proc: more robust bulk read test

/proc may not be mounted and test will exit successfully.

Ensure proc is mounted at /proc.

Link: http://lkml.kernel.org/r/20190209105613.GA10384@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/proc/read.c

@@ -26,8 +26,10 @@
 #include <dirent.h>
 #include <stdbool.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/vfs.h>
 #include <fcntl.h>
 #include <unistd.h>
 
@@ -123,10 +125,22 @@ static void f(DIR *d, unsigned int level)
 int main(void)
 {
 	DIR *d;
+	struct statfs sfs;
 
 	d = opendir("/proc");
 	if (!d)
 		return 4;
+
+	/* Ensure /proc is proc. */
+	if (fstatfs(dirfd(d), &sfs) == -1) {
+		return 1;
+	}
+	if (sfs.f_type != 0x9fa0) {
+		fprintf(stderr, "error: unexpected f_type %lx\n", (long)sfs.f_type);
+		return 2;
+	}
+
 	f(d, 0);
+
 	return 0;
 }