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Subject: Re: [PATCH] nfsd: restore owner override for truncate
From: Chuck Lever <chucklever@gmail.com>
In-Reply-To: <20170209160131.GE17622@fieldses.org>
Date: Thu, 9 Feb 2017 13:04:08 -0500
Cc: Christoph Hellwig <hch@lst.de>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Message-Id: <0F97E86D-EBF0-44DD-AA91-F4BB81CB3304@gmail.com>
References: <20170209142238.5322-1-hch@lst.de> <20170209160131.GE17622@fieldses.org>
To: "J. Bruce Fields" <bfields@fieldses.org>
Sender: linux-nfs-owner@vger.kernel.org


> On Feb 9, 2017, at 11:01 AM, J. Bruce Fields <bfields@fieldses.org> =
wrote:
>=20
> On Thu, Feb 09, 2017 at 03:22:38PM +0100, Christoph Hellwig wrote:
>> The switch to vfs_truncate in nfsd_setattr dropped the owner override
>> used for NFS permissions.  Add a copy of vfs_truncate with it =
restored
>> to the nfsd code for now as it's very late in the cycle, but there
>> should be a way to consolidate it back in the future.
>=20
> This also needs:
>=20
>=20
> diff --git a/fs/open.c b/fs/open.c
> index 9921f70bc5ca..5d8126b230d9 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -64,6 +64,7 @@ int do_truncate(struct dentry *dentry, loff_t =
length, unsigned int time_attrs,
> 	inode_unlock(dentry->d_inode);
> 	return ret;
> }
> +EXPORT_SYMBOL_GPL(do_truncate);
>=20
> long vfs_truncate(const struct path *path, loff_t length)
> {

After adding this change, I confirmed that t1050-large.sh is working
as expected.

Tested-by: Chuck Lever <chuck.lever@oracle.com>


> --b.
>=20
>>=20
>> Fixes: 41f53350 ("nfsd: special case truncates some more")
>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>> Reported-by: Chuck Lever <chucklever@gmail.com>
>> ---
>> fs/nfsd/vfs.c | 81 =
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>> 1 file changed, 80 insertions(+), 1 deletion(-)
>>=20
>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
>> index a974368026a1..fd4a32e0b0b4 100644
>> --- a/fs/nfsd/vfs.c
>> +++ b/fs/nfsd/vfs.c
>> @@ -332,6 +332,85 @@ nfsd_sanitize_attrs(struct inode *inode, struct =
iattr *iap)
>> 	}
>> }
>>=20
>> +/* copy of vfs_truncate with NFS owner override hacked in, sigh.. */
>> +static long nfsd_truncate(const struct path *path, loff_t length)
>> +{
>> +	struct inode *inode;
>> +	struct dentry *upperdentry;
>> +	long error;
>> +
>> +	inode =3D path->dentry->d_inode;
>> +
>> +	/* For directories it's -EISDIR, for other non-regulars - =
-EINVAL */
>> +	if (S_ISDIR(inode->i_mode))
>> +		return -EISDIR;
>> +	if (!S_ISREG(inode->i_mode))
>> +		return -EINVAL;
>> +
>> +	error =3D mnt_want_write(path->mnt);
>> +	if (error)
>> +		goto out;
>> +
>> +	/*
>> +	 * The file owner always gets access permission for accesses =
that
>> +	 * would normally be checked at open time. This is to make
>> +	 * file access work even when the client has done a fchmod(fd, =
0).
>> +	 *
>> +	 * However, `cp foo bar' should fail nevertheless when bar is
>> +	 * readonly. A sensible way to do this might be to reject all
>> +	 * attempts to truncate a read-only file, because a creat() call
>> +	 * always implies file truncation.
>> +	 * ... but this isn't really fair.  A process may reasonably =
call
>> +	 * ftruncate on an open file descriptor on a file with perm 000.
>> +	 * We must trust the client to do permission checking - using =
"ACCESS"
>> +	 * with NFSv3.
>> +	 */
>> +	if (!uid_eq(inode->i_uid, current_fsuid())) {
>> +		error =3D inode_permission(inode, MAY_WRITE);
>> +		if (error)
>> +			goto mnt_drop_write_and_out;
>> +	}
>> +
>> +	error =3D -EPERM;
>> +	if (IS_APPEND(inode))
>> +		goto mnt_drop_write_and_out;
>> +
>> +	/*
>> +	 * If this is an overlayfs then do as if opening the file so we =
get
>> +	 * write access on the upper inode, not on the overlay inode.  =
For
>> +	 * non-overlay filesystems d_real() is an identity function.
>> +	 */
>> +	upperdentry =3D d_real(path->dentry, NULL, O_WRONLY);
>> +	error =3D PTR_ERR(upperdentry);
>> +	if (IS_ERR(upperdentry))
>> +		goto mnt_drop_write_and_out;
>> +
>> +	error =3D get_write_access(upperdentry->d_inode);
>> +	if (error)
>> +		goto mnt_drop_write_and_out;
>> +
>> +	/*
>> +	 * Make sure that there are no leases.  get_write_access() =
protects
>> +	 * against the truncate racing with a lease-granting setlease().
>> +	 */
>> +	error =3D break_lease(inode, O_WRONLY);
>> +	if (error)
>> +		goto put_write_and_out;
>> +
>> +	error =3D locks_verify_truncate(inode, NULL, length);
>> +	if (!error)
>> +		error =3D security_path_truncate(path);
>> +	if (!error)
>> +		error =3D do_truncate(path->dentry, length, 0, NULL);
>> +
>> +put_write_and_out:
>> +	put_write_access(upperdentry->d_inode);
>> +mnt_drop_write_and_out:
>> +	mnt_drop_write(path->mnt);
>> +out:
>> +	return error;
>> +}
>> +
>> /*
>>  * Set various file attributes.  After this call fhp needs an fh_put.
>>  */
>> @@ -398,7 +477,7 @@ nfsd_setattr(struct svc_rqst *rqstp, struct =
svc_fh *fhp, struct iattr *iap,
>> 		    ((iap->ia_valid & ~(ATTR_SIZE | ATTR_MTIME)) =3D=3D =
0))
>> 			implicit_mtime =3D true;
>>=20
>> -		host_err =3D vfs_truncate(&path, iap->ia_size);
>> +		host_err =3D nfsd_truncate(&path, iap->ia_size);
>> 		if (host_err)
>> 			goto out_host_err;
>>=20
>> --=20
>> 2.11.0
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--
Chuck Lever
chucklever@gmail.com