getname_maybe_null() - the third variant of pathname copy-in

Semantics used by statx(2) (and later *xattrat(2)): without AT_EMPTY_PATH
it's standard getname() (i.e. ERR_PTR(-ENOENT) on empty string,
ERR_PTR(-EFAULT) on NULL), with AT_EMPTY_PATH both empty string and
NULL are accepted.

Calling conventions: getname_maybe_null(user_pointer, flags) returns
	* pointer to struct filename when non-empty string had been
successfully read
	* ERR_PTR(...) on error
	* NULL if an empty string or NULL pointer had been given
with AT_EMPTY_PATH in the flags argument.

It tries to avoid allocation in the last case; it's not always
able to do so, in which case the temporary struct filename instance
is freed and NULL returned anyway.

Fast path is inlined.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 23 insertions, 7 deletions

diff --git a/fs/namei.c b/fs/namei.c
index aaf3cd6c802f..2bfe476c3bd0 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -211,22 +211,38 @@ getname_flags(const char __user *filename, int flags)
 	return result;
 }
 
-struct filename *
-getname_uflags(const char __user *filename, int uflags)
+struct filename *getname_uflags(const char __user *filename, int uflags)
 {
 	int flags = (uflags & AT_EMPTY_PATH) ? LOOKUP_EMPTY : 0;
 
 	return getname_flags(filename, flags);
 }
 
-struct filename *
-getname(const char __user * filename)
+struct filename *getname(const char __user * filename)
 {
 	return getname_flags(filename, 0);
 }
 
-struct filename *
-getname_kernel(const char * filename)
+struct filename *__getname_maybe_null(const char __user *pathname)
+{
+	struct filename *name;
+	char c;
+
+	/* try to save on allocations; loss on um, though */
+	if (get_user(c, pathname))
+		return ERR_PTR(-EFAULT);
+	if (!c)
+		return NULL;
+
+	name = getname_flags(pathname, LOOKUP_EMPTY);
+	if (!IS_ERR(name) && !(name->name[0])) {
+		putname(name);
+		name = NULL;
+	}
+	return name;
+}
+
+struct filename *getname_kernel(const char * filename)
 {
 	struct filename *result;
 	int len = strlen(filename) + 1;
@@ -264,7 +280,7 @@ EXPORT_SYMBOL(getname_kernel);
 
 void putname(struct filename *name)
 {
-	if (IS_ERR(name))
+	if (IS_ERR_OR_NULL(name))
 		return;
 
 	if (WARN_ON_ONCE(!atomic_read(&name->refcnt)))