Re: [PATCH 09/11] nfsd: Allow filehandle lookup to cross internal mount points.

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Jul 28, 2021 at 08:37:45AM +1000, NeilBrown wrote:
> Enhance nfsd to detect internal mounts and to cross them without
> requiring a new export.

Why don't we want a new export?

(Honest question, it's not obvious to me what the best behavior is.)

--b.

> 
> Also ensure the fsid reported is different for different submounts.  We
> do this by xoring in the ino of the mounted-on directory.  This makes
> sense for btrfs at least.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  fs/nfsd/nfs3xdr.c |   28 +++++++++++++++++++++-------
>  fs/nfsd/nfs4xdr.c |   34 +++++++++++++++++++++++-----------
>  fs/nfsd/nfsfh.c   |    7 ++++++-
>  fs/nfsd/vfs.c     |   11 +++++++++--
>  4 files changed, 59 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c
> index 67af0c5c1543..80b1cc0334fa 100644
> --- a/fs/nfsd/nfs3xdr.c
> +++ b/fs/nfsd/nfs3xdr.c
> @@ -370,6 +370,8 @@ svcxdr_encode_fattr3(struct svc_rqst *rqstp, struct xdr_stream *xdr,
>  	case FSIDSOURCE_UUID:
>  		fsid = ((u64 *)fhp->fh_export->ex_uuid)[0];
>  		fsid ^= ((u64 *)fhp->fh_export->ex_uuid)[1];
> +		if (fhp->fh_mnt != fhp->fh_export->ex_path.mnt)
> +			fsid ^= nfsd_get_mounted_on(fhp->fh_mnt);
>  		break;
>  	default:
>  		fsid = (u64)huge_encode_dev(fhp->fh_dentry->d_sb->s_dev);
> @@ -1094,8 +1096,8 @@ compose_entry_fh(struct nfsd3_readdirres *cd, struct svc_fh *fhp,
>  	__be32 rv = nfserr_noent;
>  
>  	dparent = cd->fh.fh_dentry;
> -	exp  = cd->fh.fh_export;
> -	child.mnt = cd->fh.fh_mnt;
> +	exp  = exp_get(cd->fh.fh_export);
> +	child.mnt = mntget(cd->fh.fh_mnt);
>  
>  	if (isdotent(name, namlen)) {
>  		if (namlen == 2) {
> @@ -1112,15 +1114,27 @@ compose_entry_fh(struct nfsd3_readdirres *cd, struct svc_fh *fhp,
>  			child.dentry = dget(dparent);
>  	} else
>  		child.dentry = lookup_positive_unlocked(name, dparent, namlen);
> -	if (IS_ERR(child.dentry))
> +	if (IS_ERR(child.dentry)) {
> +		mntput(child.mnt);
> +		exp_put(exp);
>  		return rv;
> -	if (d_mountpoint(child.dentry))
> -		goto out;
> -	if (child.dentry->d_inode->i_ino != ino)
> +	}
> +	/* If child is a mountpoint, then we want to expose the fact
> +	 * so client can create a mountpoint.  If not, then a different
> +	 * ino number probably means a race with rename, so avoid providing
> +	 * too much detail.
> +	 */
> +	if (nfsd_mountpoint(child.dentry, exp)) {
> +		int err;
> +		err = nfsd_cross_mnt(cd->rqstp, &child, &exp);
> +		if (err)
> +			goto out;
> +	} else if (child.dentry->d_inode->i_ino != ino)
>  		goto out;
>  	rv = fh_compose(fhp, exp, &child, &cd->fh);
>  out:
> -	dput(child.dentry);
> +	path_put(&child);
> +	exp_put(exp);
>  	return rv;
>  }
>  
> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> index d5683b6a74b2..4dbc99ed2c8b 100644
> --- a/fs/nfsd/nfs4xdr.c
> +++ b/fs/nfsd/nfs4xdr.c
> @@ -2817,6 +2817,8 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
>  	struct kstat stat;
>  	struct svc_fh *tempfh = NULL;
>  	struct kstatfs statfs;
> +	u64 mounted_on_ino;
> +	u64 sub_fsid;
>  	__be32 *p;
>  	int starting_len = xdr->buf->len;
>  	int attrlen_offset;
> @@ -2871,6 +2873,24 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
>  			goto out;
>  		fhp = tempfh;
>  	}
> +	if ((bmval0 & FATTR4_WORD0_FSID) ||
> +	    (bmval1 & FATTR4_WORD1_MOUNTED_ON_FILEID)) {
> +		mounted_on_ino = stat.ino;
> +		sub_fsid = 0;
