Cygwin: fstatat, fchownat: support the AT_EMPTY_PATH flag

Following Linux, allow the pathname argument to be an empty string if
the AT_EMPTY_PATH flag 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 | O_NOFOLLOW.

diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
index 282d9e0ee144be59eedff153a3522627addf121f..4956b6ff57bb460079b669b5db1623cf29d3b534 100644 (file)
--- a/winsup/cygwin/syscalls.cc
+++ b/winsup/cygwin/syscalls.cc
@@ -4785,14 +4785,36 @@ 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))
-       __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
+           {
+             cygheap_fdget cfd (dirfd);
+             if (cfd < 0)
+               __leave;
+             strcpy (path, cfd->get_name ());
+             /* If dirfd refers to a symlink (which was necessarily
+                opened with O_PATH | O_NOFOLLOW), we must operate
+                directly on that symlink.. */
+             flags = AT_SYMLINK_NOFOLLOW;
+           }
+       }
       return chown_worker (path, (flags & AT_SYMLINK_NOFOLLOW)
                                 ? PC_SYM_NOFOLLOW : PC_SYM_FOLLOW, uid, gid);
     }
@@ -4808,14 +4830,27 @@ 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);