nftw: fill in stat buf for dangling links [BZ #23501]

diff --git a/io/Makefile b/io/Makefile
index 088e86da77b..82ed5a00305 100644
--- a/io/Makefile
+++ b/io/Makefile
@@ -73,7 +73,8 @@  tests		:= test-utime test-stat test-stat2 test-lfs tst-getcwd \
 		   tst-mknodat tst-mkfifoat tst-ttyname_r bug-ftw5 \
 		   tst-posix_fallocate tst-posix_fallocate64 \
 		   tst-fts tst-fts-lfs tst-open-tmpfile \
-		   tst-copy_file_range tst-getcwd-abspath tst-lockf
+		   tst-copy_file_range tst-getcwd-abspath tst-lockf \
+		   tst-ftw-lnk
 
 # This test includes the compat implementation of copy_file_range,
 # which uses internal, unexported libc functions.
diff --git a/io/ftw.c b/io/ftw.c
index 33e1a5ecab6..1ca70a55c8d 100644
--- a/io/ftw.c
+++ b/io/ftw.c
@@ -424,7 +424,31 @@  process_entry (struct ftw_data *data, struct dir_data *dir, const char *name,
       else if (data->flags & FTW_PHYS)
 	flag = FTW_NS;
       else if (d_type == DT_LNK)
-	flag = FTW_SLN;
+	{
+	  int err;
+
+	  flag = FTW_SLN;
+
+	  /* Old code left ST undefined; a clarification at the POSIX
+	     level suggests it should contain information about the
+	     link (ala lstat ()).  We do our best to fill in what data
+	     we can.  */
+	  if (dir->streamfd != -1)
+	    err = FXSTATAT (_STAT_VER, dir->streamfd, name, &st,
+			    AT_SYMLINK_NOFOLLOW);
+	  else
+	    err = LXSTAT (_STAT_VER, name, &st);
+
+	  if (err < 0)
+	    {
+	      /* Stat failed.  We have three choices: don't report
+		 this file, report with invalid data, or report with
+		 zero'd data.  Let's prefer obvious over subtle so
+		 zero the data here.  This should happen extremely
+		 rarely.  */
+	      memset (&st, 0, sizeof (st));
+	    }
+	}
       else
 	{
 	  if (dir->streamfd != -1)
diff --git a/io/tst-ftw-lnk.c b/io/tst-ftw-lnk.c
new file mode 100644
index 00000000000..8ee23186312
--- /dev/null
+++ b/io/tst-ftw-lnk.c
@@ -0,0 +1,239 @@ 
+/* Test for ftw function related to symbolic links for BZ #23501
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <ftw.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+
+#include <support/support.h>
+#include <support/check.h>
+
+#define TSTDIR "tst-ftw-lnk.d"
+
+static void
+un (const char *file)
+{
+  struct stat st;
+  /* Does the file exist?  */
+  if (lstat (file, &st) < 0
+      && errno == ENOENT)
+    return;
+
+  /* If so, try to remove it.  */
+  if (unlink (file) < 0)
+    FAIL_EXIT1 ("Unable to unlink %s", file);
+}
+
+static void
+debug_cb (const char *which, const char *fpath,
+	  const struct stat *sb, int typeflags)
+{
+  const char *sb_type = "???";
+  const char *ftw_type = "???";
+
+  /* Coding style here is intentionally "wrong" to increase readability.  */
+  if (S_ISREG (sb->st_mode))  sb_type = "REG";
+  if (S_ISDIR (sb->st_mode))  sb_type = "DIR";
+  if (S_ISLNK (sb->st_mode))  sb_type = "LNK";
+
+  if (typeflags == FTW_F)   ftw_type = "F";
+  if (typeflags == FTW_D)   ftw_type = "D";
+  if (typeflags == FTW_DNR) ftw_type = "DNR";
+  if (typeflags == FTW_DP)  ftw_type = "DP";
+  if (typeflags == FTW_NS)  ftw_type = "NS";
+  if (typeflags == FTW_SL)  ftw_type = "SL";
+  if (typeflags == FTW_SLN) ftw_type = "SLN";
+
+  printf ("%s %5d %-3s %-3s %s\n", which, (int)(sb->st_ino % 100000), sb_type, ftw_type, fpath);
+}
+
+int good_cb = 0;
+#define EXPECTED_GOOD 12
+
+/* See if the stat buffer SB refers to the file AS_FNAME.  */
+static void
+check_same_stats (const struct stat *sb, const char *as_fname)
+{
+  struct stat as;
+  if (lstat (as_fname, &as) < 0)
+    FAIL_EXIT1 ("unable to stat %s for comparison", as_fname);
+
+  if (as.st_mode == sb->st_mode
+      && as.st_ino == sb->st_ino
+      && as.st_size == sb->st_size)
+    good_cb ++;
+  else
+    printf ("statbuf data doesn't match %s\n", as_fname);
+}
+
+static int
+callback_phys (const char *fpath, const struct stat *sb, int typeflags, struct FTW *ftwbuf)
+{
+  debug_cb ("P", fpath, sb, typeflags);
+
