[3.13] gh-83869: tarfile: compute next header offset using pax size for sparse file (GH-18562)

Lib/tarfile.py

@@ -1605,17 +1605,22 @@ def _proc_pax(self, tarfile):
         if self.type in (XHDTYPE, SOLARIS_XHDTYPE):
             # Patch the TarInfo object with the extended header info.
             next._apply_pax_info(pax_headers, tarfile.encoding, tarfile.errors)
-            next.offset = self.offset
 
             if "size" in pax_headers:
                 # If the extended header replaces the size field,
                 # we need to recalculate the offset where the next
                 # header starts.
-                offset = next.offset_data
+                offset = next.offset + BLOCKSIZE
                 if next.isreg() or next.type not in SUPPORTED_TYPES:
-                    offset += next._block(next.size)
+                    try:
+                        size = PAX_NUMBER_FIELDS["size"](pax_headers["size"])
+                    except ValueError:
+                        size = 0
+                    offset += next._block(size)
                 tarfile.offset = offset
 
+            next.offset = self.offset
+
         return next
 
     def _proc_gnusparse_00(self, next, raw_headers):

Lib/test/test_tarfile.py

@@ -1348,6 +1348,37 @@ def test_sparse_file_01(self):
     def test_sparse_file_10(self):
         self._test_sparse_file("gnu/sparse-1.0")
 
+    def test_sparse_file_10_pax_size(self):
+        # gh-83869: when the pax header replaces the size field, the offset
+        # of the next header must be computed from the size of the data in
+        # the archive, not from the apparent size of the sparse file.
+        data = b"payload!" * 4
+        realsize = 1 << 20
+        smap = b"1\n%d\n%d\n" % (realsize - len(data), len(data))
+        smap += b"\0" * (-len(smap) % tarfile.BLOCKSIZE)
+
+        sparse = tarfile.TarInfo("sparse")
+        sparse.size = len(smap) + len(data)
+        sparse.pax_headers = {
+            "GNU.sparse.major": "1",
+            "GNU.sparse.minor": "0",
+            "GNU.sparse.name": "sparse",
+            "GNU.sparse.realsize": str(realsize),
+            "size": str(sparse.size),
+        }
+        buf = sparse.tobuf(tarfile.PAX_FORMAT)
+        buf += smap + data + b"\0" * (-len(data) % tarfile.BLOCKSIZE)
+
+        last = tarfile.TarInfo("last")
+        last.size = len(data)
+        buf += last.tobuf(tarfile.PAX_FORMAT)
+        buf += data + b"\0" * (-len(data) % tarfile.BLOCKSIZE)
+        buf += b"\0" * (tarfile.BLOCKSIZE * 2)
+
+        with tarfile.open(fileobj=io.BytesIO(buf)) as tar:
+            self.assertEqual(tar.getnames(), ["sparse", "last"])
+            self.assertEqual(tar.extractfile("last").read(), data)
+
     @staticmethod
     def _fs_supports_holes():
         # Return True if the platform knows the st_blocks stat attribute and
@@ -1400,6 +1431,31 @@ def test_pax_global_headers(self):
         finally:
             tar.close()
 
+    def test_offset_after_global_header(self):
+        # gh-83869: a global header is a member of its own, the member which
+        # follows it keeps the offset of its own header.
+        rec = b"30 comment=global header here\n"
+        glob = tarfile.TarInfo("././@PaxHeader")
+        glob.type = tarfile.XGLTYPE
+        glob.size = len(rec)
+        buf = glob.tobuf(tarfile.USTAR_FORMAT)
+        buf += rec + b"\0" * (-len(rec) % tarfile.BLOCKSIZE)
+
+        member = tarfile.TarInfo("member")
+        data = b"hello\n"
+        member.size = len(data)
+        offset = len(buf)
+        buf += member.tobuf(tarfile.USTAR_FORMAT)
+        buf += data + b"\0" * (-len(data) % tarfile.BLOCKSIZE)
+        buf += b"\0" * (tarfile.BLOCKSIZE * 2)
+
+        with tarfile.open(fileobj=io.BytesIO(buf)) as tar:
+            tarinfo = tar.getmember("member")
+            self.assertEqual(tarinfo.offset, offset)
+            self.assertEqual(tarinfo.pax_headers.get("comment"),
+                             "global header here")
+            self.assertEqual(tar.extractfile(tarinfo).read(), data)
+
     def test_pax_number_fields(self):
         # All following number fields are read from the pax header.
         tar = tarfile.open(tarname, encoding="iso8859-1")

Misc/NEWS.d/next/Library/2020-02-19-16-35-52.gh-issue-83869.EPD_zn.rst

@@ -0,0 +1,6 @@
+Fix :mod:`tarfile` reading an archive with a GNU sparse 1.0 member whose
+size is set in the pax extended header.
+The offset of the next header was computed from the offset of the data,
+which is already past the sparse map, and from the size of the member,
+which can be the apparent size of the sparse file.
+All following members were unreachable.