ls: check fts_children() for errors that may not surface otherwise

In particular, if one simply does a non-recursive `ls` on a directory
that is not accessible, there are some classes of errors that may cause
it to fail that wouldn't be surfaced unless we do an fts_read() that
will recurse into the inaccessible directory.  Catch those kinds of
errors here since we cannot expect to an FTS_ERR/FTS_DNR entry to follow
up on them.

PR:		287451
Reviewed by:	kib
Discusssed with:	des
Differential Revision:	https://reviews.freebsd.org/D51056

bin/ls/ls.c

@@ -707,6 +707,23 @@ traverse(int argc, char *argv[], int options)
 				output = 1;
 			}
 			chp = fts_children(ftsp, ch_options);
+			if (chp == NULL && errno != 0) {
+				warn("%s", p->fts_path);
+				rval = 1;
+
+				/*
+				 * Avoid further errors on this entry.  We won't
+				 * always get an FTS_ERR/FTS_DNR for errors
+				 * in fts_children(), because opendir could
+				 * have failed early on and that only flags an
+				 * error for fts_read() when we try to recurse
+				 * into it.  We catch both the non-recursive and
+				 * the recursive case here.
+				 */
+				(void)fts_set(ftsp, p, FTS_SKIP);
+				break;
+			}
+
 			display(p, chp, options);
 
 			if (!f_recursive && chp != NULL)

bin/ls/tests/ls_tests.sh

@@ -476,6 +476,35 @@ b_flag_body()
 	atf_check -e empty -o match:'y\\vz' -s exit:0 ls -b
 }
 
+atf_test_case childerr
+childerr_head()
+{
+	atf_set "descr" "Verify that fts_children() in pre-order errors are checked"
+	atf_set "require.user" "unprivileged"
+}
+
+childerr_body()
+{
+	atf_check mkdir -p root/dir root/edir
+	atf_check touch root/c
+
+	# Check that listing an empty directory hasn't regressed into being
+	# called an error.
+	atf_check -o match:"total 0" -e empty ls -l root/dir
+
+	atf_check chmod 0 root/dir
+
+	# If we did not abort after fts_children() properly, then stdout would
+	# have an output of the total files enumerated (0).  Thus, assert that
+	# it's empty and that we see the correct error on stderr.
+	atf_check -s not-exit:0 -e match:"Permission denied" ls -l root/dir
+
+	# Now ensure that we didn't just stop there, we printed out a directory
+	# that would've been enumerated later.
+	atf_check -s not-exit:0 -o match:"^root/edir" \
+	    -e match:"Permission denied" ls -lR root
+}
+
 atf_test_case d_flag
 d_flag_head()
 {
@@ -971,6 +1000,7 @@ atf_init_test_cases()
 	#atf_add_test_case Z_flag
 	atf_add_test_case a_flag
 	atf_add_test_case b_flag
+	atf_add_test_case childerr
 	#atf_add_test_case c_flag
 	atf_add_test_case d_flag
 	atf_add_test_case f_flag