> +		/*
> +		 * The inode number that the current mnt is mounted on is
> +		 * used for MOUNTED_ON_FILED if we are at the root,
> +		 * and for sub_fsid if mnt is not the export mnt.
> +		 */
> +		if (ignore_crossmnt == 0) {
> +			u64 moi = nfsd_get_mounted_on(mnt);
> +
> +			if (dentry == mnt->mnt_root && moi)
> +				mounted_on_ino = moi;
> +			if (mnt != exp->ex_path.mnt)
> +				sub_fsid = moi;
> +		}
> +	}
>  	if (bmval0 & FATTR4_WORD0_ACL) {
>  		err = nfsd4_get_nfs4_acl(rqstp, dentry, &acl);
>  		if (err == -EOPNOTSUPP)
> @@ -3008,6 +3028,8 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
>  		case FSIDSOURCE_UUID:
>  			p = xdr_encode_opaque_fixed(p, exp->ex_uuid,
>  								EX_UUID_LEN);
> +			if (mnt != exp->ex_path.mnt)
> +				*(u64*)(p-2) ^= sub_fsid;
>  			break;
>  		}
>  	}
> @@ -3253,20 +3275,10 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
>  		*p++ = cpu_to_be32(stat.mtime.tv_nsec);
>  	}
>  	if (bmval1 & FATTR4_WORD1_MOUNTED_ON_FILEID) {
> -		u64 ino;
> -
>  		p = xdr_reserve_space(xdr, 8);
>  		if (!p)
>  			goto out_resource;
> -		/*
> -		 * Get parent's attributes if not ignoring crossmount
> -		 * and this is the root of a cross-mounted filesystem.
> -		 */
> -		if (ignore_crossmnt == 0 && dentry == mnt->mnt_root)
> -			ino = nfsd_get_mounted_on(mnt);
> -		if (!ino)
> -			ino = stat.ino;
> -		p = xdr_encode_hyper(p, ino);
> +		p = xdr_encode_hyper(p, mounted_on_ino);
>  	}
>  #ifdef CONFIG_NFSD_PNFS
>  	if (bmval1 & FATTR4_WORD1_FS_LAYOUT_TYPES) {
> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> index 4023046f63e2..4b53838bca89 100644
> --- a/fs/nfsd/nfsfh.c
> +++ b/fs/nfsd/nfsfh.c
> @@ -9,7 +9,7 @@
>   */
>  
>  #include <linux/exportfs.h>
> -
> +#include <linux/namei.h>
>  #include <linux/sunrpc/svcauth_gss.h>
>  #include "nfsd.h"
>  #include "vfs.h"
> @@ -285,6 +285,11 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct svc_fh *fhp)
>  			default:
>  				dentry = ERR_PTR(-ESTALE);
>  			}
> +		} else if (nfsd_mountpoint(dentry, exp)) {
> +			struct path path = { .mnt = mnt, .dentry = dentry };
> +			follow_down(&path, LOOKUP_AUTOMOUNT);
> +			mnt = path.mnt;
> +			dentry = path.dentry;
>  		}
>  	}
>  	if (dentry == NULL)
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index baa12ac36ece..22523e1cd478 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -64,7 +64,7 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct path *path_parent,
>  			    .dentry = dget(path_parent->dentry)};
>  	int err = 0;
>  
> -	err = follow_down(&path, 0);
> +	err = follow_down(&path, LOOKUP_AUTOMOUNT);
>  	if (err < 0)
>  		goto out;
>  	if (path.mnt == path_parent->mnt && path.dentry == path_parent->dentry &&
> @@ -73,6 +73,13 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct path *path_parent,
>  		path_put(&path);
>  		goto out;
>  	}
> +	if (mount_is_internal(path.mnt)) {
> +		/* Use the new path, but don't look for a new export */
> +		/* FIXME should I check NOHIDE in this case?? */
> +		path_put(path_parent);
> +		*path_parent = path;
> +		goto out;
> +	}
>  
>  	exp2 = rqst_exp_get_by_name(rqstp, &path);
>  	if (IS_ERR(exp2)) {
> @@ -157,7 +164,7 @@ int nfsd_mountpoint(struct dentry *dentry, struct svc_export *exp)
>  		return 1;
>  	if (nfsd4_is_junction(dentry))
>  		return 1;
> -	if (d_mountpoint(dentry))
> +	if (d_managed(dentry))
>  		/*
>  		 * Might only be a mountpoint in a different namespace,
>  		 * but we need to check.
>