[PATCH] mount.nfs: fix ignore EBUSY problem in special scenario

["Jianhong.Yin" <yin-jianhong@163.com>]

now mount.nfs ignore EBUSY when the $dir is a mountpoint,
it's not correct in specail scenario for example:
  $dir is a mountpoint of other filesystem(let's say tmpfs)
so we must be sure: current nfs mounting has mounted on $dir, then
ignore the EBUSY for user.

- if (errno == EBUSY && is_mountpoint(mi->node))
+ if (errno == EBUSY && is_mountpoint(mi->node) && has_mounted_on(mi->spec, mi->node))

reproducer:
'''
mkdir -p /expdir /mnt/nfsmp /mnt/tmpfs
echo "/expdir *(rw,no_root_squash)" >/etc/exports
service nfs restart
mount -t tmpfs tmpfs /mnt/tmpfs
mount -osharecache localhost:/expdir /mnt/nfsmp #success
mount -osharecache localhost:/expdir /mnt/nfsmp #mounted already, return 0(pass)
mount -osharecache localhost:/expdir /mnt/tmpfs #success
mount -osharecache localhost:/expdir /mnt/tmpfs #mounted already, return 0(pass)
umount /mnt/nfsmp
umount /mnt/tmpfs

mount -ocontext=system_u:object_r:xferlog_t:s0,sharecache localhost:/expdir /mnt/nfsmp
mount -ocontext=system_u:object_r:user_home_dir_t:s0,sharecache localhost:/expdir /mnt/tmpfs
[[ $? = 0 ]] && echo "***test fail: hasn't mounted, should return fail ***"

umount /mnt/nfsmp
'''

Signed-off-by: Jianhong Yin <yin-jianhong@163.com>
---
 support/include/misc.h    |  1 +
 support/misc/mountpoint.c | 26 +++++++++++++++++++++++++-
 utils/mount/stropts.c     |  2 +-
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/support/include/misc.h b/support/include/misc.h
index 06e2a0c..bd02bfc 100644
--- a/support/include/misc.h
+++ b/support/include/misc.h
@@ -19,6 +19,7 @@ char *generic_make_pathname(const char *, const char *);
 _Bool generic_setup_basedir(const char *, const char *, char *, const size_t);
 
 extern int is_mountpoint(char *path);
+extern int has_mounted_on(const char *fsname, const char *dir);
 
 /* size of the file pointer buffers for rpc procfs files */
 #define RPC_CHAN_BUF_SIZE 32768
diff --git a/support/misc/mountpoint.c b/support/misc/mountpoint.c
index 9f9ce44..7f8a89f 100644
--- a/support/misc/mountpoint.c
+++ b/support/misc/mountpoint.c
@@ -3,9 +3,11 @@
  * check if a given path is a mountpoint 
  */
 
+#include <stdio.h>
 #include <string.h>
-#include "xcommon.h"
+#include <mntent.h>
 #include <sys/stat.h>
+#include "xcommon.h"
 #include "misc.h"
 
 int
@@ -38,3 +40,25 @@ is_mountpoint(char *path)
 	free(dotdot);
 	return rv;
 }
+
+int
+has_mounted_on(const char *fsname, const char *dir)
+{
+	struct mntent *ent;
+	FILE *fp;
+	int ret = 0;
+
+	fp = setmntent("/proc/mounts", "r");
+	if (fp == NULL) {
+		perror("[unlikely] setmntent(3) fail");
+		exit(1);
+	}
+	while (NULL != (ent = getmntent(fp))) {
+		if (!strcmp(ent->mnt_fsname, fsname) && !strcmp(ent->mnt_dir, dir)) {
+			ret = 1;
+			break;
+		}
+	}
+	endmntent(fp);
+	return ret;
+}
diff --git a/utils/mount/stropts.c b/utils/mount/stropts.c
index a093926..bffb74a 100644
--- a/utils/mount/stropts.c
+++ b/utils/mount/stropts.c
@@ -1081,7 +1081,7 @@ static int nfsmount_fg(struct nfsmount_info *mi)
 			return EX_SUCCESS;
 
 #pragma GCC diagnostic ignored "-Wdiscarded-qualifiers"
-		if (errno == EBUSY && is_mountpoint(mi->node)) {
+		if (errno == EBUSY && is_mountpoint(mi->node) && has_mounted_on(mi->spec, mi->node)) {
 #pragma GCC diagnostic warning "-Wdiscarded-qualifiers"
 			/*
 			 * EBUSY can happen when mounting a filesystem that
-- 
2.17.2