tar: ignore nonzero sizes in hard links etc

Problem reported by Antonio Teixeira.
* src/list.c (read_header): When POSIX says a size field must
be zero or does not represent a data count, treat it as zero.
* tests/extrac32.at: Update to match new behavior.
We now treat hard link sizes as zero even when the size fields are
nonzero, and this means the “injected” file is treated as valid
regardless of whether we list or extract.
* tests/extrac34.at: New test.
* tests/Makefile.am (TESTSUITE_AT), tests/testsuite.at: Add it.

NEWS

@@ -1,4 +1,4 @@
-GNU tar NEWS - User visible changes. 2026-04-04
+GNU tar NEWS - User visible changes. 2026-06-20
 Please send GNU tar bug reports to <bug-tar@gnu.org>
 
 version 1.35.90 (git)
@@ -73,6 +73,11 @@ option.
 ** tar --ignore-failed-read no longer complains about a file removed
    before reading.
 
+** tar no longer behaves erratically when reading nonzero size fields
+   in archive headers representing special files, fifos, and symlinks.
+   Although these size fields are typically zero, POSIX allows some to
+   be nonzero, and in practice they do not count data blocks.
+
 * Performance improvements
 
 ** Sparse files are now read and written with larger blocksizes.

THANKS

@@ -39,6 +39,7 @@ Andrey A. Chernov	ache@astral.msk.su
 Andy Gay		andy@rdl.co.uk
 Anthony G. Basile	blueness@gentoo.org
 Antonio Jose Coutinho	ajc@di.uminho.pt
+Antonio Teixeira	antonio.teixeira@suse.com
 Ariel Faigon		ariel@engr.sgi.com
 Arne Wichmann		aw@math.uni-sb.de
 Arnold Robbins		arnold@gnu.org

src/list.c

@@ -535,6 +535,22 @@ read_header (union block **return_block, struct tar_stat_info *info,
 	  struct posix_header const *h = &header->header;
 	  char namebuf[sizeof h->prefix + 1 + NAME_FIELD_SIZE + 1];
 
+	  switch (h->typeflag)
+	    {
+	    /* For these file types, although POSIX does not specify the
+	       meaning of the size, it does say there should be no data,
+	       so treat the size as zero.  */
+	    case BLKTYPE: case CHRTYPE: case FIFOTYPE:
+
+	    /* For these file types, POSIX requires that the size be zero.
+	       Be generous and accept any size as zero, as some
+	       nonconforming programs generate nonzero size fields along
+	       with no data.  */
+	    case LNKTYPE: case SYMTYPE:
+
+	      info->stat.st_size = 0;
+	    }
+
 	  free (recent_long_name);
 
 	  if (next_long_name)

tests/Makefile.am

@@ -143,6 +143,7 @@ TESTSUITE_AT = \
  extrac31.at\
  extrac32.at\
  extrac33.at\
+ extrac34.at\
  filerem01.at\
  filerem02.at\
  filerem03.at\

tests/extrac32.at

@@ -36,11 +36,13 @@ base64 -d < archive.in | xz -c -d > archive.tar
 AT_CHECK([tar tf archive.tar],
 [0],
 [carrier_entry
+injected.txt
 marker.txt
 ])
 AT_CHECK([tar xvf archive.tar],
 [0],
 [carrier_entry
+injected.txt
 marker.txt
 ])
 AT_CLEANUP

tests/extrac34.at

@@ -0,0 +1,40 @@
+# Check hard link with nonzero size field in tarball. -*- Autotest -*-
+
+# Copyright 2026 Free Software Foundation, Inc.
+
+# This file is part of GNU tar.
+
+# GNU tar is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+
+# GNU tar is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check extracting from a nonconforming tarball that has
+# a hard link with nonzero size.
+
+AT_SETUP([hard link with a nonzero size field])
+AT_KEYWORDS([extract extrac34 hard link])
+AT_DATA([archive.in],
+[/Td6WFoAAATm1rRGBMBwgFAhARwAAAAAAAAAAAbKrCjgJ/8AaF0AMIAzUBhoiawFdYeRen4lxlj0
+QWRLpUM+28ArRsXVY5bGv4H5kijsqiJ4Z9YIVhZd01+IppF+AkltS60aB8fuUW35Tp/3XzUx9Mq2
+4ypJFzvcgHXsSIvc9L+mmTDuHzhvJQ/oe7ya8QAASbOfD7CgZcIAAYwBgFAAAEWxohaxxGf7AgAA
+AAAEWVo=
+])
+AT_CHECK([base64 --help >/dev/null 2>&1 || AT_SKIP_TEST
+xz --help >/dev/null 2>&1 || AT_SKIP_TEST
+base64 -d < archive.in | xz -c -d > archive.tar
+])
+AT_CHECK([mkdir dir
+])
+AT_CHECK([tar -C dir -xf archive.tar || exit 1
+cmp dir/a dir/b
+])
+AT_CLEANUP

tests/testsuite.at

@@ -360,6 +360,7 @@ m4_include([extrac30.at])
 m4_include([extrac31.at])
 m4_include([extrac32.at])
 m4_include([extrac33.at])
+m4_include([extrac34.at])
 
 m4_include([backup01.at])