ls: color each symlink-to-relative-name in / properly

In order for ls --color to color each symlink, it must form the name
of each referent and then stat it to see if the link is dangling, to
a directory, to a file, etc.  When the symlink is to a relative name,
ls must concatenate the starting directory name and that relative name.
When, in addition, the starting directory was "/" or "/some-name",
the result was ill-formed, and the subsequent stat would usually fail,
making the caller color it as a dangling symlink.
* src/ls.c (make_link_name): Don't botch the case in which
dir_name(NAME) == "/" and LINKNAME is relative.
* tests/ls/root-rel-symlink-color: New file.  Test for the above.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.
Reported by Mike Frysinger in http://bugs.gnu.org/11453
Bug introduced by commit v8.16-23-gbcb9078.

4 files changed, 65 insertions, 1 deletions

diff --git a/NEWS b/NEWS
index 6c620b339..f9e9c702b 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,11 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** Bug fixes
+
+  ls --color would mis-color relative-named symlinks in /
+  [bug introduced in coreutils-8.17]
+
 
 * Noteworthy changes in release 8.17 (2012-05-10) [stable]
 
diff --git a/src/ls.c b/src/ls.c
index 397e4ea92..9494ae919 100644
--- a/src/ls.c
+++ b/src/ls.c
@@ -3213,7 +3213,14 @@ make_link_name (char const *name, char const *linkname)
     return xstrdup (linkname);
 
   char *p = xmalloc (prefix_len + 1 + strlen (linkname) + 1);
-  stpcpy (stpncpy (p, name, prefix_len + 1), linkname);
+
+  /* PREFIX_LEN usually specifies a string not ending in slash.
+     In that case, extend it by one, since the next byte *is* a slash.
+     Otherwise, the prefix is "/", so leave the length unchanged.  */
+  if ( ! ISSLASH (name[prefix_len - 1]))
+    ++prefix_len;
+
+  stpcpy (stpncpy (p, name, prefix_len), linkname);
   return p;
 }
 
diff --git a/tests/Makefile.am b/tests/Makefile.am
index a4370a6d1..0bafc5fc4 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -449,6 +449,7 @@ TESTS =						\
   ls/proc-selinux-segfault			\
   ls/readdir-mountpoint-inode			\
   ls/recursive					\
+  ls/root-rel-symlink-color			\
   ls/rt-1					\
   ls/slink-acl					\
   ls/stat-dtype					\
diff --git a/tests/ls/root-rel-symlink-color b/tests/ls/root-rel-symlink-color
new file mode 100755
index 000000000..d7954323e
--- /dev/null
+++ b/tests/ls/root-rel-symlink-color
@@ -0,0 +1,51 @@
+#!/bin/sh
+# Exercise the 8.17 ls bug with coloring relative-named symlinks in "/".
+
+# Copyright (C) 2012 Free Software Foundation, Inc.
+
+# This program 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.
+
+# This program 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/>.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+print_ver_ ls
+
+symlink_to_rel=
+for i in /*; do
+  # Skip non-symlinks:
+  env test -h "$i" || continue
+
+  # Skip dangling symlinks:
+  env test -e "$i" || continue
+
+  # Skip any symlink-to-absolute-name:
+  case $(readlink "$i") in /*) continue ;; esac
+
+  symlink_to_rel=$i
+  break
+done
+
+test -z "$symlink_to_rel" \
+  && skip_ no relative symlink in /
+
+e='\33'
+color_code='01;36'
+c_pre="$e[0m$e[${color_code}m"
+c_post="$e[0m"
+printf "$c_pre$symlink_to_rel$c_post\n" > exp || framework_failure_
+
+env TERM=xterm LS_COLORS="ln=$color_code:or=1;31;42" \
+  ls -d --color=always "$symlink_to_rel" > out || fail=1
+
+compare exp out || fail=1
+
+Exit $fail