[PATCH 3/3] Cygwin: fchownat and fstatat: support the AT_EMPTY_PATH flag

[Ken Brown <kbrown at cornell dot edu>]

Following Linux, allow the pathname argument to be empty if the
AT_EMPTY_PATH is specified.  In this case the dirfd argument can refer
to any type of file, not just a directory, and the call operates on
that file.  In particular, dirfd can refer to a symlink that was
opened with O_PATH and O_NOFOLLOW.

Add a new optional argument to gen_full_path_at to help implement
this.
---
 winsup/cygwin/syscalls.cc | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
index 2be8693c9..1bc856268 100644
--- a/winsup/cygwin/syscalls.cc
+++ b/winsup/cygwin/syscalls.cc
@@ -4635,7 +4635,8 @@ pclose (FILE *fp)
 
 static int
 gen_full_path_at (char *path_ret, int dirfd, const char *pathname,
-		  bool null_pathname_allowed = false)
+		  bool null_pathname_allowed = false,
+		  bool at_empty_path_flag = false)
 {
   /* Set null_pathname_allowed to true to allow GLIBC compatible behaviour
      for NULL pathname.  Only used by futimesat. */
@@ -4644,20 +4645,25 @@ gen_full_path_at (char *path_ret, int dirfd, const char *pathname,
       set_errno (EFAULT);
       return -1;
     }
+  bool empty_path = false;
   if (pathname)
     {
       if (!*pathname)
 	{
-	  set_errno (ENOENT);
-	  return -1;
+	  empty_path = true;
+	  if (!at_empty_path_flag)
+	    {
+	      set_errno (ENOENT);
+	      return -1;
+	    }
 	}
-      if (strlen (pathname) >= PATH_MAX)
+      else if (strlen (pathname) >= PATH_MAX)
 	{
 	  set_errno (ENAMETOOLONG);
 	  return -1;
 	}
     }
-  if (pathname && isabspath (pathname))
+  if (pathname && !empty_path && isabspath (pathname))
     stpcpy (path_ret, pathname);
   else
     {
@@ -4674,12 +4680,14 @@ gen_full_path_at (char *path_ret, int dirfd, const char *pathname,
 	  cygheap_fdget cfd (dirfd);
 	  if (cfd < 0)
 	    return -1;
-	  if (!cfd->pc.isdir ())
+	  if (!empty_path && !cfd->pc.isdir ())
 	    {
 	      set_errno (ENOTDIR);
 	      return -1;
 	    }
 	  p = stpcpy (path_ret, cfd->get_name ());
+	  if (empty_path)
+	    return 0;
 	}
       if (!p)
 	{
@@ -4785,13 +4793,14 @@ fchownat (int dirfd, const char *pathname, uid_t uid, gid_t gid, int flags)
   tmp_pathbuf tp;
   __try
     {
-      if (flags & ~AT_SYMLINK_NOFOLLOW)
+      if (flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))
 	{
 	  set_errno (EINVAL);
 	  __leave;
 	}
       char *path = tp.c_get ();
-      if (gen_full_path_at (path, dirfd, pathname))
+      if (gen_full_path_at (path, dirfd, pathname, false,
+			    flags & AT_EMPTY_PATH))
 	__leave;
       return chown_worker (path, (flags & AT_SYMLINK_NOFOLLOW)
 				 ? PC_SYM_NOFOLLOW : PC_SYM_FOLLOW, uid, gid);
@@ -4808,13 +4817,14 @@ fstatat (int dirfd, const char *__restrict pathname, struct stat *__restrict st,
   tmp_pathbuf tp;
   __try
     {
-      if (flags & ~AT_SYMLINK_NOFOLLOW)
+      if (flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))
 	{
 	  set_errno (EINVAL);
 	  __leave;
 	}
       char *path = tp.c_get ();
-      if (gen_full_path_at (path, dirfd, pathname))
+      if (gen_full_path_at (path, dirfd, pathname, false,
+			    flags & AT_EMPTY_PATH))
 	__leave;
       path_conv pc (path, ((flags & AT_SYMLINK_NOFOLLOW)
 			   ? PC_SYM_NOFOLLOW : PC_SYM_FOLLOW)
-- 
2.21.0