linux-kernel - Re: [PATCH 1/3] VFS: introduce MAY_ACT_AS

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Date:   Fri, 05 Oct 2018 07:52:19 +1000
From:   NeilBrown <neilb@...e.com>
To:     David Howells <dhowells@...hat.com>
Cc:     dhowells@...hat.com, "J. Bruce Fields" <bfields@...ldses.org>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Jan Harkes <jaharkes@...cmu.edu>, linux-nfs@...r.kernel.org,
        Miklos Szeredi <miklos@...redi.hu>,
        Jeff Layton <jlayton@...nel.org>, linux-kernel@...r.kernel.org,
        linux-afs@...ts.infradead.org, coda@...cmu.edu,
        linux-fsdevel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/3] VFS: introduce MAY_ACT_AS_OWNER

On Thu, Oct 04 2018, David Howells wrote:

> NeilBrown <neilb@...e.com> wrote:
>
>> diff --git a/fs/afs/security.c b/fs/afs/security.c
>> index 81dfedb7879f..ac2e39de8bff 100644
>> --- a/fs/afs/security.c
>> +++ b/fs/afs/security.c
>> @@ -349,6 +349,16 @@ int afs_permission(struct inode *inode, int mask)
>>  	if (mask & MAY_NOT_BLOCK)
>>  		return -ECHILD;
>>  
>> +	/* Short-circuit for owner */
>> +	if (mask & MAY_ACT_AS_OWNER) {
>> +		if (inode_owner_or_capable(inode))
>
> You don't know that inode->i_uid in meaningful.  You may have noticed that
> afs_permission() ignores i_uid and i_gid entirely.  It queries the server (if
> this information is not otherwise cached) to ask what permits the user is
> granted - where the user identity is defined by the key returned from
> afs_request_key()[*].
>
> So, NAK for the afs piece.

Thanks for the review.
As afs doesn't use the generic xattr code and doesn't call
setattr_prepare(), this is all largely irrelevant for afs.

afs_permission() will probably only get MAY_ACT_AS_OWNER passed when
someone uses fcntl(F_SETFL) to set the O_NOATIME flag.
Currently a permission test based on UID is performed which, as you say,
is wrong.  My patch simply preserved this current (wrong) behaviour.
Shall I change it to always allow access, like with NFS?
Probably O_NOATIME is ignored, in which case f_op->check_flags should
probably report -EINVAL (???) ... or might that cause a regression?

Thanks,
NeilBrown

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