exec selftests: test ->recursion_depth

Test that trivially recursing script onto itself doesn't work.

Note: this is different test from ELOOP tests in execveat.c Those test
that execveat(2) doesn't follow symlinks when told to do so.

Link: http://lkml.kernel.org/r/20190423192720.GA21433@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/exec/.gitignore

@@ -6,4 +6,5 @@ execveat.moved
 execveat.path.ephemeral
 execveat.ephemeral
 execveat.denatured
-xxxxxxxx*
+/recursion-depth
+xxxxxxxx*

tools/testing/selftests/exec/Makefile

@@ -1,11 +1,15 @@
 # SPDX-License-Identifier: GPL-2.0
 CFLAGS = -Wall
+CFLAGS += -Wno-nonnull
+CFLAGS += -D_GNU_SOURCE
 
 TEST_GEN_PROGS := execveat
 TEST_GEN_FILES := execveat.symlink execveat.denatured script subdir
 # Makefile is a run-time dependency, since it's accessed by the execveat test
 TEST_FILES := Makefile
 
+TEST_GEN_PROGS += recursion-depth
+
 EXTRA_CLEAN := $(OUTPUT)/subdir.moved $(OUTPUT)/execveat.moved $(OUTPUT)/xxxxx*
 
 include ../lib.mk

tools/testing/selftests/exec/recursion-depth.c

@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2019 Alexey Dobriyan <adobriyan@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+/* Test that pointing #! script interpreter to self doesn't recurse. */
+#include <errno.h>
+#include <sched.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <sys/mount.h>
+#include <unistd.h>
+
+int main(void)
+{
+	if (unshare(CLONE_NEWNS) == -1) {
+		if (errno == ENOSYS || errno == EPERM) {
+			fprintf(stderr, "error: unshare, errno %d\n", errno);
+			return 4;
+		}
+		fprintf(stderr, "error: unshare, errno %d\n", errno);
+		return 1;
+	}
+	if (mount(NULL, "/", NULL, MS_PRIVATE|MS_REC, NULL) == -1) {
+		fprintf(stderr, "error: mount '/', errno %d\n", errno);
+		return 1;
+	}
+	/* Require "exec" filesystem. */
+	if (mount(NULL, "/tmp", "ramfs", 0, NULL) == -1) {
+		fprintf(stderr, "error: mount ramfs, errno %d\n", errno);
+		return 1;
+	}
+
+#define FILENAME "/tmp/1"
+
+	int fd = creat(FILENAME, 0700);
+	if (fd == -1) {
+		fprintf(stderr, "error: creat, errno %d\n", errno);
+		return 1;
+	}
+#define S "#!" FILENAME "\n"
+	if (write(fd, S, strlen(S)) != strlen(S)) {
+		fprintf(stderr, "error: write, errno %d\n", errno);
+		return 1;
+	}
+	close(fd);
+
+	int rv = execve(FILENAME, NULL, NULL);
+	if (rv == -1 && errno == ELOOP) {
+		return 0;
+	}
+	fprintf(stderr, "error: execve, rv %d, errno %d\n", rv, errno);
+	return 1;
+}