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, 13 insertions, 2 deletions

diff --git a/fs/cifs/cifsproto.h b/fs/cifs/cifsproto.h
index ed6ea0c59cb9..c8faa3e82fe7 100644
--- a/fs/cifs/cifsproto.h
+++ b/fs/cifs/cifsproto.h
@@ -69,9 +69,20 @@ extern int init_cifs_idmap(void);
 extern void exit_cifs_idmap(void);
 extern int init_cifs_spnego(void);
 extern void exit_cifs_spnego(void);
-extern const char *build_path_from_dentry(struct dentry *);
+extern const char *build_path_from_dentry(struct dentry *, void *);
 extern char *build_path_from_dentry_optional_prefix(struct dentry *direntry,
-						    bool prefix);
+						    void *page, bool prefix);
+static inline void *alloc_dentry_path(void)
+{
+	return __getname();
+}
+
+static inline void free_dentry_path(void *page)
+{
+	if (page)
+		__putname(page);
+}
+
 extern char *cifs_build_path_to_root(struct smb3_fs_context *ctx,
 				     struct cifs_sb_info *cifs_sb,
 				     struct cifs_tcon *tcon,