xar: Avoid infinite link loop

[stoeckmann]

A file may have only one link target at a time. Otherwise the internal link structure could loop. Besides, a hard link realistically can only link to one file, not multiple ones.

Consider such an archive invalid.

Proof of Concept:

1. Create archive which contains multiple files with multiple links

$ base64 -d > archive.xar << EOF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EOF

2. Try to list content

$ bsdtar -tf archive.xar

An infinite loop is entered with 100 % CPU usage which will never complete (and neither fail due to memory constraints etc.).

[kientzle]

Thanks! The fix looks reasonable to me, but I'd like to include a test as well. Can you explain how to re-create your test archive?

[stoeckmann]

Thanks! The fix looks reasonable to me, but I'd like to include a test as well. Can you explain how to re-create your test archive?

I have created the archive by patching libarchive to create invalid xar files due to duplicate link entries:

diff --git a/libarchive/archive_write_set_format_xar.c b/libarchive/archive_write_set_format_xar.c
index 2cf655d..bd46f8e 100644
--- a/libarchive/archive_write_set_format_xar.c
+++ b/libarchive/archive_write_set_format_xar.c
@@ -1241,6 +1241,8 @@ make_file_entry(struct archive_write *a, xmlTextWriterPtr writer,
 			filetype = "file";
 		break;
 	}
+	r = xmlwrite_string_attr(a, writer, "type", filetype,
+	    filelink, linkto.s);
 	r = xmlwrite_string_attr(a, writer, "type", filetype,
 	    filelink, linkto.s);
 	archive_string_free(&linkto);

With such a modified bsdtar, the commands are:

touch file1
ln file1 file2
./bsdtar --format xar -cf archive.xar file1 file2

[mmatuska]

Merged