[PATCH 4/4] listmount: allow continuing

[Miklos Szeredi <mszeredi@redhat.com>]

Rename mnt_id_req.request_mask to .param to allow using it for listmount(2)
as well.

1) If the buffer is full don't return EOVERFLOW, instead return the buffer
size.  This still allows detecting a full buffer.

2) listing is continued after the ID contained in .param.  This allows
listing the mount IDs in multiple listmount() invocations without having to
resize buffer.  If .param is zero, then the listing is started from the
beginning, just like previously.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/namespace.c             | 17 ++++++++---------
 include/uapi/linux/mount.h |  9 ++++++++-
 2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 10cd651175b5..5c1455c4b53b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4942,7 +4942,7 @@ static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 		return -EFAULT;
 
 	*ks = (struct kstatmount){
-		.mask		= kreq->request_mask,
+		.mask		= kreq->param,
 		.buf		= buf,
 		.bufsize	= bufsize,
 		.seq = {
@@ -5020,14 +5020,11 @@ static ssize_t do_listmount(struct mount *first, struct path *orig, u64 mnt_id,
 	if (err)
 		return err;
 
-	for (ctr = 0, r = first; r; r = listmnt_next(r)) {
+	for (ctr = 0, r = first; r && ctr < bufsize; r = listmnt_next(r)) {
 		if (r->mnt_id_unique == mnt_id)
 			continue;
 		if (!is_path_reachable(r, r->mnt.mnt_root, orig))
 			continue;
-
-		if (ctr >= bufsize)
-			return -EOVERFLOW;
 		ctr = array_index_nospec(ctr, bufsize);
 		if (put_user(r->mnt_id_unique, buf + ctr))
 			return -EFAULT;
@@ -5044,7 +5041,7 @@ SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,
 	struct mnt_id_req kreq;
 	struct mount *first;
 	struct path root, orig;
-	u64 mnt_id;
+	u64 mnt_id, last_mnt_id;
 	ssize_t ret;
 
 	if (flags)
@@ -5052,9 +5049,8 @@ SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,
 
 	if (copy_from_user(&kreq, req, sizeof(kreq)))
 		return -EFAULT;
-	if (kreq.request_mask != 0)
-		return -EINVAL;
 	mnt_id = kreq.mnt_id;
+	last_mnt_id = kreq.param;
 
 	down_read(&namespace_sem);
 	get_fs_root(current->fs, &root);
@@ -5067,7 +5063,10 @@ SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,
 			goto err;
 		orig.dentry = orig.mnt->mnt_root;
 	}
-	first = node_to_mount(rb_first(&ns->mounts));
+	if (!last_mnt_id)
+		first = node_to_mount(rb_first(&ns->mounts));
+	else
+		first = mnt_find_id_at(ns, last_mnt_id + 1);
 
 	ret = do_listmount(first, &orig, mnt_id, buf, bufsize, &root);
 err:
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index f6b35a15b7dd..dc9a0112d819 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -176,9 +176,16 @@ struct statmount {
 	char str[];		/* Variable size part containing strings */
 };
 
+/*
+ * Structure for passing mount ID and miscellaneous parameters to statmount(2)
+ * and listmount(2).
+ *
+ * For statmount(2) @param represents the request mask.
+ * For listmount(2) @param represents the last listed mount id (or zero).
+ */
 struct mnt_id_req {
 	__u64 mnt_id;
-	__u64 request_mask;
+	__u64 param;
 };
 
 /*
-- 
2.41.0