMLINKS", "0", 0);
-		return 1; /* "UNSAFE!" */
 	}
+}
 
-	dot = target;
-	for (;;) {
-		dot = strchr(dot, '.');
-		if (!dot)
-			return 0; /* safe target */
+void FAST_FUNC create_symlinks_from_list(llist_t *list)
+{
+	while (list) {
+		char *target;
 
-		/* Is it a path component starting with ".."? */
-		if ((dot[1] == '.')
-		 && (dot == target || dot[-1] == '/')
-		    /* Is it exactly ".."? */
-		 && (dot[2] == '/' || dot[2] == '\0')
-		) {
-			goto unsafe;
+		target = list->data + strlen(list->data) + 1;
+		if (symlink(target, list->data)) {
+			/* shared message */
+			bb_error_msg_and_die("can't create %slink '%s' to '%s'",
+				"sym",
+				list->data, target
+			);
 		}
-		/* NB: it can even be trailing ".", should only add 1 */
-		dot += 1;
+		list = list->link;
 	}
 }
diff --git a/archival/tar.c b/archival/tar.c
index 9ed3821..415ebde 100644
--- a/archival/tar.c
+++ b/archival/tar.c
@@ -1244,6 +1244,8 @@ int tar_main(int argc UNUSED_PARAM, char **argv)
 	while (get_header_tar(tar_handle) == EXIT_SUCCESS)
 		bb_got_signal = EXIT_SUCCESS; /* saw at least one header, good */
 
+	create_symlinks_from_list(tar_handle->symlink_placeholders);
+
 	/* Check that every file that should have been extracted was */
 	while (tar_handle->accept) {
 		if (!find_list_entry(tar_handle->reject, tar_handle->accept->data)
diff --git a/archival/unzip.c b/archival/unzip.c
index da4b2a5..0d00d8d 100644
--- a/archival/unzip.c
+++ b/archival/unzip.c
@@ -345,7 +345,9 @@ static void unzip_create_leading_dirs(const char *fn)
 }
 
 #if ENABLE_FEATURE_UNZIP_CDF
-static void unzip_extract_symlink(zip_header_t *zip, const char *dst_fn)
+static void unzip_extract_symlink(llist_t **symlink_placeholders,
+		zip_header_t *zip,
+		const char *dst_fn)
 {
 	char *target;
 
@@ -370,15 +372,9 @@ static void unzip_extract_symlink(zip_header_t *zip, const char *dst_fn)
 		target[xstate.mem_output_size] = '\0';
 #endif
 	}
-	if (!unsafe_symlink_target(target)) {
-//TODO: libbb candidate
-		if (symlink(target, dst_fn)) {
-			/* shared message */
-			bb_perror_msg_and_die("can't create %slink '%s' to '%s'",
-				"sym", dst_fn, target
-			);
-		}
-	}
+	create_or_remember_symlink(symlink_placeholders,
+			target,
+			dst_fn);
 	free(target);
 }
 #endif
@@ -490,6 +486,9 @@ int unzip_main(int argc, char **argv)
 	llist_t *zaccept = NULL;
 	llist_t *zreject = NULL;
 	char *base_dir = NULL;
+#if ENABLE_FEATURE_UNZIP_CDF
+	llist_t *symlink_placeholders = NULL;
+#endif
 	int i;
 	char key_buf[80]; /* must match size used by my_fgets80 */
 
@@ -954,7 +953,7 @@ int unzip_main(int argc, char **argv)
 #if ENABLE_FEATURE_UNZIP_CDF
 			if (S_ISLNK(file_mode)) {
 				if (dst_fd != STDOUT_FILENO) /* not -p? */
-					unzip_extract_symlink(&zip, dst_fn);
+					unzip_extract_symlink(&symlink_placeholders, &zip, dst_fn);
 			} else
 #endif
 			{
@@ -990,6 +989,10 @@ int unzip_main(int argc, char **argv)
 		total_entries++;
 	}
 
+#if ENABLE_FEATURE_UNZIP_CDF
+	create_symlinks_from_list(symlink_placeholders);
+#endif
+
 	if ((opts & OPT_l) && quiet <= 1) {
 		if (!verbose) {
 			//	"  Length      Date    Time    Name\n"