cifs: allocate buffer in the caller of build_path_from_dentry()

build_path_from_dentry() open-codes dentry_path_raw().  The reason
we can't use dentry_path_raw() in there (and postprocess the
result as needed) is that the callers of build_path_from_dentry()
expect that the object to be freed on cleanup and the string to
be used are at the same address.  That's painful, since the path
is naturally built end-to-beginning - we start at the leaf and
go through the ancestors, accumulating the pathname.

Life would be easier if we left the buffer allocation to callers.
It wouldn't be exact-sized buffer, but none of the callers keep
the result for long - it's always freed before the caller returns.
So there's no need to do exact-sized allocation; better use
__getname()/__putname(), same as we do for pathname arguments
of syscalls.  What's more, there's no need to do allocation under
spinlocks, so GFP_ATOMIC is not needed.

Next patch will replace the open-coded dentry_path_raw() (in
build_path_from_dentry_optional_prefix()) with calling the real
thing.  This patch only introduces wrappers for allocating/freeing
the buffers and switches to new calling conventions:
	build_path_from_dentry(dentry, buf)
expects buf to be address of a page-sized object or NULL,
return value is a pathname built inside that buffer on success,
ERR_PTR(-ENOMEM) if buf is NULL and ERR_PTR(-ENAMETOOLONG) if
the pathname won't fit into page.  Note that we don't need to
check for failure when allocating the buffer in the caller -
build_path_from_dentry() will do the right thing.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Steve French <stfrench@microsoft.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/fs/cifs/ioctl.c b/fs/cifs/ioctl.c
index aba573dd86ac..08d99fec593e 100644
--- a/fs/cifs/ioctl.c
+++ b/fs/cifs/ioctl.c
@@ -43,12 +43,15 @@ static long cifs_ioctl_query_info(unsigned int xid, struct file *filep,
 	struct cifs_tcon *tcon = cifs_sb_master_tcon(cifs_sb);
 	struct dentry *dentry = filep->f_path.dentry;
 	const unsigned char *path;
+	void *page = alloc_dentry_path();
 	__le16 *utf16_path = NULL, root_path;
 	int rc = 0;
 
-	path = build_path_from_dentry(dentry);
-	if (path == NULL)
-		return -ENOMEM;
+	path = build_path_from_dentry(dentry, page);
+	if (IS_ERR(path)) {
+		free_dentry_path(page);
+		return PTR_ERR(path);
+	}
 
 	cifs_dbg(FYI, "%s %s\n", __func__, path);
 
@@ -73,7 +76,7 @@ static long cifs_ioctl_query_info(unsigned int xid, struct file *filep,
  ici_exit:
 	if (utf16_path != &root_path)
 		kfree(utf16_path);
-	kfree(path);
+	free_dentry_path(page);
 	return rc;
 }