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.

1 files changed, 8 insertions, 1 deletions

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;
 }