+  /* This callback is for when the FTW_PHYS flag is set.  The results
+     should reflect the physical filesystem entry, not what it might
+     point to.  */
+
+  /* link1-bad is a dangling symlink, but we're reporting on the link
+     anyway (ala lstat ()).  */
+  if (strcmp (fpath, "./link1-bad") == 0)
+    {
+      if (S_ISLNK (sb->st_mode) && typeflags == FTW_SL)
+	good_cb ++;
+      else
+	printf ("link1-bad had wrong phys stats\n");
+
+      check_same_stats (sb, "link1-bad");
+    }
+
+  /* link2-ok is a regular non-dangling symlink.  */
+  if (strcmp (fpath, "./link2-ok") == 0)
+    {
+      if (S_ISLNK (sb->st_mode) && typeflags == FTW_SL)
+	good_cb ++;
+      else
+	printf ("link2-ok had wrong phys stats\n");
+
+      check_same_stats (sb, "link2-ok");
+    }
+
+  /* This is the file link2-ok points to.  */
+  if (strcmp (fpath, "./link2-tgt") == 0)
+    {
+      if (S_ISREG (sb->st_mode) && typeflags == FTW_F)
+	good_cb ++;
+      else
+	printf ("link2-tgt had wrong phys stats\n");
+
+      check_same_stats (sb, "link2-tgt");
+    }
+
+  return 0;
+}
+
+static int
+callback_log (const char *fpath, const struct stat *sb, int typeflags, struct FTW *ftwbuf)
+{
+  debug_cb ("L", fpath, sb, typeflags);
+
+  /* This callback is for when the FTW_PHYS flags is NOT set.  The
+     results should reflect the logical file, i.e. symlinks should be
+     followed.  */
+
+  /* We would normally report what link1-bad links to, but link1-bad
+     is a dangling symlink.  This is an exception to FTW_PHYS in that
+     we report FTW_SLN (dangling symlink) but the stat data is
+     correctly set to the link itself (ala lstat ()).  */
+  if (strcmp (fpath, "./link1-bad") == 0)
+    {
+      if (S_ISLNK (sb->st_mode) && typeflags == FTW_SLN)
+	good_cb ++;
+      else
+	printf ("link1-bad had wrong logical stats\n");
+
+      check_same_stats (sb, "link1-bad");
+    }
+
+  /* link2-ok points to link2-tgt, so we expect data reflecting
+     link2-tgt (ala stat ()).  */
+  if (strcmp (fpath, "./link2-ok") == 0)
+    {
+      if (S_ISREG (sb->st_mode) && typeflags == FTW_F)
+	good_cb ++;
+      else
+	printf ("link2-ok had wrong logical stats\n");
+
+      check_same_stats (sb, "link2-tgt");
+    }
+
+  /* This is the file link2-ok points to.  */
+  if (strcmp (fpath, "./link2-tgt") == 0)
+    {
+      if (S_ISREG (sb->st_mode) && typeflags == FTW_F)
+	good_cb ++;
+      else
+	printf ("link2-tgt had wrong logical stats\n");
+
+      check_same_stats (sb, "link2-tgt");
+    }
+
+  return 0;
+}
+
+static int
+do_test (void)
+{
+  struct stat st;
+
+  if (chdir (support_objdir_root) < 0)
+    FAIL_EXIT1 ("cannot chdir to objdir root");
+
+  if (chdir ("io") < 0)
+    FAIL_EXIT1 ("cannot chdir to objdir/io subdir");
+
+  if (stat (TSTDIR, &st) >= 0)
+    {
+      /* Directory does exist, delete any potential conflicts. */
+      if (chdir (TSTDIR) < 0)
+	FAIL_EXIT1 ("cannot chdir to %s\n", TSTDIR);
+      un ("link1-bad");
+      un ("link1-tgt");
+      un ("link2-ok");
+      un ("link2-tgt");
+    }
+  else
+    {
+      /* Directory does not exist, create it.  */
+      mkdir (TSTDIR, 0777);
+      if (chdir (TSTDIR) < 0)
+	FAIL_EXIT1 ("cannot chdir to %s\n", TSTDIR);
+    }
+
+  /* At this point, we're inside our test directory, and need to
+     prepare it.  */
+
+  if (symlink ("link1-tgt", "link1-bad") < 0)
+    FAIL_EXIT1 ("symlink link1-bad failed");
+  if (symlink ("link2-tgt", "link2-ok") < 0)
+    FAIL_EXIT1 ("symlink link2-ok failed");
+  if (open ("link2-tgt", O_RDWR|O_CREAT, 0777) < 0)
+    FAIL_EXIT1 ("create of link2-tgt failed");
+
+  /* Now we run the tests.  */
+
+  nftw (".", callback_phys, 10, FTW_PHYS);
+  nftw (".", callback_log, 10, 0);
+
+  /* Did we see the expected number of correct callbacks? */
+
+  if (good_cb != EXPECTED_GOOD)
+    {
+      FAIL_EXIT1 ("Saw %d good callbacks, expected %d\n",
+		  good_cb, EXPECTED_GOOD);
+    }
+
+  return 0;
+}
+
+#include <support/test-driver.c>