tar: reject -C x --one-top-dir=/abs/olute

It’s not clear what ‘-C x --one-top-dir=/abs/olute’ should mean,
so for now let’s play it safe and reject that combination.
* src/misc.c (chdir_arg): Diagnose attempts to combine
-C with --one-top-level=/abs/olute.
* tests/onetop06.at: Move last test to onetop07.at,
since it now differs in behavior and it’s better to
test it separately.
* tests/onetop07.at: New file.
* tests/Makefile.am (TESTSUITE_AT): Add it.
* tests/testsuite.at: Include it.

NEWS

@@ -38,10 +38,11 @@ option.
 ** The --one-top-level option now requires either -x or -d mode.
    Previously the behavior was unspecified in other operation modes.
 
-** If DIR is absolute, the --one-top-level=DIR option now extracts all
-   files relative to DIR, without stripping leading DIR from file names.
-   Any -C options are ignored.  Formerly, the behavior was
-   undocumented and differed from one tar version to the next.
+** If DIR is absolute, the --one-top-level=DIR option is now incompatible
+   with the --directory (-C) option, and it now extracts all files
+   relative to DIR, without stripping leading DIR from file names.
+   Formerly, the behavior was undocumented and differed from one tar
+   version to the next.
 
 * Bug fixes
 

doc/tar.texi

@@ -3291,8 +3291,7 @@ Extract files relative to @var{dir},
 ignoring any @var{dir} prefixes in the archive.
 An absent @var{dir} defaults to
 the base name of the archive minus any recognized archive suffix.
-If @var{dir} is relative, it is relative to the working directory;
-if absolute, any @option{--directory} (@option{-C}) options are irrelevant.
+If @var{dir} is relative, it is relative to the working directory.
 @xref{directory}.
 
 @opsummary{overwrite}
@@ -9465,8 +9464,7 @@ Recognized file name suffixes are @samp{.tar},
 and any compression suffixes recognizable by @xref{--auto-compress}.
 
 If @var{dir} is absolute, extract all files under that directory;
-any @option{-C} options are irrelevant,
-and @var{dir} itself is exempt from the usual safety rules
+@var{dir} itself is exempt from the usual safety rules
 prohibiting extraction outside the working directory.
 The two main differences between @samp{--one-top-level=/abs/olute}
 and @samp{-C /abs/olute} are that the former affects all files
@@ -9477,7 +9475,9 @@ whereas in the latter @file{/abs/olute} must already exist.
 
 The @option{--one-top-level} option can be used only when extracting,
 comparing, or reading from the archive.
-It is incompatible with the @option{--absolute-names} (@option{-P}) option.
+It is incompatible with the @option{--absolute-names} (@option{-P}) option,
+and if @var{dir} is absolute it is incompatible with the
+@option{--directory} (@option{-C}) option.
 @end table
 
 All filename arguments appearing after a

src/misc.c

@@ -979,7 +979,10 @@ struct wd
    directory.  Ordinarily the remaining entries are for -C options.
    But if --one-top-level, each entry is followed by another entry for
    its --one-top-level counterpart, so that ordinary entries are
-   even-numbered and --one-top-level entries are odd-numbered.  */
+   even-numbered and --one-top-level entries are odd-numbered.
+   And if --one-top-level specifies an absolute directory,
+   there are just two entries, one for the initial working directory
+   and one for the absolute directory.  */
 static struct wd *wd;
 
 /* The number of working directories in the vector.  */
@@ -1046,13 +1049,18 @@ ensure_wd (void)
 }
 
 /* DIR is the operand of a -C option; add it to vector of chdir targets,
-   and return the index of its location.  If --one-top-level-dir, add
-   two targets to the vector.  However, if DIR is "." or an equivalent,
-   or if --one-top-level-dir is an absolute file name,
+   and return the index of its location.  If --one-top-level-dir=ONETOP
+   is specified, add two targets to the vector if ONETOP is relative
+   and report an error otherwise.  However, if DIR is "." or an equivalent,
    just reuse the last item in the vector.  */
 idx_t
 chdir_arg (char *dir)
 {
+  /* Unless this is the trivial chdir_arg (".") at start, this is an
+     error when combined with --one-top-level=X where X is absolute.  */
+  if (wd && one_top_level_dir && IS_ABSOLUTE_FILE_NAME (one_top_level_dir))
+    paxfatal (0, _("-C and --one-top-level='/...' are incompatible"));
+
   ensure_wd ();
 
   /* Optimize the common special case of the working directory,
@@ -1064,10 +1072,6 @@ chdir_arg (char *dir)
 	return wd_count - 1;
     }
 
-  /* Optimize --one-top-level=X where X is an absolute file name.  */
-  if (one_top_level_dir && IS_ABSOLUTE_FILE_NAME (one_top_level_dir))
-    return wd_count - 1;
-
   ptrdiff_t shortage = 1 + !!one_top_level_dir - (wd_alloc - wd_count);
   if (0 < shortage)
     wd = xpalloc (wd, &wd_alloc, shortage, -1, sizeof *wd);

tests/Makefile.am

@@ -205,6 +205,7 @@ TESTSUITE_AT = \
  onetop04.at\
  onetop05.at\
  onetop06.at\
+ onetop07.at\
  opcomp01.at\
  opcomp02.at\
  opcomp03.at\

tests/onetop06.at

@@ -32,8 +32,6 @@ diff d e/d/d || exit
 tar -cf test1.tar e/c || exit
 tar -xf test1.tar -C f --one-top-level=e/c || exit
 diff e/c/d f/e/c/d || exit
-tar -xf test1.tar -C f --one-top-level=$PWD/g || exit
-diff e/c/d g/e/c/d || exit
 ])
 
 AT_CLEANUP

tests/onetop07.at

@@ -0,0 +1,35 @@
+# Process this file with autom4te to create testsuite. -*- Autotest -*-
+#
+# Test suite for GNU tar.
+# 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/>.
+#
+AT_SETUP([tar -C and --one-top-level=/abs/olute])
+AT_KEYWORDS([extract onetop onetop07])
+
+AT_TAR_CHECK([
+mkdir d f && genfile --file d/file || framework_failure_
+tar -cf test.tar d || exit
+tar -xf test.tar -C f --one-top-level=$PWD/g
+],
+[2],
+[],
+[tar: -C and --one-top-level='/...' are incompatible
+tar: Error is not recoverable: exiting now
+])
+
+AT_CLEANUP

tests/testsuite.at

@@ -544,6 +544,7 @@ m4_include([onetop03.at])
 m4_include([onetop04.at])
 m4_include([onetop05.at])
 m4_include([onetop06.at])
+m4_include([onetop07.at])
 
 AT_BANNER([Star tests])
 m4_include([star/gtarfail.at])