Cygwin: introduce isabspath_strict macro

isabspath handles a path "X:", without trailing slash or backslash,
as absolute path.  This breaks some scenarios with relative paths
starting with "X:".  For instance, fstatat will mishandle a call
with valid dirfd and "c:" as path.

The reason is that gen_full_path_at() will check for isabspath("C:")
which returns true.  So the path will be used verbatim in fstatat,
rather than being converted to a path "<dirfd-path>/c:".

So, introduce isabspath_strict, which returns true for paths starting
with "X:" only if the next char is actually a slash or backslash.
Use it from gen_full_path_at().

This still fixes only half the problem.  The right thing would have been
to disallow using DOS paths in the first place.  Unfortunately it's much
too late for that.

Addresses: https://cygwin.com/pipermail/cygwin/2021-November/249837.html
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/syscalls.cc

@@ -4714,7 +4714,7 @@ gen_full_path_at (char *path_ret, int dirfd, const char *pathname,
 	  return -1;
 	}
     }
-  if (pathname && isabspath (pathname))
+  if (pathname && isabspath_strict (pathname))
     stpcpy (path_ret, pathname);
   else
     {

winsup/cygwin/winsup.h

@@ -139,9 +139,17 @@ extern int cygserver_running;
 #undef issep
 #define issep(ch) (strchr (" \t\n\r", (ch)) != NULL)
 
+/* Treats "X:" as absolute path.
+   FIXME: We should drop the notion that "X:" is a valid absolute path.
+   Only "X:/" and "X:\\" should be (see isabspath_strict below).  The
+   problem is to find out if we have code depending on this behaviour. */
 #define isabspath(p) \
   (isdirsep (*(p)) || (isalpha (*(p)) && (p)[1] == ':' && (!(p)[2] || isdirsep ((p)[2]))))
 
+/* Treats "X:/" and "X:\\" as absolute paths, but not "X:" */
+#define isabspath_strict(p) \
+  (isdirsep (*(p)) || (isalpha (*(p)) && (p)[1] == ':' && isdirsep ((p)[2])))
+
 /******************** Initialization/Termination **********************/
 
 class per_process;