Allow valid links in extracted tar archives

The check is intended to ensure paths don't traverse outside
the extracted directory.  However, the check was too strict: it
banned all links, even relative links that still point inside the
target directory.  This CL relaxes the requirement to allow valid
links.  This is required to allow library symlinks for the
instrumented libraries.  Examples:
libpcre.so -> libpcre.so.3.13.3
libpcre.so.3 -> libpcre.so.3.13.3
libpixman-1.so -> libpixman-1.so.0.38.4
libpixman-1.so.0 -> libpixman-1.so.0.38.4
libpng16.so -> libpng16.so.16.37.0
libpng16.so.16 -> libpng16.so.16.37.0

Bug: 320564950
Change-Id: I2aae18b86b1f1cc3d73a1b80c06d757af782f700
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5369132
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>

download_from_google_storage.py

@@ -225,7 +225,14 @@ def _validate_tar_file(tar, prefix):
     def _validate(tarinfo):
         """Returns false if the tarinfo is something we explicitly forbid."""
         if tarinfo.issym() or tarinfo.islnk():
+            # For links, check if the destination is valid.
+            if os.path.isabs(tarinfo.linkname):
                 return False
+            link_target = os.path.normpath(
+                os.path.join(os.path.dirname(tarinfo.name), tarinfo.linkname))
+            if not link_target.startswith(prefix):
+                return False
+
         if ('../' in tarinfo.name or '..\\' in tarinfo.name
                 or not tarinfo.name.startswith(prefix)):
             return False