Subject: Re: [PATCH] mount.nfs: fix ignore EBUSY problem in special scenario
To:     "Jianhong.Yin" <yin-jianhong@163.com>
Cc:     linux-nfs@vger.kernel.org, jiyin@redhat.com
References: <20190228050842.15827-1-yin-jianhong@163.com>
From:   Steve Dickson <SteveD@RedHat.com>
Message-ID: <813b5167-8c4a-dd64-eb95-d29b4463d54a@RedHat.com>
Date:   Tue, 5 Mar 2019 13:00:02 -0500
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Hello,

I apologies for taking so long to get to this... 

On 2/28/19 12:08 AM, Jianhong.Yin wrote:
> 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
What real world problem is this solving or better yet what real world 
situation would something like this be done??

I see this more of a pilot error... no one uses tmpfs to make mount to fail
(or not fail in this case)... It's just not how tmpfs is used.

Plus who uses -ocontext= this way? If some one is that sophisticated to
use mount like that.... and get wrong... they need to be a little 
more sophisticated ;-)

Finally, when tmpfs is not in the picture things work as expected....

Maybe I'm missing something... I don't this as a problem or a situation
the people will run into... again... maybe I'm missing something. 

steved. 
> '''
> 
> 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
>