selftests/bpf: umount children of TDIR in test_bpffs

Currently this tests tries to umount /sys/kernel/debug (TDIR) but the
system it is running on may have mounts below.

For example, danobi/vmtest [0] VMs have
    mount -t tracefs tracefs /sys/kernel/debug/tracing
as part of their init.

This change list mounts and will umount any mounts below TDIR before
umounting TDIR itself.

Note that it is not umounting recursively, so in the case of a sub-mount
of TDIR  having another sub-mount, this will fail as mtab is ordered.

Test:

Originally:

    $ vmtest -k $KERNEL_REPO/arch/x86_64/boot/bzImage "./test_progs -vv -a test_bpffs"
    => bzImage
    ===> Booting
    ===> Setting up VM
    ===> Running command
    [    2.138818] bpf_testmod: loading out-of-tree module taints kernel.
    [    2.140913] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
    bpf_testmod.ko is already unloaded.
    Loading bpf_testmod.ko...
    Successfully loaded bpf_testmod.ko.
    test_test_bpffs:PASS:clone 0 nsec
    fn:PASS:unshare 0 nsec
    fn:PASS:mount / 0 nsec
    fn:FAIL:umount /sys/kernel/debug unexpected error: -1 (errno 16)
    bpf_testmod.ko is already unloaded.
    Loading bpf_testmod.ko...
    Successfully loaded bpf_testmod.ko.
    test_test_bpffs:PASS:clone 0 nsec
    test_test_bpffs:PASS:waitpid 0 nsec
    test_test_bpffs:FAIL:bpffs test  failed 255#282     test_bpffs:FAIL
    Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED
    Successfully unloaded bpf_testmod.ko.
    Command failed with exit code: 1

After this change:

    $ vmtest -k $KERNEL_REPO/arch/x86_64/boot/bzImage "./test_progs -vv -a test_bpffs"
    => bzImage
    ===> Booting
    ===> Setting up VM
    ===> Running command
    [    2.035210] bpf_testmod: loading out-of-tree module taints kernel.
    [    2.036510] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
    bpf_testmod.ko is already unloaded.
    Loading bpf_testmod.ko...
    Successfully loaded bpf_testmod.ko.
    test_test_bpffs:PASS:clone 0 nsec
    fn:PASS:unshare 0 nsec
    fn:PASS:mount / 0 nsec
    fn:PASS:accessing /etc/mtab 0 nsec
    fn:PASS:umount /sys/kernel/debug/tracing 0 nsec
    fn:PASS:umount /sys/kernel/debug 0 nsec
    fn:PASS:mount tmpfs 0 nsec
    fn:PASS:mkdir /sys/kernel/debug/fs1 0 nsec
    fn:PASS:mkdir /sys/kernel/debug/fs2 0 nsec
    fn:PASS:mount bpffs /sys/kernel/debug/fs1 0 nsec
    fn:PASS:mount bpffs /sys/kernel/debug/fs2 0 nsec
    fn:PASS:reading /sys/kernel/debug/fs1/maps.debug 0 nsec
    fn:PASS:reading /sys/kernel/debug/fs2/progs.debug 0 nsec
    fn:PASS:creating /sys/kernel/debug/fs1/a 0 nsec
    fn:PASS:creating /sys/kernel/debug/fs1/a/1 0 nsec
    fn:PASS:creating /sys/kernel/debug/fs1/b 0 nsec
    fn:PASS:create_map(ARRAY) 0 nsec
    fn:PASS:pin map 0 nsec
    fn:PASS:stat(/sys/kernel/debug/fs1/a) 0 nsec
    fn:PASS:renameat2(/fs1/a, /fs1/b, RENAME_EXCHANGE) 0 nsec
    fn:PASS:stat(/sys/kernel/debug/fs1/b) 0 nsec
    fn:PASS:b should have a's inode 0 nsec
    fn:PASS:access(/sys/kernel/debug/fs1/b/1) 0 nsec
    fn:PASS:stat(/sys/kernel/debug/fs1/map) 0 nsec
    fn:PASS:renameat2(/fs1/c, /fs1/b, RENAME_EXCHANGE) 0 nsec
    fn:PASS:stat(/sys/kernel/debug/fs1/b) 0 nsec
    fn:PASS:b should have c's inode 0 nsec
    fn:PASS:access(/sys/kernel/debug/fs1/c/1) 0 nsec
    fn:PASS:renameat2(RENAME_NOREPLACE) 0 nsec
    fn:PASS:access(/sys/kernel/debug/fs1/b) 0 nsec
    bpf_testmod.ko is already unloaded.
    Loading bpf_testmod.ko...
    Successfully loaded bpf_testmod.ko.
    test_test_bpffs:PASS:clone 0 nsec
    test_test_bpffs:PASS:waitpid 0 nsec
    test_test_bpffs:PASS:bpffs test  0 nsec
    torvalds#282     test_bpffs:OK
    Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
    Successfully unloaded bpf_testmod.ko.

[0] https://github.com/danobi/vmtest

Signed-off-by: Manu Bretelle <chantr4@gmail.com>

tools/testing/selftests/bpf/prog_tests/test_bpffs.c

@@ -3,12 +3,14 @@
 #define _GNU_SOURCE
 #include <stdio.h>
 #include <sched.h>
+#include <mntent.h>
 #include <sys/mount.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <test_progs.h>
 
 #define TDIR "/sys/kernel/debug"
+#define MTAB "/etc/mtab"
 
 static int read_iter(char *file)
 {
@@ -32,6 +34,8 @@ static int read_iter(char *file)
 
 static int fn(void)
 {
+	/* A buffer to store logging messages */
+	char buf[1024];
 	struct stat a, b, c;
 	int err, map;
 
@@ -43,6 +47,30 @@ static int fn(void)
 	if (!ASSERT_OK(err, "mount /"))
 		goto out;
 
+	/* TDIR may have mounts below. unount them first */
+	FILE *mtab = setmntent(MTAB, "r");
+
+	if (!ASSERT_TRUE(mtab != NULL, "accessing " MTAB)) {
+		err = errno;
+		goto out;
+	}
+
+	struct mntent *mnt = NULL;
+
+	while ((mnt = getmntent(mtab)) != NULL) {
+		if (strlen(mnt->mnt_dir) > strlen(TDIR) &&
+			strncmp(TDIR, mnt->mnt_dir, strlen(TDIR)) == 0) {
+			snprintf(buf, sizeof(buf) - 1, "umount %s", mnt->mnt_dir);
+			err = umount(mnt->mnt_dir);
+			if (!ASSERT_OK(err, buf)) {
+				endmntent(mtab);
+				goto out;
+			}
+		}
+	}
+	// Ignore any error here
+	endmntent(mtab);
+
 	err = umount(TDIR);
 	if (!ASSERT_OK(err, "umount " TDIR))
 		goto out;