linux-kernel - Re: [PATCH 2/3] fs: namei: Allow follow

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Message-ID: <459307a8-cb64-265e-7112-feafaec6a32f@themaw.net>
Date:   Fri, 18 Nov 2022 07:37:30 +0800
From:   Ian Kent <raven@...maw.net>
To:     Jeff Layton <jlayton@...nel.org>,
        Richard Weinberger <richard@....at>, linux-nfs@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        chuck.lever@...cle.com, anna@...nel.org,
        trond.myklebust@...merspace.com, viro@...iv.linux.org.uk,
        chris.chilvers@...sbroker.com, david.young@...sbroker.com,
        luis.turcitu@...sbroker.com, david@...ma-star.at
Subject: Re: [PATCH 2/3] fs: namei: Allow follow_down() to uncover auto mounts


On 18/11/22 05:01, Jeff Layton wrote:
> On Thu, 2022-11-17 at 20:11 +0100, Richard Weinberger wrote:
>> This function is only used by NFSD to cross mount points.
>> If a mount point is of type auto mount, follow_down() will
>> not uncover it. Add LOOKUP_AUTOMOUNT to the lookup flags
>> to have ->d_automount() called when NFSD walks down the
>> mount tree.
>>
>> Signed-off-by: Richard Weinberger <richard@....at>
>> ---
>>   fs/namei.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/namei.c b/fs/namei.c
>> index 578c2110df02..000c4b84e6be 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -1462,7 +1462,7 @@ int follow_down(struct path *path)
>>   {
>>   	struct vfsmount *mnt = path->mnt;
>>   	bool jumped;
>> -	int ret = traverse_mounts(path, &jumped, NULL, 0);
>> +	int ret = traverse_mounts(path, &jumped, NULL, LOOKUP_AUTOMOUNT);
>>   
>>   	if (path->mnt != mnt)
>>   		mntput(mnt);
>
> What happens when CROSSMOUNT isn't enabled and someone tries to stroll
> into an automount point? I'm guessing the automount happens but the
> export is denied? It seems like LOOKUP_AUTOMOUNT ought to be conditional
> on the parent export having CROSSMOUNT set.
>
> There's also another caller of follow_down too, the UNIX98 pty code.
> This may be harmless for it, but it'd be best not to perturb that if we
> can help it.
>
> Maybe follow_down can grow a lookupflags argument?es, I think that's needed too.


Changing the core VFS unconditionally ricks breaking things.


For example this:

         if (!(lookup_flags & (LOOKUP_PARENT | LOOKUP_DIRECTORY |
                            LOOKUP_OPEN | LOOKUP_CREATE | 
LOOKUP_AUTOMOUNT)) &&
             dentry->d_inode)

will never be true now so that, at the least, the handling of this case

will change for automount(8). I don't remember now the reasons behind

doing this but I do remember there was a special case that needed to

be handled by it.


Ian