[PATCH v3 3/3] Cygwin: fstatat: support the AT_EMPTY_PATH flag

[Ken Brown]

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.
---
 winsup/cygwin/syscalls.cc | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
index 3d87fd685..e7298fd43 100644
--- a/winsup/cygwin/syscalls.cc
+++ b/winsup/cygwin/syscalls.cc
@@ -4823,14 +4823,27 @@ 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))
-	__leave;
+      int res = gen_full_path_at (path, dirfd, pathname);
+      if (res)
+	{
+	  if (!(errno == ENOENT && (flags & AT_EMPTY_PATH)))
+	    __leave;
+	  /* pathname is an empty string.  Operate on dirfd. */
+	  if (dirfd == AT_FDCWD)
+	    {
+	      cwdstuff::cwd_lock.acquire ();
+	      strcpy (path, cygheap->cwd.get_posix ());
+	      cwdstuff::cwd_lock.release ();
+	    }
+	  else
+	    return fstat (dirfd, st);
+	}
       path_conv pc (path, ((flags & AT_SYMLINK_NOFOLLOW)
 			   ? PC_SYM_NOFOLLOW : PC_SYM_FOLLOW)
 			  | PC_POSIX | PC_KEEP_HANDLE, stat_suffixes);
-- 
2.21.0