Fix #75102: PharData says invalid checksum for valid tar

Apparently, there are broken tarballs out there which are actually in
ustar format, but did not write the `ustar` marker.  Since popular tar
tools like GNU tar and 7zip have no issues dealing with such tarballs,
Phar should also be more resilient.

Thus, when the first checksum check of a tarball in (presumed) in old-
style format fails, we check whether the checksum would be suitable for
ustar format; if so, we treat the tarball as being in ustar format.

Closes GH-6479.

NEWS

@@ -26,6 +26,7 @@ PHP                                                                        NEWS
 
 - Phar:
   . Fixed bug #73809 (Phar Zip parse crash - mmap fail). (cmb)
+  . Fixed #75102 (`PharData` says invalid checksum for valid tar). (cmb)
 
 - Phpdbg:
   . Fixed bug #76813 (Access violation near NULL on source operand). (cmb)

ext/phar/tar.c

@@ -271,6 +271,15 @@ int phar_parse_tarfile(php_stream* fp, char *fname, size_t fname_len, char *alia
 		memset(hdr->checksum, ' ', sizeof(hdr->checksum));
 		sum2 = phar_tar_checksum(buf, old?sizeof(old_tar_header):sizeof(tar_header));
 
+		if (old && sum2 != sum1) {
+			uint32_t sum3 = phar_tar_checksum(buf, sizeof(tar_header));
+			if (sum3 == sum1) {
+				/* apparently a broken tar which is in ustar format w/o setting the ustar marker */
+				sum2 = sum3;
+				old = 0;
+			}
+		}
+
 		size = entry.uncompressed_filesize = entry.compressed_filesize =
 			phar_tar_number(hdr->size, sizeof(hdr->size));
 

ext/phar/tests/bug75102.phpt

@@ -0,0 +1,13 @@
+--TEST--
+Bug #75102 (`PharData` says invalid checksum for valid tar)
+--SKIPIF--
+<?php
+if (!extension_loaded('phar')) die('skip phar extension not available');
+?>
+--FILE--
+<?php
+$phar = new PharData(__DIR__ . '/bug75102.tar');
+var_dump(file_get_contents($phar['test.txt']->getPathName()));
+?>
+--EXPECT--
+string(9) "yada yada"